# Circuit to determine the capability of a power source to deliver current

#### ianhem

Joined Jul 8, 2016
3
I'm looking for some help from somebody with better knowledge than myself.

I wish to build a circuit to integrate into a piece of test equipment.

The circuit to be tested is a 24v 3a supply. I want to test that when delivering 3a (Through a high power resistor probably) it can maintain a voltage of 21v.
Then simply illuminate an LED.

I don't have a great knowledge of electronics more of a mediocre diyer.

I think a tl431 could be used but if someone could scribble me a diagram it'd be really helpful.

#### Papabravo

Joined Feb 24, 2006
18,108
It should deliver 3 Amperes of current at 24VDC, for example into a 8Ω resistor, and you should measure a bit less than 24V across the resistor to account for the source resistance of the power supply. What exactly is the purpose of the high power resistor? I don't quite follow your thinking. Perhaps a diagram would help. I'm not clear on the difference between the power supply, the application or load for the power supply, and the piece of test equipment.

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#### crutschow

Joined Mar 14, 2008
29,515
Below is my take on a circuit using the TL431.
LED D2 indicates power is applied.
LED D1 indicates that the voltage is at least 21V with the load applied.
Pot U2 adjusts D1's threshold voltage.
The LTspice simulation with the pot wiper near its center position shows that D1 conducts (turns ON) when the battery voltage is ≥21V.
Note that the load resistor, R_Load, needs to dissipate over 80W so should be at least a 100W resistor.
(You can put several lower wattage resistors in series or parallel to get that value if you like, for example seven 1 ohm, 15W resistors in series would give 7 ohms at 105W total.)

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#### #12

Joined Nov 30, 2010
18,223
Below is my take on this:

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#### crutschow

Joined Mar 14, 2008
29,515
#12's simplified comparator circuit will work, of course. Thanks for the KISS update.
I had originally designed the circuit to power a grounded load, but that's obviously not needed here.
Below is the simulation with that mod.
Note the simulation shows that the TL431 starts to conduct slightly (due to its operating bias current) before the threshold voltage is reached.
If that's a problem in the real circuit, causing the LED to light dimly, a resistor (2kΩ or so) can be placed across (in parallel with) the LED, which should eliminate that effect.

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#### #12

Joined Nov 30, 2010
18,223
#12's simplified comparator circuit will work, of course. Thanks for the KISS update.
Not intending to be in competition with you. I have some difficulty understanding the OP request, so I just did a concept drawing. It may, or may not, satisfy the requirements of the OP.

My first iteration had the LED coming on for low voltage. Then I considered that no voltage at all would show no LED, therefore, a false validation of the power supply, so I reversed the logic.

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