Bench PSU - buy or build?

J

James Harris

Guest
Hi. Am looking for a bench power supply that has positive and
negative supplies. Ideal would be a device that supplies +12,
+5, -5, -12 and a variable 0-30V DC, preferably with current
limiting.

I have been thinking about making such from a computer power
supply (to get the +/-12 and +/-5) and a torroidal with 2x23V
RMS to get circa 30V with full-wave rectification. I could then
use a semiconductor regulator (eg. L200x on the positive and
maybe something on the negative side also) to adjust the output.

Frankly the above is a bit daunting. I am OK with the safety but
unsure of the details of the electronics. It seems to be
expensive to get any kind of panel meter, let alone the two or
four required (V and A on both variable outputs). And a decent
case will be costly also.

Would I be better buying? I have seen this one,
http://www.cybermarket.co.uk/ishop/923/shopscr1565.html, which
is the Skytronic unit here,
http://www.skytronic.com/uk/prod/search.php?s=650.685. It has
one +5V and two variable 0-30V for UKP200. It has -, 0, and +
outputs so does that means it will supply up to (-15 to +15) on
each output? I've read the manual on the web site which is,
frankly, dismal. This is a little offputting but other devices
with similar specs are at least twice the price.....

Should I buy or build? Any guidance much appreciated.
- James
 
"James Harris" <no.email.please> wrote in message
news:3f9c31b8$0$12686$fa0fcedb@lovejoy.zen.co.uk...
Hi. Am looking for a bench power supply that has positive and
negative supplies. Ideal would be a device that supplies +12,
+5, -5, -12 and a variable 0-30V DC, preferably with current
limiting.

I have been thinking about making such from a computer power
supply (to get the +/-12 and +/-5) and a torroidal with 2x23V
RMS to get circa 30V with full-wave rectification. I could then
use a semiconductor regulator (eg. L200x on the positive and
maybe something on the negative side also) to adjust the output.

Frankly the above is a bit daunting. I am OK with the safety but
unsure of the details of the electronics. It seems to be
expensive to get any kind of panel meter, let alone the two or
four required (V and A on both variable outputs). And a decent
case will be costly also.

Would I be better buying? I have seen this one,
http://www.cybermarket.co.uk/ishop/923/shopscr1565.html, which
is the Skytronic unit here,
http://www.skytronic.com/uk/prod/search.php?s=650.685. It has
one +5V and two variable 0-30V for UKP200. It has -, 0, and +
outputs so does that means it will supply up to (-15 to +15) on
each output? I've read the manual on the web site which is,
frankly, dismal. This is a little offputting but other devices
with similar specs are at least twice the price.....

Should I buy or build? Any guidance much appreciated.
- James
Click to expand...
Watch EBAY for power supplies. You will occasionally find an old bench
supply for a reasonable price.

Building linear power supplies is pretty easy, at least the fixed voltage
ones. Just get some transformers of the right size, and use the appropriate
78xx regulator (with bridge, caps, etc.)

The variable supply is also fairly easy using something like an LM317.
However, you waste lots of power (and thus generate lots of heat) when using
something like this at 3VDC and 2A.

EBAY occasionally has variacs on sale, which is a good solution to the
waste/heat issue; I've seen them for as low as $16 US (which I lost to a
sniper, drat!)

One thing is that unless you are a craftsman, your enclosure won't look as
nice. Also, the displays, current limiting, control knobs, etc that you get
with a typical bench supply are features you'll eventually want.

Regards,
Bob Monsen
 
"James Harris" <no.email.please> wrote in message
news:3f9c31b8$0$12686$fa0fcedb@lovejoy.zen.co.uk...

I have been thinking about making such from a computer power
supply
Frankly the above is a bit daunting. I am OK with the safety but
unsure of the details of the electronics.
Click to expand...
Years ago I built all my own equipment
it looked like crap & barely functioned
but it was so much fun.
You won't learn anything buying
'off the shelf'
 
If you want to build it yourself, go ahead and do it, but be warned that you
will probably be better off in the long run with buying a pre-built supply.
I built a variable dual 3 amp supply (LM317 iirc), with an additional 5V
supply built in, in a rack mount chassis. I put in a digital panel meter
and a dial to read V or A on any of the 3 supplies. It is big, heavy, and
was not cheap to built. The panel meter is non operational now because you
need a separate supply to power it (I may buy a DC-DC converter to power the
meter, that will isolate the supply voltage). I recently bought an RC car
and all the associated gear. The battery charger I bought came with an
AC/DC switching power supply much like the typical "wall wart" only this one
is switching, has a fan, and puts out 12V at 7A! It is VERY lightweight, I
almost can't believe it. The supply and smart charger were only ~$60. This
is not what you are looking for, but just an example of what power supply
technology has become. My advice, buy a nice modern pre-built supply.

--Dan



"James Harris" <no.email.please> wrote in message
news:3f9c31b8$0$12686$fa0fcedb@lovejoy.zen.co.uk...
Hi. Am looking for a bench power supply that has positive and
negative supplies. Ideal would be a device that supplies +12,
+5, -5, -12 and a variable 0-30V DC, preferably with current
limiting.

I have been thinking about making such from a computer power
supply (to get the +/-12 and +/-5) and a torroidal with 2x23V
RMS to get circa 30V with full-wave rectification. I could then
use a semiconductor regulator (eg. L200x on the positive and
maybe something on the negative side also) to adjust the output.

Frankly the above is a bit daunting. I am OK with the safety but
unsure of the details of the electronics. It seems to be
expensive to get any kind of panel meter, let alone the two or
four required (V and A on both variable outputs). And a decent
case will be costly also.

Would I be better buying? I have seen this one,
http://www.cybermarket.co.uk/ishop/923/shopscr1565.html, which
is the Skytronic unit here,
http://www.skytronic.com/uk/prod/search.php?s=650.685. It has
one +5V and two variable 0-30V for UKP200. It has -, 0, and +
outputs so does that means it will supply up to (-15 to +15) on
each output? I've read the manual on the web site which is,
frankly, dismal. This is a little offputting but other devices
with similar specs are at least twice the price.....

Should I buy or build? Any guidance much appreciated.
- James
Click to expand...
 
Your comments about wall warts reminded me of this link,
http://my.integritynet.com.au/purdic/power3.htm. Seems a good
compromise!

I was interested in this one,
http://members.cox.net/berniekm/Bench.html, which seems to read
voltage and current and drive a 3-digit LED display for a song.
(7-segment LED displays are pennies, the sensing circuits are
op-amps) The work is all done in software on a PICmicro. Seems
very clever.
- James

"dg" <dan_gus@hotmail.com> wrote in message
news:%Fcnb.6376$1D7.2818@newssvr27.news.prodigy.com...
If you want to build it yourself, go ahead and do it, but be
warned that you
will probably be better off in the long run with buying a
pre-built supply.
I built a variable dual 3 amp supply (LM317 iirc), with an
additional 5V
supply built in, in a rack mount chassis. I put in a digital
panel meter
and a dial to read V or A on any of the 3 supplies. It is
big, heavy, and
was not cheap to built. The panel meter is non operational
now because you
need a separate supply to power it (I may buy a DC-DC
converter to power the
meter, that will isolate the supply voltage). I recently
bought an RC car
and all the associated gear. The battery charger I bought
came with an
AC/DC switching power supply much like the typical "wall wart"
only this one
is switching, has a fan, and puts out 12V at 7A! It is VERY
lightweight, I
almost can't believe it. The supply and smart charger were
only ~$60. This
is not what you are looking for, but just an example of what
power supply
technology has become. My advice, buy a nice modern pre-built
supply.

--Dan



"James Harris" <no.email.please> wrote in message
news:3f9c31b8$0$12686$fa0fcedb@lovejoy.zen.co.uk...
Hi. Am looking for a bench power supply that has positive
and
negative supplies. Ideal would be a device that supplies
+12,
+5, -5, -12 and a variable 0-30V DC, preferably with current
limiting.

I have been thinking about making such from a computer power
supply (to get the +/-12 and +/-5) and a torroidal with
2x23V
RMS to get circa 30V with full-wave rectification. I could
then
use a semiconductor regulator (eg. L200x on the positive and
maybe something on the negative side also) to adjust the
output.

Frankly the above is a bit daunting. I am OK with the safety
but
unsure of the details of the electronics. It seems to be
expensive to get any kind of panel meter, let alone the two
or
four required (V and A on both variable outputs). And a
decent
case will be costly also.

Would I be better buying? I have seen this one,
http://www.cybermarket.co.uk/ishop/923/shopscr1565.html,
which
is the Skytronic unit here,
http://www.skytronic.com/uk/prod/search.php?s=650.685. It
has
one +5V and two variable 0-30V for UKP200. It has -, 0, and
+
outputs so does that means it will supply up to (-15 to +15)
on
each output? I've read the manual on the web site which is,
frankly, dismal. This is a little offputting but other
devices
with similar specs are at least twice the price.....

Should I buy or build? Any guidance much appreciated.
- James
Click to expand...
 
"Robert Monsen" <postmaster@BulkingPro.com> wrote in message
news:dUZmb.39031$HS4.156034@attbi_s01...
"James Harris" <no.email.please> wrote in message
news:3f9c31b8$0$12686$fa0fcedb@lovejoy.zen.co.uk...
Hi. Am looking for a bench power supply that has positive
and
negative supplies. Ideal would be a device that supplies
+12,
+5, -5, -12 and a variable 0-30V DC, preferably with current
limiting.

I have been thinking about making such from a computer power
supply (to get the +/-12 and +/-5) and a torroidal with
2x23V
RMS to get circa 30V with full-wave rectification. I could
then
use a semiconductor regulator (eg. L200x on the positive and
maybe something on the negative side also) to adjust the
output.

Frankly the above is a bit daunting. I am OK with the safety
but
unsure of the details of the electronics. It seems to be
expensive to get any kind of panel meter, let alone the two
or
four required (V and A on both variable outputs). And a
decent
case will be costly also.

Would I be better buying? I have seen this one,
http://www.cybermarket.co.uk/ishop/923/shopscr1565.html,
which
is the Skytronic unit here,
http://www.skytronic.com/uk/prod/search.php?s=650.685. It
has
one +5V and two variable 0-30V for UKP200. It has -, 0, and
+
outputs so does that means it will supply up to (-15 to +15)
on
each output? I've read the manual on the web site which is,
frankly, dismal. This is a little offputting but other
devices
with similar specs are at least twice the price.....

Should I buy or build? Any guidance much appreciated.
- James




Watch EBAY for power supplies. You will occasionally find an
old bench
supply for a reasonable price.

Building linear power supplies is pretty easy, at least the
fixed voltage
ones. Just get some transformers of the right size, and use
the appropriate
78xx regulator (with bridge, caps, etc.)

The variable supply is also fairly easy using something like
an LM317.
However, you waste lots of power (and thus generate lots of
heat) when using
something like this at 3VDC and 2A.

EBAY occasionally has variacs on sale, which is a good
solution to the
waste/heat issue; I've seen them for as low as $16 US (which I
lost to a
sniper, drat!)

One thing is that unless you are a craftsman, your enclosure
won't look as
nice. Also, the displays, current limiting, control knobs, etc
that you get
with a typical bench supply are features you'll eventually
want.

Regards,
Bob Monsen
Click to expand...
Variacs sound good to cut down on power and heat. I would buy
from a retailer but the variacs I can see seem designed to work
up to mains voltage. I guess they could just be used at the
lower end of the range. I'll give it some thought. I can see
myself ending up making more than one PSU.

I wonder why variacs don't isolate from the mains. I can't think
of a reason the secondaries need to be linked to the primary.

Yes, am very very far from a craftsman.
Cheers,
James
 
"James Harris" <no.email.please> wrote in message
news:3f9c31b8$0$12686$fa0fcedb@lovejoy.zen.co.uk...
Hi. Am looking for a bench power supply that has positive and
negative supplies. Ideal would be a device that supplies +12,
+5, -5, -12 and a variable 0-30V DC, preferably with current
limiting.

snip
Click to expand...
A computer psu might be a good choice, it has most protections already
built-in. But it does not provide voltage regulation facilities. I'll
describe an idea how to construct an (a little advanced) linear psu, if you
have further questions about linear regulators, please ask.

The choice whether to build or buy a psu is yours, but be warned that
building one may take more time and be more expensive than one may want.
However you won't learn anything (except hating the prices) by buying one
while constructing your own psu is likely to be a lot more interesting. If
you have some spare time left, my advice would be: Try it. If you have old
components salvaged from some no-more-use equipment, constructing a psu
might even be cheap (I built mine entirely from this sort of things, it's
working reliably for 3 years now, params: 1 to 30V, 8A). The enclosure will
be one of the hardest parts to find, so a poor man's solution may be a
speaker box with transformer and heatsink on top unless you have the means
and skills to create a more suitable one. In your post you only gave the
voltages, no maximal current values, so I can hardly assume anything about
the power throughput you need. So for the simple reason of not disappointing
you with a 'weak' psu let's assume 7A approx current at 30V. There are 3
classical approaches to such a thing: 1: Linear, 2: Switching, 3:
Triac-controlled transformer with linear fine-regulation electronics. The
first solution is the easiest, but the most inefficient. The second one is
the most complicated and exceptionally hard to build at home (do not try
unless you are really experienced), but the most efficient. And finally the
third one might be a viable medium-complicated solution with acceptable
efficiency. The concept of this device is to roughly pre-control the voltage
on the primary of a transformer by using a triac (rated at two times the
maximal transformer power), then transform the voltage down, rectify it
full-bridge, filter it, and finally use a linear regulator to fine-tune the
voltage, limit the current (if necessary), implement a short circuit
protection and this like. The disadvantage of this design is that there is a
need to operate 3 (if you are skilled, maybe 2) transformers independently
of each other while only one of them delivers useful power. Since I have
only built similar, but not exactly the same sort of power supplies I cannot
give you a very exact description, nevertheless I'll try to outline a likely
possible way of handling this. Everyone who is more experienced is
encouraged to correct me. First, the easiest-to-test structure of any device
is a modular one, so I'll try to make use of it. The psu shall thus consist
of 3 parts (apart from the xformers): 1: the triac circuit, 2: the linear
regulation circuit and 3: the control circuit. First the triac circuit. Note
that it is not isolated and operates at mains voltage. To control the
primary of a transformer, a triac is placed in series with it. To prevent
the triac from tripping accidentally, a resistor (around 500k) should be
connected to the cathode and control pins of it. The voltage to 'fire' the
triac can be taken off a capacitor that is charged from the mains through a
resistor (resistor value depending on the capacitor used, the more
capacitance, the less resistance, but not less than 30k, you'll need to do
some testing). To make the thing work more reliable, 2 zeners in series, but
with opposite directions and a voltage of about 6 V can be used in series
with the triac gate. Controlling is done by shunting the cap with a
transistor (bridge rectifier needed), this will delay the time until the
triac triggers, if the transistor is open completely, the triac will shut
down. The transistor base voltage / current is provided by an additional
power source (very small trafo, maybe half a watt) and controlled via an
optocoupler (Isolation required!).
Schematics in ASCII:

-------x------------------------------
from | R__ to trafo
mains '--|__|-.
C |
from .--||---x--------.
mains | | to trafo
-------x-------|><|-------------------
| / Triac |
| R__ | |
x-|__|-x-|<|-|>|-x
| 2 Zeners|
| .-------------'
| |
---------
|rectifier| .---. R__
--------- | | .------|__|---- -
-| |+ | C \ B | R__ opto
| '-----' |---x-|__|-. .---- +
| E</NPN | C | |
| | '-||---x '---- +
| | | 6V DC
'------------x------------x------- -

When the LED part of the optocoupler is on, the transistor will be on, it
will shunt the triac-controlling cap and the triac will stay off, the less
current will pass through the opto LED, the more power will reach the main
transformer.

The next part would be a linear regulator. with its control circuitry. I'll
leave it up to you to choose one appropriate for your needs and draw it as a
simple square in the schematics. Note that if you use a custom regulation
system (a couple of 2N3055 with appropriate control or this like), an
additional transformer will be needed to power the control circuit.

.-------.
| regu |
----------|-----|-----x---x--| lator |--x---x-----
from | rec | + C_|_ | | (lin) | | _|_ +
trafo | tif | ___ | '-------' | ___ OUT
secondary | ier | - | | | | | -
----------|-----|-----x----------x----------x-----
| |
| R__ |Zener 4V
'-|__|-. '-|>|-----
1K | -
| To opto LED
| +
'----------------

Depending on the voltage difference between the connections before and after
the regulator the triac circuit will be auto-adjusted using the optocoupler.
When the voltage rises, the LED will light stronger and the optocoupler will
lower the triac throughput thus limiting the voltage.

That's all. The whole thing can be repeated with 2 transformers if a
positive and a negative voltage are needed simultaneously. It is also
possible to use one power transformer and connect the 2 optocoupler sensors
in series to control one triac. This may be less efficient though.

Please note that this design is certainly a waste of parts and time if less
than some 5A at 30V output are needed. For a less powerful psu just use
linear regulation with no triacs. Don't forget a BIG heatsink in either
case.

More questions -> please ask here,
you can also email me (address valid).

Dimitrij
 
"Chris Oates" <CUTchrisoates99@hotmail.com> wrote in message
news:3f9c7934$0$9466$cc9e4d1f@news.dial.pipex.com...
"James Harris" <no.email.please> wrote in message
news:3f9c31b8$0$12686$fa0fcedb@lovejoy.zen.co.uk...

I have been thinking about making such from a computer power
supply
Frankly the above is a bit daunting. I am OK with the safety
but
unsure of the details of the electronics.

Years ago I built all my own equipment
it looked like crap & barely functioned
but it was so much fun.
You won't learn anything buying
'off the shelf'
Click to expand...
Agreed. This is the one I saw about using an old computer PSU,
http://academic1.bellevue.edu/robots/powerSupply/. What do you
think about the use of just 2Ohms between +5 and ground to
preload the PSU. This seems an incredibly small value to me,
passing a large 2.5A just to get the PSU working.

Another point. Would it be naiive to add zeners on all except
the +5 to ensure the outputs don't exceed the rated voltages by
too much?
- James
 
James Harris <no.email.please> wrote:
I wonder why variacs don't isolate from the mains. I can't think
of a reason the secondaries need to be linked to the primary.
Click to expand...
Just economics -- you don't need two windings if your VARIAC isn't isolated!
You certainly can buy isolated VARIACs, though -- I own one, and I believe
they're still readily available. Other than cost, I can't think of a good
reason you wouldn't buy one! :)

---Joel Kolstad
 
"James Harris" <no.email.please> wrote:

Agreed. This is the one I saw about using an old computer PSU,
http://academic1.bellevue.edu/robots/powerSupply/. What do you
think about the use of just 2Ohms between +5 and ground to
preload the PSU. This seems an incredibly small value to me,
passing a large 2.5A just to get the PSU working.
Try a higher resistance. A lot of PC PSUs will work with a small load or
Click to expand...
sometimes even no load at all. Don't waste 2.5A

Another point. Would it be naiive to add zeners on all except
the +5 to ensure the outputs don't exceed the rated voltages by
too much?
Usually the don't exceed the specs much anyway. If they do, use some other
Click to expand...
protection. Do not add zeners, a current of up to 10A (+ 12V line) is
likely to kill them. Instead try to find out where the 5V loopback controls
the PSU power and add there a 12V-controlled protection circuit.

Dimitrij
 
"James Harris" <no.email.please> wrote:

Variacs sound good to cut down on power and heat. I would buy
from a retailer but the variacs I can see seem designed to work
up to mains voltage. I guess they could just be used at the
lower end of the range. I'll give it some thought. I can see
myself ending up making more than one PSU.

I wonder why variacs don't isolate from the mains. I can't think
of a reason the secondaries need to be linked to the primary.
Click to expand...
Use a variac that goes up to mains voltage w/o isolation. Connect a stepdown
transformer to it. This will provide isolation and a better efficiency for
low voltages and high currents. Note that it would be less efficient than a
variac only for high voltages and low currents.

As to why a lot of variacs don't isolate: They have no secondary, the (only
existing) primary is used in auto-transformer mode. There are exceptions
with (most likely isolated) secondaries though.

See also my reply about triac control. You may want to substitute the triac
with a variac and adjust it manually instead of by optocoupler. In this
case a voltage meter in the place of the opto LED will show whether variac
adjustments are necessary. It does not have to be exact, just use the
cheapest one available.

Dimitrij
 
Sorry, there was a mistake in my post. Not a real one, but it could end up
with a less efficient device. Use a 2V Zener instead of a 4V one.

| R__ |Zener 2V
'-|__|-. '-|>|-----
1K | -
| To opto LED
| +
'----------------

Part of original message:
The next part would be a linear regulator. with its control circuitry.
I'll leave it up to you to choose one appropriate for your needs and draw
it as a simple square in the schematics. Note that if you use a custom
regulation system (a couple of 2N3055 with appropriate control or this
like), an additional transformer will be needed to power the control
circuit.

.-------.
| regu |
----------|-----|-----x---x--| lator |--x---x-----
from | rec | + C_|_ | | (lin) | | _|_ +
trafo | tif | ___ | '-------' | ___ OUT
secondary | ier | - | | | | | -
----------|-----|-----x----------x----------x-----
| |
| R__ |Zener 4V
'-|__|-. '-|>|-----
1K | -
| To opto LED
| +
'----------------

Depending on the voltage difference between the connections before and
after the regulator the triac circuit will be auto-adjusted using the
optocoupler. When the voltage rises, the LED will light stronger and the
optocoupler will lower the triac throughput thus limiting the voltage.
Click to expand...
Dimitrij
 
"James Harris" <no.email.please> wrote in message
news:3f9c31b8$0$12686$fa0fcedb@lovejoy.zen.co.uk...
Click to expand...
"dg" <dan_gus@hotmail.com> wrote in message
news:%Fcnb.6376$1D7.2818@newssvr27.news.prodigy.com...

Hi. Am looking for a bench power supply that has positive and
negative supplies. Ideal would be a device that supplies +12,
+5, -5, -12 and a variable 0-30V DC, preferably with current
limiting.

I have been thinking about making such from a computer power
supply (to get the +/-12 and +/-5) and a torroidal with 2x23V
RMS to get circa 30V with full-wave rectification. I could then
use a semiconductor regulator (eg. L200x on the positive and
maybe something on the negative side also) to adjust the output.

Frankly the above is a bit daunting. I am OK with the safety but
unsure of the details of the electronics. It seems to be
expensive to get any kind of panel meter, let alone the two or
four required (V and A on both variable outputs). And a decent
case will be costly also.

Would I be better buying? I have seen this one,
http://www.cybermarket.co.uk/ishop/923/shopscr1565.html, which
is the Skytronic unit here,
http://www.skytronic.com/uk/prod/search.php?s=650.685. It has
one +5V and two variable 0-30V for UKP200. It has -, 0, and +
outputs so does that means it will supply up to (-15 to +15) on
each output? I've read the manual on the web site which is,
frankly, dismal. This is a little offputting but other devices
with similar specs are at least twice the price.....

Should I buy or build? Any guidance much appreciated.
- James





If you want to build it yourself, go ahead and do it, but be warned that
you
will probably be better off in the long run with buying a pre-built
supply.
I built a variable dual 3 amp supply (LM317 iirc), with an additional 5V
supply built in, in a rack mount chassis. I put in a digital panel meter
and a dial to read V or A on any of the 3 supplies. It is big, heavy, and
was not cheap to built. The panel meter is non operational now because
you
need a separate supply to power it (I may buy a DC-DC converter to power
the
meter, that will isolate the supply voltage). I recently bought an RC car
and all the associated gear. The battery charger I bought came with an
AC/DC switching power supply much like the typical "wall wart" only this
one
is switching, has a fan, and puts out 12V at 7A! It is VERY lightweight,
I
almost can't believe it. The supply and smart charger were only ~$60.
This
is not what you are looking for, but just an example of what power supply
technology has become. My advice, buy a nice modern pre-built supply.

--Dan
Click to expand...
The thing about modern supplies is that they are usually switching supplies.
That is how they get away with such small transformers (they use higher
frequency transformers, which require much less metal.)

One problem with these, however, is that they are really noisy. If you are
doing work with an oscilloscope, you can end up with lots of noise in the
signal of the device under test from the supply. I was using a switch mode
PSU from an old cable modem for a bit, and it was great in terms of
generating lots of different output voltages (35V, 12V, 5V, -5V, -12V, all
at different amperages.) However, I couldn't get away from the 250k 10mV
signal it was adding into everything. It didn't even have to be connected,
it was radiating the signal so intensely that my probes would pick it up.

Switching <g> to a linear power supply did the trick. Much cleaner signals.

Regards,
Bob Monsen
 
"Dimitrij Klingbeil" <dklingb@cs.tu-berlin.de> wrote in message
news:3f9d9c28_1@news.arcor-ip.de...
"James Harris" <no.email.please> wrote in message
news:3f9c31b8$0$12686$fa0fcedb@lovejoy.zen.co.uk...
Hi. Am looking for a bench power supply that has positive
and
negative supplies. Ideal would be a device that supplies
+12,
+5, -5, -12 and a variable 0-30V DC, preferably with current
limiting.

snip

A computer psu might be a good choice, it has most protections
already
built-in. But it does not provide voltage regulation
facilities. I'll
describe an idea how to construct an (a little advanced)
linear psu, if you
have further questions about linear regulators, please ask.

The choice whether to build or buy a psu is yours, but be
warned that
building one may take more time and be more expensive than one
may want.
However you won't learn anything (except hating the prices) by
buying one
while constructing your own psu is likely to be a lot more
interesting. If
you have some spare time left, my advice would be: Try it. If
you have old
components salvaged from some no-more-use equipment,
constructing a psu
might even be cheap (I built mine entirely from this sort of
things, it's
working reliably for 3 years now, params: 1 to 30V, 8A). The
enclosure will
be one of the hardest parts to find, so a poor man's solution
may be a
speaker box with transformer and heatsink on top unless you
have the means
and skills to create a more suitable one. In your post you
only gave the
voltages, no maximal current values, so I can hardly assume
anything about
the power throughput you need. So for the simple reason of not
disappointing
you with a 'weak' psu let's assume 7A approx current at 30V.
There are 3
classical approaches to such a thing: 1: Linear, 2: Switching,
3:
Triac-controlled transformer with linear fine-regulation
electronics. The
first solution is the easiest, but the most inefficient. The
second one is
the most complicated and exceptionally hard to build at home
(do not try
unless you are really experienced), but the most efficient.
And finally the
third one might be a viable medium-complicated solution with
acceptable
efficiency. The concept of this device is to roughly
pre-control the voltage
on the primary of a transformer by using a triac (rated at two
times the
maximal transformer power), then transform the voltage down,
rectify it
full-bridge, filter it, and finally use a linear regulator to
fine-tune the
voltage, limit the current (if necessary), implement a short
circuit
protection and this like. The disadvantage of this design is
that there is a
need to operate 3 (if you are skilled, maybe 2) transformers
independently
of each other while only one of them delivers useful power.
Since I have
only built similar, but not exactly the same sort of power
supplies I cannot
give you a very exact description, nevertheless I'll try to
outline a likely
possible way of handling this. Everyone who is more
experienced is
encouraged to correct me. First, the easiest-to-test structure
of any device
is a modular one, so I'll try to make use of it. The psu shall
thus consist
of 3 parts (apart from the xformers): 1: the triac circuit, 2:
the linear
regulation circuit and 3: the control circuit. First the triac
circuit. Note
that it is not isolated and operates at mains voltage. To
control the
primary of a transformer, a triac is placed in series with it.
To prevent
the triac from tripping accidentally, a resistor (around 500k)
should be
connected to the cathode and control pins of it. The voltage
to 'fire' the
triac can be taken off a capacitor that is charged from the
mains through a
resistor (resistor value depending on the capacitor used, the
more
capacitance, the less resistance, but not less than 30k,
you'll need to do
some testing). To make the thing work more reliable, 2 zeners
in series, but
with opposite directions and a voltage of about 6 V can be
used in series
with the triac gate. Controlling is done by shunting the cap
with a
transistor (bridge rectifier needed), this will delay the time
until the
triac triggers, if the transistor is open completely, the
triac will shut
down. The transistor base voltage / current is provided by an
additional
power source (very small trafo, maybe half a watt) and
controlled via an
optocoupler (Isolation required!).
Schematics in ASCII:

-------x------------------------------
from | R__ to trafo
mains '--|__|-.
C |
from .--||---x--------.
mains | | to trafo
-------x-------|><|-------------------
| / Triac |
| R__ | |
x-|__|-x-|<|-|>|-x
| 2 Zeners|
| .-------------'
| |
---------
|rectifier| .---. R__
--------- | | .------|__|---- -
-| |+ | C \ B | R__ opto
| '-----' |---x-|__|-. .---- +
| E</NPN | C | |
| | '-||---x '---- +
| | | 6V DC
'------------x------------x------- -

When the LED part of the optocoupler is on, the transistor
will be on, it
will shunt the triac-controlling cap and the triac will stay
off, the less
current will pass through the opto LED, the more power will
reach the main
transformer.

The next part would be a linear regulator. with its control
circuitry. I'll
leave it up to you to choose one appropriate for your needs
and draw it as a
simple square in the schematics. Note that if you use a custom
regulation
system (a couple of 2N3055 with appropriate control or this
like), an
additional transformer will be needed to power the control
circuit.

.-------.
| regu |
----------|-----|-----x---x--| lator |--x---x-----
from | rec | + C_|_ | | (lin) | | _|_ +
trafo | tif | ___ | '-------' | ___ OUT
secondary | ier | - | | | | | -
----------|-----|-----x----------x----------x-----
| |
| R__ |Zener 4V
'-|__|-. '-|>|-----
1K | -
| To opto LED
| +
'----------------

Depending on the voltage difference between the connections
before and after
the regulator the triac circuit will be auto-adjusted using
the optocoupler.
When the voltage rises, the LED will light stronger and the
optocoupler will
lower the triac throughput thus limiting the voltage.

That's all. The whole thing can be repeated with 2
transformers if a
positive and a negative voltage are needed simultaneously. It
is also
possible to use one power transformer and connect the 2
optocoupler sensors
in series to control one triac. This may be less efficient
though.

Please note that this design is certainly a waste of parts and
time if less
than some 5A at 30V output are needed. For a less powerful psu
just use
linear regulation with no triacs. Don't forget a BIG heatsink
in either
case.

More questions -> please ask here,
you can also email me (address valid).

Dimitrij
Click to expand...
Dimitrij,

Thanks for all of the info. I get at least some of the
principles but I think this is a bit above me for now. I'll keep
it, though, for future reference - maybe a second project! I do
have a question, though, about displaying the voltage and
current. It would be essential to avoid dotting my meter between
circuit and PSU. Can you recommend a cost-effective but
reasonably accurate solution? If possible I would like to
display V and A at the same time.

Thanks,
James
 
"James Harris" <no.email.please> wrote
snip
Thanks for all of the info. I get at least some of the
principles but I think this is a bit above me for now. I'll keep
it, though, for future reference - maybe a second project! I do
have a question, though, about displaying the voltage and
current. It would be essential to avoid dotting my meter between
circuit and PSU. Can you recommend a cost-effective but
reasonably accurate solution? If possible I would like to
display V and A at the same time.

Thanks,
James
Click to expand...
Sorry, I don't have much experience with meters. I actually have only one
and using it for all sorts of testing (V, A, Ohm, hfe, F, H, Hz, diode test,
duty cycle, etc) I find myself disconnecting and reconnecting it in another
mode over and over :). It was not an especially cheap one either.

I assume, the more experienced users here will help you out.
 
In article <3f9c31b8$0$12686$fa0fcedb@lovejoy.zen.co.uk>, "James
Harris" <no.email.please> mentioned...
Hi. Am looking for a bench power supply that has positive and
negative supplies. Ideal would be a device that supplies +12,
+5, -5, -12 and a variable 0-30V DC, preferably with current
limiting.

I have been thinking about making such from a computer power
supply (to get the +/-12 and +/-5) and a torroidal with 2x23V
RMS to get circa 30V with full-wave rectification. I could then
use a semiconductor regulator (eg. L200x on the positive and
maybe something on the negative side also) to adjust the output.

Frankly the above is a bit daunting. I am OK with the safety but
unsure of the details of the electronics. It seems to be
expensive to get any kind of panel meter, let alone the two or
four required (V and A on both variable outputs). And a decent
case will be costly also.

Would I be better buying? I have seen this one,
http://www.cybermarket.co.uk/ishop/923/shopscr1565.html, which
is the Skytronic unit here,
http://www.skytronic.com/uk/prod/search.php?s=650.685. It has
one +5V and two variable 0-30V for UKP200. It has -, 0, and +
outputs so does that means it will supply up to (-15 to +15) on
each output? I've read the manual on the web site which is,
frankly, dismal. This is a little offputting but other devices
with similar specs are at least twice the price.....

Should I buy or build? Any guidance much appreciated.
- James
Click to expand...
I think it's more economical to buy than to build. I bought four HP
66312A 0-20V, 2A power supplies for $200 each, I could never come
close to getting the capabilities on anything I build. Here in the
U.S., inexpensive PSes are available, for an example see URL
http://www.mpja.com/productview.asp?product=14600+PS


--
@@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@h@e@r@e@@
###Got a Question about ELECTRONICS? Check HERE First:###
http://users.pandora.be/educypedia/electronics/databank.htm
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 <at> hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
@@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@u@e@n@t@@
 
In article <3f9c7934$0$9466$cc9e4d1f@news.dial.pipex.com>,
CUTchrisoates99@hotmail.com mentioned...
"James Harris" <no.email.please> wrote in message
news:3f9c31b8$0$12686$fa0fcedb@lovejoy.zen.co.uk...

I have been thinking about making such from a computer power
supply
Frankly the above is a bit daunting. I am OK with the safety but
unsure of the details of the electronics.

Years ago I built all my own equipment
it looked like crap & barely functioned
but it was so much fun.
You won't learn anything buying
'off the shelf'
Click to expand...
There's not a whole lot to learn when you assemble a PS made out of a
Xfmr, a FW bridge, a filter cap, a LM317, two resistors and a meter.
Just about all the design is done already.



--
@@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@h@e@r@e@@
###Got a Question about ELECTRONICS? Check HERE First:###
http://users.pandora.be/educypedia/electronics/databank.htm
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 <at> hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
@@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@u@e@n@t@@
 
In article <3f9d9c28_1@news.arcor-ip.de>, dklingb@cs.tu-berlin.de
mentioned...
"James Harris" <no.email.please> wrote in message
news:3f9c31b8$0$12686$fa0fcedb@lovejoy.zen.co.uk...
Hi. Am looking for a bench power supply that has positive and
negative supplies. Ideal would be a device that supplies +12,
+5, -5, -12 and a variable 0-30V DC, preferably with current
limiting.

snip

A computer psu might be a good choice, it has most protections already
built-in. But it does not provide voltage regulation facilities. I'll
describe an idea how to construct an (a little advanced) linear psu, if you
have further questions about linear regulators, please ask.

The choice whether to build or buy a psu is yours, but be warned that
building one may take more time and be more expensive than one may want.
However you won't learn anything (except hating the prices) by buying one
while constructing your own psu is likely to be a lot more interesting. If
you have some spare time left, my advice would be: Try it. If you have old
components salvaged from some no-more-use equipment, constructing a psu
might even be cheap (I built mine entirely from this sort of things, it's
working reliably for 3 years now, params: 1 to 30V, 8A). The enclosure will
be one of the hardest parts to find, so a poor man's solution may be a
speaker box with transformer and heatsink on top unless you have the means
and skills to create a more suitable one. In your post you only gave the
voltages, no maximal current values, so I can hardly assume anything about
the power throughput you need. So for the simple reason of not disappointing
you with a 'weak' psu let's assume 7A approx current at 30V. There are 3
classical approaches to such a thing: 1: Linear, 2: Switching, 3:
Triac-controlled transformer with linear fine-regulation electronics. The
first solution is the easiest, but the most inefficient. The second one is
the most complicated and exceptionally hard to build at home (do not try
unless you are really experienced), but the most efficient. And finally the
third one might be a viable medium-complicated solution with acceptable
efficiency. The concept of this device is to roughly pre-control the voltage
on the primary of a transformer by using a triac (rated at two times the
maximal transformer power), then transform the voltage down, rectify it
full-bridge, filter it, and finally use a linear regulator to fine-tune the
voltage, limit the current (if necessary), implement a short circuit
protection and this like. The disadvantage of this design is that there is a
need to operate 3 (if you are skilled, maybe 2) transformers independently
of each other while only one of them delivers useful power.
Click to expand...
Another serious disadvantage of a PS with a TRIAC or switch mode PS is
that they make a lot of EMI and RFI, so if you are using these kinds
of PS to power a low level amplifier, you will probably see some
interference in the circuit. Linear PSes don't have this kind of
problem. But Dimitrij has some good ideas if you don't plan on using
the PS for low level amplifiers.


Since I have
only built similar, but not exactly the same sort of power supplies I cannot
give you a very exact description, nevertheless I'll try to outline a likely
possible way of handling this. Everyone who is more experienced is
encouraged to correct me. First, the easiest-to-test structure of any device
is a modular one, so I'll try to make use of it. The psu shall thus consist
of 3 parts (apart from the xformers): 1: the triac circuit, 2: the linear
regulation circuit and 3: the control circuit. First the triac circuit. Note
that it is not isolated and operates at mains voltage. To control the
primary of a transformer, a triac is placed in series with it. To prevent
the triac from tripping accidentally, a resistor (around 500k) should be
connected to the cathode and control pins of it. The voltage to 'fire' the
triac can be taken off a capacitor that is charged from the mains through a
resistor (resistor value depending on the capacitor used, the more
capacitance, the less resistance, but not less than 30k, you'll need to do
some testing). To make the thing work more reliable, 2 zeners in series, but
with opposite directions and a voltage of about 6 V can be used in series
with the triac gate. Controlling is done by shunting the cap with a
transistor (bridge rectifier needed), this will delay the time until the
triac triggers, if the transistor is open completely, the triac will shut
down. The transistor base voltage / current is provided by an additional
power source (very small trafo, maybe half a watt) and controlled via an
optocoupler (Isolation required!).
Schematics in ASCII:

-------x------------------------------
from | R__ to trafo
mains '--|__|-.
C |
from .--||---x--------.
mains | | to trafo
-------x-------|><|-------------------
| / Triac |
| R__ | |
x-|__|-x-|<|-|>|-x
| 2 Zeners|
| .-------------'
| |
---------
|rectifier| .---. R__
--------- | | .------|__|---- -
-| |+ | C \ B | R__ opto
| '-----' |---x-|__|-. .---- +
| E</NPN | C | |
| | '-||---x '---- +
| | | 6V DC
'------------x------------x------- -

When the LED part of the optocoupler is on, the transistor will be on, it
will shunt the triac-controlling cap and the triac will stay off, the less
current will pass through the opto LED, the more power will reach the main
transformer.

The next part would be a linear regulator. with its control circuitry. I'll
leave it up to you to choose one appropriate for your needs and draw it as a
simple square in the schematics. Note that if you use a custom regulation
system (a couple of 2N3055 with appropriate control or this like), an
additional transformer will be needed to power the control circuit.

.-------.
| regu |
----------|-----|-----x---x--| lator |--x---x-----
from | rec | + C_|_ | | (lin) | | _|_ +
trafo | tif | ___ | '-------' | ___ OUT
secondary | ier | - | | | | | -
----------|-----|-----x----------x----------x-----
| |
| R__ |Zener 4V
'-|__|-. '-|>|-----
1K | -
| To opto LED
| +
'----------------

Depending on the voltage difference between the connections before and after
the regulator the triac circuit will be auto-adjusted using the optocoupler.
When the voltage rises, the LED will light stronger and the optocoupler will
lower the triac throughput thus limiting the voltage.

That's all. The whole thing can be repeated with 2 transformers if a
positive and a negative voltage are needed simultaneously. It is also
possible to use one power transformer and connect the 2 optocoupler sensors
in series to control one triac. This may be less efficient though.

Please note that this design is certainly a waste of parts and time if less
than some 5A at 30V output are needed. For a less powerful psu just use
linear regulation with no triacs. Don't forget a BIG heatsink in either
case.

More questions -> please ask here,
you can also email me (address valid).

Dimitrij
Click to expand...
--
@@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@h@e@r@e@@
###Got a Question about ELECTRONICS? Check HERE First:###
http://users.pandora.be/educypedia/electronics/databank.htm
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 <at> hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
@@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@u@e@n@t@@
 
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