# Current preset circuit.

#### rahul411

Joined Feb 19, 2018
260
I want a circuit which will allow to preset the current flowing through some device, just like a power supply where we can set max current and the power supply will only allow that much current to flow.
I've tried the google but everywhere they are showing how to limit the current for certain value and if the current value overflows the circuit will stop powering the connected device.

#### iimagine

Joined Dec 20, 2010
483
Thats just not possible, drawing too much currents means voltage drops. Which power supply can do this?

#### rahul411

Joined Feb 19, 2018
260
Thats just not possible, drawing too much currents means voltage drops. Which power supply can do this?
Like in lab bench power supply we can limit the current, it is possible there then why not we can something like that which will just limit the current.

#### MrChips

Joined Oct 2, 2009
24,241
You cannot control the current and voltage of the power supply at the same time. Current and voltage are interrelated by Ohm's Law

I = V / R

where R is the resistance of the load.

What you are seeking is a constant current source.

Current and voltage settings on a bench power supply are limits. At all times the power supply is operating at one of these two limits, either current limited or voltage limited.

#### AlbertHall

Joined Jun 4, 2014
11,506
Some current limits fold back as described but they don't all. Somee just maintain the current at the limit value.

Lots of power supplies do allow you to view the current limit setting.
Modern supplies probably do this by displaying the setting of the controls.
Older supplies (non-digital) can do this by having a button which shorts the output of the supply and then, of course, the ammeter will display the current which is flowing which will be the limit. I have one of these supplies.

I am in the process of making a control box for electroplating which uses a constant current source and an ammeter to display the actual current. This can be preset by shorting the current output leads and adjusting until the ammeter shows the desire current.

#### AlbertHall

Joined Jun 4, 2014
11,506
I want a circuit which will allow to preset the current flowing through some device, just like a power supply where we can set max current and the power supply will only allow that much current to flow.
I've tried the google but everywhere they are showing how to limit the current for certain value and if the current value overflows the circuit will stop powering the connected device.
The LM317 can be used as a current limit. See the datasheet for details.

#### MrChips

Joined Oct 2, 2009
24,241
Like in lab bench power supply we can limit the current, it is possible there then why not we can something like that which will just limit the current.
As the man says, when the current limit is reached, the voltage falls.
Ohm's Law still applies:

V = I x R

#### ci139

Joined Jul 11, 2016
1,696
preset the current flowing through some device
.
? Do you have a practical example (device/load) you need to keep the preset current going through

#### AnalogKid

Joined Aug 1, 2013
9,362
It is not clear from your question exactly what you are looking for.

IF you have a device powered by a typical constant-voltage source (industrial power supply, wall wart, whatever), and you want to add a circuit between the source and your device such that the circuit allows normal operation up to a certain current value, and above that value it limits the current to that value,

THEN when the current limiting kicks in, the voltage seen by the device will decrease. You cannot have a constant voltage and a constant current for a variable load. When the equivalent resistance of the load changes - and it must be changing or you wouldn't need a current limiter - then the total power in the load changes which means the source voltage, source current, or both must change. If you hold the voltage constant, the current will change; if you hold the current constant, the voltage will change.

ak

#### MrChips

Joined Oct 2, 2009
24,241
To make it a bit more clear, consider this example.
Suppose we set up a bench power supply unit (PSU) to output 10V @ 1A max.

1) With no load attached, the PSU will show 0A @ 10V.

2) Now we connect a 20Ω load. I = V / R. The PSU shows 0.5A @ 10V.

3) With a load R just less than 10Ω, the PSU shows slightly under 1A @ 10V.

4) With a load R = 5Ω, V = I x R. The PSU will show 1A @ 5V, i.e. the PSU is in current limit mode and the voltage must fall according to Ohm's Law.

If you want the PSU to shut down when the current limit is exceeded, then you will need a RESET button on the PSU to resume power, otherwise the PSU will oscillate between on and off. An alternative is a crowbar circuit which fires an SCR across the power rails and blows the fuse. (I am not sure that that is what you desire.)

Incidentally, I built an electronic circuit breaker that does exactly that. I can set the threshold current level above which the power relays will open and remove AC power. I use this when I don't want to be constantly replacing fuses while I diagnose faulty equipment that keeps blowing fuses.

#### BobTPH

Joined Jun 5, 2013
3,694
Has anyone other than than I read the two posts by the TS? Everyone wants to tell him he cannot have constant voltage and constant current, but he never asked for that. Basically, he asked for a settable constant current source.

Bob

#### MrChips

Joined Oct 2, 2009
24,241
Has anyone other than than I read the two posts by the TS? Everyone wants to tell him he cannot have constant voltage and constant current, but he never asked for that. Basically, he asked for a settable constant current source.

Bob
TS asks for a power supply that shuts down when the max current is exceeded.
That is called a fuse or circuit breaker.

#### AlbertHall

Joined Jun 4, 2014
11,506
TS asks for a power supply that shuts down when the max current is exceeded.
That is called a fuse or circuit breaker.
Please read post #1 again - that is exactly what TS does not want.

#### MrChips

Joined Oct 2, 2009
24,241
Please read post #1 again - that is exactly what TS does not want.
Sorry. You are correct.
Then, what does the TS want if it is not constant current power supply?

#### BobTPH

Joined Jun 5, 2013
3,694
Sorry. You are correct.
Then, what does the TS want if it is not constant current power supply?
IMHO that IS what he wants. He just didn’t know the name of it.

Bob

#### MrChips

Joined Oct 2, 2009
24,241
IMHO that IS what he wants. He just didn’t know the name of it.

Bob
A constant current (CC) power supply with a twist.
A CC power supply will keep increasing the voltage until the current is reached.

Maybe TS wants a constant voltage (CV) supply that limits the current when the current limit is reached. In that case TS wants a typical CC/CV bench power supply. TS should google CC/CV power supply circuit.

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,090
You have CC or Constant Current and CV Constant Voltage Power supplies, In that context, You have voltage and current compliance values and crossover occurs between the two modes. e.d Apply 200 mA, but use no more than 100V.

For a CV supply, there is
Current limit - Not precise regulation
Constant Current - regulation
"Foldback Current Limiting" - Harder one to explain.

Look for "Agilent DC power supply handbook" Agilent my be replaced by keysight of HP. It's free.

#### ci139

Joined Jul 11, 2016
1,696
every idea implementable in real world's time-space × physics
or not implementable

has

a trigger

and a target

if we know these it would be more likely for anyone to provide a better guidance if/how to proceed

Joined Mar 10, 2018
4,057
You cannot control the current and voltage of the power supply at the same time. Current and voltage are interrelated by Ohm's Law

I = V / R

where R is the resistance of the load.

What you are seeking is a constant current source.

Current and voltage settings on a bench power supply are limits. At all times the power supply is operating at one of these two limits, either current limited or voltage limited.
Constant power control -