# Current Supply

Discussion in 'General Electronics Chat' started by fullNelson, Jan 25, 2012.

1. ### fullNelson Thread Starter Member

Nov 14, 2011
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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.

2. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,791
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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 likes this.
3. ### fullNelson Thread Starter Member

Nov 14, 2011
46
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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?

4. ### MrChips Moderator

Oct 2, 2009
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3,457
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.

5. ### crutschow Expert

Mar 14, 2008
13,505
3,382
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).

6. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
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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.

7. ### fullNelson Thread Starter Member

Nov 14, 2011
46
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1 ohm is pretty darn small. So why 1 ohm?

8. ### fullNelson Thread Starter Member

Nov 14, 2011
46
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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.

9. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
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Just look at the supply. If the front panel has a current knob, it will do CC mode.

10. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
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Li batteries actually require both CC and CV mode of operation.

11. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998

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.

12. ### Audioguru New Member

Dec 20, 2007
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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.

13. ### crutschow Expert

Mar 14, 2008
13,505
3,382
Oops, my mental calculator failed. The resistor value should be 5 ohms obviously.

14. ### Brownout Well-Known Member

Jan 10, 2012
2,375
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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.