# DC Bench power supply - basics

#### Mark Flint

Joined Jun 11, 2017
145
I'm wanting to know if my PSU is operating "normally" or if there is a problem with it.
I'm powering a 12v motor - the heater-blower type from a car. This can draw over 10 amps at 12v, but I only need this powered at about 4A at 12v.

When I allow max amps from the supply (5A) the voltage will only get to 3.95v. The supply is rated at 150w (30v @ 5A). So why will it stop at about 12w maximum?

Am I using the power supply (pictured below) incorrectly?

Thanks,
Mark

#### LesJones

Joined Jan 8, 2017
4,223
The supply is rated at 5 amps and the motor requires more than that so the power supply shuts down to protect itself.

Les.

#### djsfantasi

Joined Apr 11, 2010
9,185
Your power supply appears to be working as expected. You have set it to control both current and voltage. You know (or should) that according to ohms law, this is impossible. The way a power supply handles that situation is when the maximum current you’ve set is exceeded, it reduces the voltage.

#### MrChips

Joined Oct 2, 2009
31,001
Ohm's Law applies.
Let's assume that the heater takes about 150W. Hence the resistance is about 1Ω.
Power P = V x V / R = 12 x 12 / 1 = 144W
(According to the PSU reading, the heater is about 4V/5A = 0.8Ω)

You cannot have both 4A @ 12V at the same time going into 1Ω.
The PSU reaches a limit of 5A first and holds it there.
If you want about 48W output then you need a PSU that can deliver at least 7A.

#### Mark Flint

Joined Jun 11, 2017
145

#### BobTPH

Joined Jun 5, 2013
9,190
Trying to run a device requiring 10A at 12V with 4A at 12V is sort of like trying to fill a gallon milk jug with 1/2 gallon of milk.

Bob

#### MrChips

Joined Oct 2, 2009
31,001
TS might have been thinking of running with 48W from a 150W PSU.
He ignored the resistance of the load and Ohm's Law.
If he wants 48W into 1Ω he needs at least 7A.
So 10A PSU at a voltage of at least 5-12V will do it. It's better to get a 30V 10A PSU.

Something like this £17.58
https://www.aliexpress.com/item/4000189286797.html

#### Audioguru again

Joined Oct 21, 2019
6,767
The Chinese power supply is missing a safety certification label. Then it might catch on fire or electrocute you.
It does not say 10A continuous, it might produce 10A only for a moment.