Bench Power Supply question

[lilimike]

On THIS power supply there are two displays; current and voltage. It is my understanding that if you power up a circuit with this power supply, after selecting the voltage you would get the current consumed by the circuit on the current display. Therefore what is the pot for the current used for?

Mike

 

[crutschow]

You can adjust the current pot to either limit the current to a particular value and/or use the power supply in a constant-current mode.

In the constant-current mode the supply adjusts the output voltage to keep the output current constant. The current remains constant up to the point that the load resistance is high enough to exceed the voltage setting of the power supply for the particular current setting. For example if you had the voltage set to 10V and the current set to 1A, then the output current would stay at 1A for any load resistance less than 10Ω. It's useful for testing things like LEDs that require a constant current for their operation, or charging batteries at a constant current.

 

[lilimike]

Thanks for the response,
However I am having a hard time understanding. Since there is a relation between Voltage, current and resistance and if I can not change the resistance within the circuit and I set the supply to 10V. how can I possibly change the current?

If I set to 10V and the resistance of my circuit is 9Ω how can I have anything other than 900mA?

Mike

 

[MikeMl]

...if i set to 10v and the resistance of my circuit is 9Ω how can i have anything other than 900ma?

i = e/r = 10/9 = 1.11111a

If the supply's current limit was previously set to 1.00A, and the open circuit voltage was set to 10V, when you connect a 9Ω resistor, the supply reverts to constant-current mode, and the voltage drops to e = ir = 1*9 = 9V.

 

[crutschow]

Thanks for the response,
However I am having a hard time understanding. Since there is a relation between Voltage, current and resistance and if I can not change the resistance within the circuit and I set the supply to 10V. how can I possibly change the current?

Of course V, I, and R can not all be set independently. You can vary two of the three parameters, which determines the third.

The voltage and current settings on the power supply can be looked upon as limits. Which limit is in effect depends upon the load. Thus say you vary the load resistance with the voltage set to 10V and the current set to 1A. For these settings the current limit will be in effect as the resistance goes from 0Ω to 10Ω, with the voltage varying from 0V to 10V to maintain 1A. As the load increases above 10Ω, the voltage will be limited to 10V, so the current will now start to drop since the current equals 10V divided by the load resistance.

 

[lilimike]

MikeML,
Yes, sorry I made a mistake, good thing posts do not include cam shots because I am all red right now

Carl,
Thanks for clarifying I understand now.

Mike

 

[MrAl]

Hello there,


Some bench (maybe most) power supplies dont like to act like true current generators though. The catch is that many of them like to try to detect a short circuit on the output. Well, a short circuit is perfectly acceptable for a current generator, so if you set the current limit to say 1 amp and short the output, the current does not go to 1 amp it goes to whatever the supply was designed to put out with a short circuit on the output, such as 0.1 amps or maybe 0.010 amps. That's a pain sometimes, but to get around it you can use a series resistor of say 1 or 2 or maybe 5 ohms in series with the very low ohms load and that will power it ok. The only catch is that the voltmeter on the power supply will no longer indicate the true voltage of the load but rather the load and the extra resistor combined.

 

[MikeMl]

Mr Al, you have accurately described a "current fold-back-limited" power supply; not a dual mode, "constant voltage", "constant current" supply such as was posted by the OP.

 

[MrAl]

Mr Al, you have accurately described a "current fold-back-limited" power supply; not a dual mode, "constant voltage", "constant current" supply such as was posted by the OP.

Hello,


Well, that's nice Lucky i used the word, "some".
I noticed that not all of them advertise which is which either.

 

[MikeMl]

...I noticed that not all of them advertise which is which either.

I have used a Mastec like the one pictured.

 

[MrAl]

Hi Mike,

Oh good, did you like the way it worked?

I use a Mastec 0 to 30v, 0 to 10 amp, which doesnt like to work with loads down below about 2 ohms.

 

[MikeMl]

At my last job, we bought about 20 Mastech supplies, some were a "linear" model, and some were "switchers". AFIK, they all had smooth current limiting, and somewere dual power supplies. I think most were the 0-30V, 0-3A models.

I frequently used them in constant-current mode to charge batteries, and in constant-voltage mode to power robotics controllers and misc circuits. The current-limiting saved burning something up when initially powering un-debugged systems...

 

[MrAl]

Hello again,


Oh yes, the current limiting feature is nice for that.
I've used my Mastec for charging batteries too, but that's not the same as a 1 ohm resistor for example which it thinks is a short circuit.
This one has a switcher front end and linear back end.