constant voltage/current question

K

Ken Williams

Guest
I just got a bench power supply
(http://www.circuitspecialists.com/prod.itml/icOid/8770). I have some
general questions. I'm a complete novice.

1) What is Constant Voltage and Constant Current? As opposed to just
normal current and voltage? I mean I have it outputting say 9V, whats
the different between 9V and 9V "constant voltage". why do I care?

2) Am I suppose to be able to adjust amps the same way I adjust voltage
when I connect a multimeter to this/any power supply? I can adjust the
voltage with the dial and the multimeter adjusts accordingly, but when I
just to adjust the current (with the current adjustment buttons) nothing
happens. the multimeter says its always 3.5 amps for whatever reason.
I'm a little lost as to why I can't simply select 9V at say 1A.

Thanks for any help.
kenw232@yahoo.com
 
Ken Williams wrote:
2) Am I suppose to be able to adjust amps the same way I adjust voltage
when I connect a multimeter to this/any power supply? I can adjust the
voltage with the dial and the multimeter adjusts accordingly, but when I
just to adjust the current (with the current adjustment buttons) nothing
happens. the multimeter says its always 3.5 amps for whatever reason.
I'm a little lost as to why I can't simply select 9V at say 1A.
Click to expand...
The amount of current that can flow is entirely determined by the
resistance of the circuit. It sounds like you are short-circuiting the
supply, in which case it will either blow up or (if it has current
limiting) will be limited to the maximum current that it can deliver.
You can work out how much current I will flow in a circuit of resistance
R when you apply a voltage V with Ohm's law:

V = I * R

Cheers,
Nicholas Sherlock
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On Sun, 25 Jan 2009 02:53:49 -0500, Ken Williams <kenw232@yahoo.com>
wrote:

I just got a bench power supply
(http://www.circuitspecialists.com/prod.itml/icOid/8770). I have some
general questions. I'm a complete novice.

1) What is Constant Voltage and Constant Current? As opposed to just
normal current and voltage? I mean I have it outputting say 9V, whats
the different between 9V and 9V "constant voltage". why do I care?
Click to expand...
A "constant voltage" power supply is the kind of gizmo you'd normally
think of as a "power supply." E.g., build a 5 VDC supply with a 7805
regulator and you have a constant voltage supply, one that will maintain
a constant 5 volt output regardless of how much current the load draws,
within the limits of the device's capabilities. There will be real-world
limits on the maximum permitted current draw, perhaps also on the
minimum, and some variation on the output voltage with variations on
supply and load, but to a first approximation, it's constant voltage.

Similarly, you can have a "constant current" supply that will provide,
say, 100 mA into any load, regardless of how what terminal voltage may
be required; it can push 100 mA through a bare wire using almost no
discernable voltage on the connectors, or if you hook up a 100 ohm load
it will automatically adjust the output voltage up to 10 V to maintain
that 100 mA current flow. As with constant voltage, constant current has
to meet real world limitations, such as not being able to deliver the 10
KV needed to push 100 mA through a 100 K load.

When put together in a benchtop supply, one usually sets up the voltage
and current limits to work together. For example, say you'll be powering
a dev board that takes 3.3 V at a nominal 40 mA.

With the power supply leads unconnected, set the supply's output voltage
to 3.3 V.

Now, short the supply leads together (voltage will drop to near zero)
and set the supply's current limit to, say, 80 mA. That doesn't have to
be exact but should be large enough for the board to turn on normally
and small enough that it will act as a clamp if something's wrong.

That way, if the board is correct then the power supply will operate in
"constant voltage mode" supplying 40 mA and all is well. If, however, a
diode was in backwards, then the power supply will go into current limit
at 80 mA and begin to operate in constant current mode, reducing the
terminal voltage as necessary to avoid exceeding 80 mA. Doing so may
prevent any damage to the board long enough for you to note that
"something ain't right" and disconnect the supply.

2) Am I suppose to be able to adjust amps the same way I adjust voltage
when I connect a multimeter to this/any power supply? I can adjust the
voltage with the dial and the multimeter adjusts accordingly, but when I
just to adjust the current (with the current adjustment buttons) nothing
happens. the multimeter says its always 3.5 amps for whatever reason.
I'm a little lost as to why I can't simply select 9V at say 1A.
Click to expand...
It looks like you may need to enable the current limit mode with the
front panel CC/CV switch. Haven't used this particular supply.

--
Rich Webb Norfolk, VA
 
On Sun, 25 Jan 2009 02:53:49 -0500, Ken Williams <kenw232@yahoo.com>
wrote:

I just got a bench power supply
(http://www.circuitspecialists.com/prod.itml/icOid/8770). I have some
general questions. I'm a complete novice.

1) What is Constant Voltage and Constant Current? As opposed to just
normal current and voltage? I mean I have it outputting say 9V, whats
the different between 9V and 9V "constant voltage". why do I care?
Click to expand...
A "9V power supply" is a constant voltage power supply.

2) Am I suppose to be able to adjust amps the same way I adjust voltage
when I connect a multimeter to this/any power supply? I can adjust the
voltage with the dial and the multimeter adjusts accordingly, but when I
just to adjust the current (with the current adjustment buttons) nothing
happens. the multimeter says its always 3.5 amps for whatever reason.
I'm a little lost as to why I can't simply select 9V at say 1A.
Click to expand...
Your power supply can either limit voltage or current. If you
increase the voltage, at some point the current will limit. The
supply is now in "constant current" mode. The current adjust will now
set the current.

A better way to see the constant current capabilities of your supply
is to short the output. You can then adjust the current over its full
range, though note the output voltage will be zero. Exactly the
inverse of an "open" output, varying they voltage (current will be
zero). Not though that the shorted output will create heat somewhere
so be careful.
 
On Jan 24, 11:53 pm, Ken Williams <kenw...@yahoo.com> wrote:
I just got a bench power supply
(http://www.circuitspecialists.com/prod.itml/icOid/8770).   I have some
general questions.  I'm a complete novice.

1) What is Constant Voltage and Constant Current?  As opposed to just
normal current and voltage?  I mean I have it outputting say 9V, whats
the different between 9V and 9V "constant voltage".  why do I care?
Click to expand...
Every thing that provides power, provides some knowable voltage
in each case of known current drain; the current vs. voltage curve
is called the 'load line'. And, each device that consumes power
has its own consumption curve, which can be graphed as
current vs. voltage; where the two curves intersect, that
determines the operating condition when the source and load
are connected.

If the source has a high dI/dV (slope) of the current vs. voltage
curve, higher than the dI/dV of the consumption curve, the
source can be approximated as a voltage source (i.e. constant
voltage). Your AC outlet is such a voltage source,
it delivers 120VAC (or whatever is your national standard) alike
to 50W and to 100W light bulbs.

If the source has low dI/dV compared to the consuming device,
it is a current source. A string of Xmas tree lights is like this:
a good light bulb, or a short circuit, in one socket will result
in nearly identical current in the series-connected string.

One can consider any source to be a voltage source if it
is feeding a very high impedance, and a current source
if it feeds a very low impedance; voltmeters are almost
always high-impedance devices, and ammeters are almost
always low-impedance devices, to fulfill the requirements of
this approximation. And, it IS an approximation, there
is no pure voltage source nor current source in nature.
 
On Sun, 25 Jan 2009 02:53:49 -0500, Ken Williams wrote:

I just got a bench power supply
(http://www.circuitspecialists.com/prod.itml/icOid/8770). I have some
general questions. I'm a complete novice.

1) What is Constant Voltage and Constant Current? As opposed to just
normal current and voltage? I mean I have it outputting say 9V, whats
the different between 9V and 9V "constant voltage". why do I care?
Click to expand...
Constant voltage means that the voltage is fixed while the current is
dictated by the voltage and the load (I=V/R), subject to any chosen
current limit.

Constant current means that the current is fixed while the voltage is
dictated by the current and the load (V=I*R), subject to any chosen
voltage limit.

2) Am I suppose to be able to adjust amps the same way I adjust voltage
when I connect a multimeter to this/any power supply? I can adjust the
voltage with the dial and the multimeter adjusts accordingly, but when I
just to adjust the current (with the current adjustment buttons) nothing
happens. the multimeter says its always 3.5 amps for whatever reason.
I'm a little lost as to why I can't simply select 9V at say 1A.
Click to expand...
You can't choose both simultaneously. For any given load resistance,
voltage and current are related by Ohm's law: V=IR (=> I=V/R).

You can fix one and limit the other (for constant voltage, if the current
limit is reached, the voltage will be reduced in order to keep the current
below the limit; conversely for constant current with a a voltage limit).
 
Thanks for the answer. I think it answered my mental problem.

I was assuming because I could adjust the voltage and current that I
could just set them both to whatever I wanted, then when I measured
either one with a multimeter it should show up on the multi meter.
which it does for voltage, but current is always a random number on my
multimeter. I guess when I hook up a real load the current can never
go above my setting. But before that its always some random number,
usually 0. Unless my multi meter is hooked up.


Nobody wrote:
On Sun, 25 Jan 2009 02:53:49 -0500, Ken Williams wrote:

I just got a bench power supply
(http://www.circuitspecialists.com/prod.itml/icOid/8770). I have some
general questions. I'm a complete novice.

1) What is Constant Voltage and Constant Current? As opposed to just
normal current and voltage? I mean I have it outputting say 9V, whats
the different between 9V and 9V "constant voltage". why do I care?

Constant voltage means that the voltage is fixed while the current is
dictated by the voltage and the load (I=V/R), subject to any chosen
current limit.

Constant current means that the current is fixed while the voltage is
dictated by the current and the load (V=I*R), subject to any chosen
voltage limit.

2) Am I suppose to be able to adjust amps the same way I adjust voltage
when I connect a multimeter to this/any power supply? I can adjust the
voltage with the dial and the multimeter adjusts accordingly, but when I
just to adjust the current (with the current adjustment buttons) nothing
happens. the multimeter says its always 3.5 amps for whatever reason.
I'm a little lost as to why I can't simply select 9V at say 1A.

You can't choose both simultaneously. For any given load resistance,
voltage and current are related by Ohm's law: V=IR (=> I=V/R).

You can fix one and limit the other (for constant voltage, if the current
limit is reached, the voltage will be reduced in order to keep the current
below the limit; conversely for constant current with a a voltage limit).
Click to expand...
 
On Mon, 26 Jan 2009 02:38:51 -0500, Ken Williams wrote:
(top posting fixed)
Nobody wrote:
On Sun, 25 Jan 2009 02:53:49 -0500, Ken Williams wrote:

I just got a bench power supply
(http://www.circuitspecialists.com/prod.itml/icOid/8770). I have
some general questions. I'm a complete novice.

1) What is Constant Voltage and Constant Current? As opposed to just
normal current and voltage? I mean I have it outputting say 9V, whats
the different between 9V and 9V "constant voltage". why do I care?

Constant voltage means that the voltage is fixed while the current is
dictated by the voltage and the load (I=V/R), subject to any chosen
current limit.

Constant current means that the current is fixed while the voltage is
dictated by the current and the load (V=I*R), subject to any chosen
voltage limit.

2) Am I suppose to be able to adjust amps the same way I adjust
voltage when I connect a multimeter to this/any power supply? I can
adjust the voltage with the dial and the multimeter adjusts
accordingly, but when I just to adjust the current (with the current
adjustment buttons) nothing happens. the multimeter says its always
3.5 amps for whatever reason. I'm a little lost as to why I can't
simply select 9V at say 1A.

You can't choose both simultaneously. For any given load resistance,
voltage and current are related by Ohm's law: V=IR (=> I=V/R).

You can fix one and limit the other (for constant voltage, if the
current limit is reached, the voltage will be reduced in order to keep
the current below the limit; conversely for constant current with a a
voltage limit).

Thanks for the answer. I think it answered my mental problem.

I was assuming because I could adjust the voltage and current that I
could just set them both to whatever I wanted, then when I measured
either one with a multimeter it should show up on the multi meter. which
it does for voltage, but current is always a random number on my
multimeter. I guess when I hook up a real load the current can never
go above my setting. But before that its always some random number,
usually 0. Unless my multi meter is hooked up.
Click to expand...
If your supply is correct the current isn't random. (It's certainly not
a random _number_, because current is current, not a number).

For all but oddball loads, the current vs. voltage relationship is
deterministic, and for many loads it's a simple monotonically increasing
function. Your supply allows you to choose either the voltage or the
current, your load picks the other.

--
http://www.wescottdesign.com
 
On Jan 25, 11:38 pm, Ken Williams <kenw...@yahoo.com> wrote:


I was assuming because I could adjust the voltage and current that I
could just set them both to whatever I wanted, then when I measured
either one with a multimeter it should show up on the multi meter.
Click to expand...
No, of course not. If you set the current to 1A, and the
voltage to 1V, a 1Mohm resistor won't magically get
1A (and a million volts) across its terminals. And, a short circuit
(OK, call it 1 milliohm) won't somehow draw 1000A and
achieve 1V across that milliohm.

Both V and I settings are LIMITS. The load line for the
bench power supply starts at I=0 V=Vset, turns a
corner at I=Iset, V=Vset, and goes to I=Iset, V=0.

Get some graph paper, and draw it.

Any actual item connected to the power supply, like a
resistor, has its own V-I characteristic (V=I*R for
a resistor). Draw the resistor's 'curve' (actually a
straight line) on the load line graph, and the
intersection is what actually happens when you
put that resistor on the power supply.

It can be a meter you connect, instead of a resistor.
A voltmeter = a very high resistance.
An ammeter = a very low resistance.
What will those meters show?
 
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