Hi

I wanna test some PSUs rated 12VDC 1A, to make sure they can deliver the rated current.

I was thinking about a solution with an LED would light up if current draw is at least 900mA.

But how can I make a circuit to draw the current amount, and light, up if satisfied?

Thanks in advance

 

Simpler, connect a 12Ω, 12W+, resistor and measure the voltage (if the PSU doesn't include a voltmeter).

 

It's meant for non-technical persons can test PSUs quickly by looking at an LED.
The PSUs are just like the PSU shipping with your favorite home router.

 

This would do it. For the switch use a push button or biased toggle switch to limit the heat generated in R5. Adjust the pot so the LED just lights at the minimum allowed voltage.

 

It's meant for non-technical persons can test PSUs quickly by looking at an LED.
The PSUs are just like the PSU shipping with your favorite home router.

Search for "window comparator" circuits - you need a dummy load for the PSU as others have stated.

 

For the switch use a push button or biased toggle switch to limit the heat generated in R5.

Make sure the switch contacts are rated for at least 2 A. Also, Even though the power dissipation is intermittent, I still would have R5 rated for at least 25 W.

ak

 

This would do it. For the switch use a push button or biased toggle switch to limit the heat generated in R5. Adjust the pot so the LED just lights at the minimum allowed voltage.
View attachment 139725

You might want to check component orientation.

 

Make sure the switch contacts are rated for at least 2 A. Also, Even though the power dissipation is intermittent, I still would have R5 rated for at least 25 W.

ak

Those power resistors potted in aluminium extrusions are a favourite for dummy loads.

Most have mounting lugs for heat sink mounting and a cooling fan can hide many evils.

 

You might want to check component orientation.

Oops! Corrected version:

 

Thanks for your quick responses.

So S1 is just to make sure the circuit is not "on" for long time to protect R5?

If I want an 2Amp edition also, I just have to swap R5 with an 24Ohm or two 12Ohm in series?

Is it R5 value alone controlling how much current is necessary for the LED to light up?

 

A 2A version would need a 6Ω, 24W load resistor.
With the switch off you can check for 12V with no load then, when you operate the switch, you can see whether the voltage is maintained under load.
If the switch is a push button or biased toggle then you can get away with a resistor rated with no margin on the power rating as the load resistor will only be connected for a limited time. If the tester gets distracted they cannot leave the load connected permanently.

R5 determines the load current. The rest of the circuit is to check the voltage and draws about 10mA, mostly the LED current.

 

Thanks for your help - I've ordered the needed components and will test it soon.

 

A 2A version would need a 6Ω, 24W load resistor.
With the switch off you can check for 12V with no load then, when you operate the switch, you can see whether the voltage is maintained under load.
If the switch is a push button or biased toggle then you can get away with a resistor rated with no margin on the power rating as the load resistor will only be connected for a limited time. If the tester gets distracted they cannot leave the load connected permanently.

R5 determines the load current. The rest of the circuit is to check the voltage and draws about 10mA, mostly the LED current.

My ATX dummy load uses various car bulbs. The H4 headlamp bulbs are run well under voltage, but you do have to be mindful of radiated heat can be a fire hazard.

 

I have now assembled it and done some testing.

If I use 6Ohm R5 (for 2A edition) and connect 12V/1A, I meassure 1.6A but the LED still lights up.

Can I control this somehow?

 

Did you adjust the pot to set the voltage threshold?

 

I tried adjusting, but it went from normal light to very bright.

Will the voltage not be the same as the input voltage is 12V?

 

Did you test at what voltage the LED switches on?
Whether the load is 6Ω or 12Ω won't affect that threshold voltage.

 

The LED is always on no matter which power supply I use.

 

While connected to a 12V supply, adjusting the pot will not turn off the LED?
In that case first check that you have the correct connections to the TL431. The anode should be connected to 0V and the reference should be connected to the wiper of the pot.