AC, DC, Power Supply follow up

[Uriah]

I had posted a question a few days ago about needing to power a bill
acceptor that requires +12DC, -12VDC, +5VDC, and 6VAC and there is
only one common ground wire. I needed to hook up a AC Bench Power
supply along with a triple output DC bench power supply so that they
share the common ground wire. With some help from this group I got it
to work. Of course I ran into problems and could use some more help
so that I don't damage my bench top power supplies. My DC bench top
power supply has three outputs, one constant 5VDC and two variable
power sources. The bill acceptor used a unregulated DC power supply
and when I wire in the constant 5VDC it drops it down to 3.2VDC
because of the regulation in bill acceptor. So the constant 5VDC PS
won't work. I need a higher DCV source.

I have a few questions.

1. Can I use two separate DC power supplies and have them share the
common ground wire to power the bill acceptor? This is where I am
worried about damaging the power supplies.
2. Is there anyway to get -12VDC and +12VDC from one of the variable
sources and then I can use the other one for the 5VDC, which I have to
bring up to around 7VDC?
3. The triple output DC bench PS I have has a green earth connection
between the black and red connections on each of the variable
sources. Is there a way to use this to get the -12VDC and +12VDC using
this green earth ground wire? (I doubt it but I have to ask)

I hope this isn't to confusing. Thanks again for the help.
Russ

 

[pimpom]

Uriah wrote:

I had posted a question a few days ago about needing to power a
bill
acceptor that requires +12DC, -12VDC, +5VDC, and 6VAC and there
is
only one common ground wire. I needed to hook up a AC Bench
Power
supply along with a triple output DC bench power supply so
that they
share the common ground wire. With some help from this group I
got it
to work. Of course I ran into problems and could use some more
help
so that I don't damage my bench top power supplies. My DC
bench top
power supply has three outputs, one constant 5VDC and two
variable
power sources. The bill acceptor used a unregulated DC power
supply
and when I wire in the constant 5VDC it drops it down to 3.2VDC
because of the regulation in bill acceptor. So the constant
5VDC PS
won't work. I need a higher DCV source.

Something needs clarification here. When you specified +5VDC, is

that the supply rail needed by internal circuitry *after*
regulation or is +5VDC needed from the power supply? Using a
regulated source to supply an input that normally uses an
unregulated supply should not make any difference as long as the
two voltages are the same.

I have a few questions.

1. Can I use two separate DC power supplies and have them share
the
common ground wire to power the bill acceptor? This is where I
am
worried about damaging the power supplies.

Yes. As long as the two power supplies are completely
independent, isolated from the mains and do not share any other
connections except the mains input, there's no problem or risk of
damage because of the common ground.

2. Is there anyway to get -12VDC and +12VDC from one of the
variable
sources and then I can use the other one for the 5VDC, which I
have to
bring up to around 7VDC?

Since all supply voltages share a common ground, you should be
able to use the +12V line to supply *both* the +12V and the +5VDC
inputs without any modification. *However,* in dropping from +12V
to +5V, the +5V regulator may have to dissipate more power than
it was designed for and overheat. This may be why it's designed
to have a separate, lower input. There's no way to be sure
without knowing details about the machine.

3. The triple output DC bench PS I have has a green earth
connection
between the black and red connections on each of the variable
sources. Is there a way to use this to get the -12VDC and
+12VDC using
this green earth ground wire? (I doubt it but I have to ask)

No.

 

[John Fields]

On Sat, 14 Nov 2009 22:36:54 -0800 (PST), Uriah <uriahsky@hotmail.com>
wrote:

I had posted a question a few days ago about needing to power a bill
acceptor that requires +12DC, -12VDC, +5VDC, and 6VAC and there is
only one common ground wire. I needed to hook up a AC Bench Power
supply along with a triple output DC bench power supply so that they
share the common ground wire. With some help from this group I got it
to work. Of course I ran into problems and could use some more help
so that I don't damage my bench top power supplies. My DC bench top
power supply has three outputs, one constant 5VDC and two variable
power sources. The bill acceptor used a unregulated DC power supply
and when I wire in the constant 5VDC it drops it down to 3.2VDC
because of the regulation in bill acceptor. So the constant 5VDC PS
won't work. I need a higher DCV source.

I have a few questions.

1. Can I use two separate DC power supplies and have them share the
common ground wire to power the bill acceptor?

---
Yes, just make sure the outputs of the supplies are floating, and
connect them to the bill acceptor like this:


ACCEPTOR
+--------+
+--[-SUPPLY1+]-----|+12VDC |
| | |
+------------------|GND |
| | |
+--[+SUPPLY2-]-----|-12VDC |
+--------+

This is where I am
worried about damaging the power supplies.

2. Is there anyway to get -12VDC and +12VDC from one of the variable
sources and then I can use the other one for the 5VDC, which I have to
bring up to around 7VDC?

---
No, but if the positive supply can supply the current and the regulator
in the acceptor can handle the dissipation, you could just connect the
+12 input to the +5 input:

ACCEPTOR
+--------+
+--[-SUPPLY1+]--+--|+12VDC |
| | | |
| +--|+5VDC |
| |
+------------------|GND |
| | |
+--[+SUPPLY2-]-----|-12VDC |
+--------+


Another way to do it would be to half-wave rectify the 6VAC, smooth it
with a big cap, and then feed the output of the cap into the 5V input of
the acceptor:

ACCEPTOR
+--------+
+--[-SUPPLY1+]-----|+12VDC |
| | |
+------------------|GND |
| | |
+--[+SUPPLY2-]-----|-12VDC |
| | |
+--[~SUPPLY3~]-+---|6VAC |
| | | |
| [DIODE]| |
| |K | |
+----[BFC+]----+---|+5VDC |
/ +--------+
8VDC

In order to determine the value of the BFC use:


I dt
C = ------
dV

where C is the capacitance in farads,
I is the steady-state load current,
dt is the period of the rectified waveform, and
dV is the allowable ripple across the cap

Now, since the 6VRMS is 8.48VPK and we'll lose about 0.5V across the
diode (use a Schottky), that'll leave us with about 8VDC when the cap is
charged to its peak.

Then, if you need about 7V into the 5V regulator in order to get 5V out,
that means the voltage on the cap can never fall below 7V, so the ripple
across the cap can never be more than 8V - 7V = 1V

Finally, assuming a 500 mA load on the 5V and 60Hz mains, we can say:

0.5A * 0.017s
C = --------------- = 0.0085F = 850ľF
1V

Since most aluminum electrolytics have a +/-20% tolerance, the smallest
value you'd want to use which would assure getting at least 850ľF would
be about 1063ľF; I'd use 2000ľF/16VDC
---

3. The triple output DC bench PS I have has a green earth connection
between the black and red connections on each of the variable
sources. Is there a way to use this to get the -12VDC and +12VDC using
this green earth ground wire? (I doubt it but I have to ask)

---
No, it isn't. Those green connections are there in case it's necessary
to reference either the positive or negative output of either variable
source to earth ground.
---

I hope this isn't to confusing. Thanks again for the help.
Russ

---
You're welcome.

JF

 

[Uriah]

Mr Pimpom and Mr Fields. I want to thank the both of you for taking
the time to answer my questions so thoroughly, and so quickly. I
spent about four days working on these things and without your help it
would have been a week or two. Thanks again I really appreciate your
help.
Russ.