# Current Supply

#### fullNelson

Joined Nov 14, 2011
46
How can a benchtop psu supply both voltage and current?

I thought only a voltage can motivate a current through a circuit. If thats true, and I know it is, then why is there a knob to adjust current on a power supply?

Some supplies, it looks like, can do 30v max and 5amps.

To me it seems as if the 5amps is a limiter, so that if the resistance through the circuit is low, then the 30v can't possibly push more than 5 amps.

#### mcgyvr

Joined Oct 15, 2009
5,394
Many have 2 modes..
Constant Current Mode=you set a current level and the power supply adjusts the voltage automatically to only supply the unit you are powering with that current level you requested.
Constant Voltage Mode=you set a voltage level and the power supply provides exactly that and whatever current you require.

Also be aware of the wattage spec on benchtop supplies.. Many say (for example) 0-30V and 0 to 5 Amps but then also throw in 90W max... So you can do 30V @3 Amps max or 18V @ 5 Amps but 30V @5Amps would be 150W which would exceed the rating of the supply.

#### fullNelson

Joined Nov 14, 2011
46
Ah, so you really aren't setting both at the same time. Its based on a mode.

So is the limitation a build fault or is it intended?

#### MrChips

Joined Oct 2, 2009
30,448
This is not a fault but actually a benefit.
The manufacturer could have sold you a power supply rated at 18V @ 3A = 54W
even though it is designed to give 90W output,
or 30V@3A or 18V@5V, you can choose.

#### crutschow

Joined Mar 14, 2008
34,002
Ah, so you really aren't setting both at the same time. Its based on a mode.

So is the limitation a build fault or is it intended?
It's not a fault. There is no way to regulate both current and voltage at the same time, it's one or the other depending upon the load.

For example, if you set the power supply voltage at 5V and the current at 1A and slowly reduce the load resistance on the supply output, the load voltage will stay at 5V until the resistance reaches 1 ohm. Below 1 ohm the current will stay at 1A and the voltage will then have to start to reduce (to maintain the 1A through whatever the resistance is).

#### bountyhunter

Joined Sep 7, 2009
2,512
Cheap supplies typically have only the CV constant voltage mode and a "current limit" which is protection. You can't set the current limit level. Good quality lab supplies hve a knob to adjust the constant current level so you can use them as a precise current source for battery charging or similar.

#### fullNelson

Joined Nov 14, 2011
46
It's not a fault. There is no way to regulate both current and voltage at the same time, it's one or the other depending upon the load.

For example, if you set the power supply voltage at 5V and the current at 1A and slowly reduce the load resistance on the supply output, the load voltage will stay at 5V until the resistance reaches 1 ohm. Below 1 ohm the current will stay at 1A and the voltage will then have to start to reduce (to maintain the 1A through whatever the resistance is).
1 ohm is pretty darn small. So why 1 ohm?

#### fullNelson

Joined Nov 14, 2011
46
Cheap supplies typically have only the CV constant voltage mode and a "current limit" which is protection. You can't set the current limit level. Good quality lab supplies hve a knob to adjust the constant current level so you can use them as a precise current source for battery charging or similar.
Is a cheap supply in the > $100 range typically? Ive looked at a few mastech psus that claim to be "lab quality". One other question, based on your last statement: Does charging a battery require a "precise" current source? So if I had a LiPo I was charging with the psu rather than straight off the mains ac. #### bountyhunter Joined Sep 7, 2009 2,512 Is a cheap supply in the >$100 range typically? Ive looked at a few mastech psus that claim to be "lab quality"..
Just look at the supply. If the front panel has a current knob, it will do CC mode.

#### bountyhunter

Joined Sep 7, 2009
2,512
One other question, based on your last statement: Does charging a battery require a "precise" current source? So if I had a LiPo I was charging with the psu rather than straight off the mains ac.
Li batteries actually require both CC and CV mode of operation.

#### Brownout

Joined Jan 10, 2012
2,390
1 ohm is pretty darn small. So why 1 ohm?

I think he means 5 ohms. If the voltage is set to 5V and current limit to 1 amp, then the power supply will go into current mode when the load is:

5V/1amp=5ohms. Any load lower than 5 ohms will cause 1amp from the power supply, because it's in constant current. If you adjust the load resistor above 5ohms, then the power supply will be back in constant voltage. Get it? It stays in constant voltage as long as the current is below the current limit setting.

#### Audioguru

Joined Dec 20, 2007
11,248
1 ohm is pretty darn small. So why 1 ohm?
Ohm's law says that a resistance of 1 ohm draws 5A when it has 5V across it. Like a 5V motor that draws 1A.
Resistor values of less than 1 ohm are also common.

#### crutschow

Joined Mar 14, 2008
34,002
I think he means 5 ohms. If the voltage is set to 5V and current limit to 1 amp, then the power supply will go into current mode when the load is:

5V/1amp=5ohms. Any load lower than 5 ohms will cause 1amp from the power supply, because it's in constant current. If you adjust the load resistor above 5ohms, then the power supply will be back in constant voltage. Get it? It stays in constant voltage as long as the current is below the current limit setting.
Oops, my mental calculator failed. The resistor value should be 5 ohms obviously.

#### Brownout

Joined Jan 10, 2012
2,390
Hi Carl,

I knew it was just a mis-fire. Didn't mean to jump in there on you, I just wasn't doing anything else.