Login Join Maker Pro

# I don’t understand current source in a power supply.

#### DickFreed

Aug 6, 2021
42
I have a simple DC power supply unit that has an adjustment knob for voltage and a separate knob for amperage. Doesn’t the voltage determine your current, along with whatever you may be supplying? As in, if the circuit I was supplying required 5v, wouldn’t I just set the supply to 5v and let it rip? The circuit under test would determine what the current was, right?

i guess the current control side is basically a pot that drops the built inresistance of the unit itself, which would change current.

#### Bluejets

Oct 5, 2014
7,097
Think you will find it is a current limit setting.

For example, if you set the current to say 50mA, then current in whatever you connect will be maximum limited to 50mA.

#### DickFreed

Aug 6, 2021
42
Think you will find it is a current limit setting.

For example, if you set the current to say 50mA, then current in whatever you connect will be maximum limited to 50mA.
Makes sense, other than if I just hook a resistor up to it that it doesnt let me adjust current at all.

#### Bluejets

Oct 5, 2014
7,097
Makes sense, other than if I just hook a resistor up to it that it doesnt let me adjust current at all.
No idea what you mean by this....

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,809
Makes sense, other than if I just hook a resistor up to it that it doesnt let me adjust current at all.
Current is determined by I = V / R.
The current limiting will set in if you try to draw more current from the power supply than the set limit. The power supply will then reduce the current.

Example:
Set V = 10 V
Set I = 10 mA (current limit)
Apply a 10 kΩ resistor -> I = V / R = 10 V / 10000 Ω = 1 mA -> o.k.
Apply a 1 kΩ resistor -> I = V / R = 10 V / 1000 Ω = 10 mA -> o.k.
Apply a 100 Ω resistor -> I = V / R = 10 V / 100 Ω = 100 mA -> not o.k.! This would exceed your set limit of 10 mA. The power supply will then drop the voltage such that V = I × R = 10 mA ×100 Ω = 1 V

#### Martaine2005

May 12, 2015
4,970
Adding to the above posts.
You have to set the current limit yourself.
This is (probably) achieved by shorting the power supply leads and setting your desired current limit.
Some of these PSU's require a higher voltage setting first in order to set the current limit.
Example: Set voltage to 5V, short the leads and set the current. If the current wont adjust, you will have to adjust the voltage higher first. Try 9V etc.
But better still, is read the manual for correct settings.

Martin

#### DickFreed

Aug 6, 2021
42
Current is determined by I = V / R.
The current limiting will set in if you try to draw more current from the power supply than the set limit. The power supply will then reduce the current.

Example:
Set V = 10 V
Set I = 10 mA (current limit)
V

Ok, I hooked up each resistor as you said after setting voltage to 10v. Display showed nothing on first resistor due to not having a single digit mili place on readout I presume. Next two pics show what happened with the other two. I was never able to adjust current while this was taking place though.
https://imgur.com/a/BO3S0L0

#### DickFreed

Aug 6, 2021
42
Adding to the above posts.
You have to set the current limit yourself.
I can adjust current with leads shorted as you said. When I do, the voltage setting completely disappears though, don’t know if that means anything. The supply is a Eventek kps305d. I haven’t found a manual, none came with it. I don’t click on sites I know nothing about that offer manuals though.

#### DickFreed

Aug 6, 2021
42
No idea what you mean by this....

As others have mentioned, I guess I short leads and adjust for current first. I thought I would be able to adjust it while having leads hooked to each end of a resistor.

#### DBingaman

Jun 27, 2021
103
There are two modes the power supply can be used in Constant voltage and constant current. As long as the current drawn is less than the limit, the power supply acts as a constant voltage source and will adjust the output current to keep the voltage constant. If the current is higher than the limit set the supply goes into constant current mode. This is useful if you need a constant current source. In this case the voltage of the power supply will change up to the voltage limit set in order to supply a constant current to the circuit. A third possible setting for a power supply is current limit protection (this is not the same as constant current). What happens in this mode is if the current exceeds the limit, the power supply will latch in the off state (acting like a circuit breaker) in order to protect the device being powered. The cool thing to take away from this is you can use your power supply as a constant current source if needed!
In order to use the power supply as a constant voltage source, set the voltage for what you want with the power supply leads open. If the current exceeds the limit it will act as a constant current source with a compliance of 0 to the voltage limit.
In order to use the power supply as a constant current source, short the power supply leads and adjust the current for what you want. Set the voltage limit will set the compliance of the constant current source from 0 to the voltage limit.
Hope that helps.

Last edited:

#### DickFreed

Aug 6, 2021
42
I believe I’ve got a grasp on it all now. Thanks everyone.

C
Replies
8
Views
2K
Rich Grise
R
Replies
14
Views
2K
Replies
16
Views
2K
M
Replies
7
Views
3K
M. Hamed
M
Replies
4
Views
35K