# Confirming a power supplies Amp output

Thread Starter

#### shezza

Joined Mar 19, 2014
54
So I bought an 8A power supply off eBay. Sadly, the setup I a putting together is far from ready, so I can't test it against it. Short of finding something that needs close to 8A to run, is there any way to confirm its potential output? The reason I ask is because the supply is extremely light! My laptops power supply is 6A and weighs 3 times as much.

#### dl324

Joined Mar 30, 2015
12,498
The only way to confirm the maximum current spec is to apply a load that draws 8A.

If it came was cheap and came from China, don't be surprised if it comes up short.

#### shteii01

Joined Feb 19, 2010
4,644
Easy enough. Lets say it produces 8A at 12V. 12*8=96W

Buy 100W resistor, put good dmm in series with the resistor, set the dmm to measure current, apply 12V from power supply to the circuit.

#### crutschow

Joined Mar 14, 2008
26,413
Or buy a 12V, 100W bulb as a load.

#### Reloadron

Joined Jan 15, 2015
5,781
As crutschow mentions. Here is an example for about $6.00 US. Using a lamp like this just make sure the glass is clean, any finger oil on the bulb will boil and the bulb will crack. I clean them with a clean paper towel and isopropanol alcohol. Ron #### ebp Joined Feb 8, 2018 2,332 An issue with using a lamp for a load is that when the filament is cold the resistance will typically be a tenth or less of what it is at normal operating temperature. What will happen depends on the design of the power supply. Some may current limit in a way that allows the supply to start over a period of tens to hundreds of milliseconds. Others may detect the load as grossly excessive, turn off, restart, detect the load as grossly excessive ... Many "ordinary" switchers don't have any mechanism for secondary-side current limiting and primary side limiting is set up to do some measure of protection of the entire power path. This sort of supply is more likely to behave in the stop-and-retry mode. If "soft start" is implemented and of long enough duration, the supply will likely light the lamp. #### be80be Joined Jul 5, 2008 2,051 @ebp if a 12 watt come's on from the start the power supply is likely more then able to put out 12 amps But your right the bulb when cold pulls lot's more current 12 watt bulb uses 1 amp for short bit it will try to get 12 amps maybe lower but not more But i've use bulbs for load alot never seen them shut a switching supply off tho. As matter of fact I used a 40 watt Halogen bulb with a 3 amp switcher it ramp up from dim to fully on. But it did come on and pulled 4 amps from a 3 amp switcher I have like 10 or so of these supply they where for routers there nice supplys. But any thing over 4 amps and the supply voltage drops out to where the supply limit's to 3 amps so I think they have a current limit builtin, #### -live wire- Joined Dec 22, 2017 912 Or buy a 12V, 100W bulb as a load. But they often have much higher startup currents. So unless he has a CC board, it may be better to use a power resistor or other constant load. Edit: I didn't mean to repeat what others have said. #### -live wire- Joined Dec 22, 2017 912 Also, there are often capacitors at the output that can supply much greater currents until they are discharged. This may help a little bit, but unless they are really, really large, the bulb will probably not be able to fully start up. #### Reloadron Joined Jan 15, 2015 5,781 While I believe everyone would agree placing a resistive load using resistors would likely be the best approach to loading a power supply it is likely not the least expensive way to go about it. Using 12 Volts and 8 Amps as an example you are looking at a 1.5 Ohm load discounting any lead resistance. You are also looking at dissipating about 96 Watts. You can buy a pair of 3.0 Ohm (LM YN 100 Watt 3 Ohm 5% Wirewound Resistor) for about$8.00 on Amazon. I have no clue how good these resistors are, likely from China or go with Ohmite for about $13.00 each. Load the power supply with one for about 4 Amps and then add a second one in parallel making for about a 1.5 Ohm load and 8.0 Amps of current draw. Pretty much a matter of what one wants to spend on a one time simple test of a power supply. Personally I would try a cheap lamp first but the possibilities are endless along with the cost. I also agree with: If it came was cheap and came from China, don't be surprised if it comes up short. Ron #### -live wire- Joined Dec 22, 2017 912 While I believe everyone would agree placing a resistive load using resistors would likely be the best approach to loading a power supply it is likely not the least expensive way to go about it. Using 12 Volts and 8 Amps as an example you are looking at a 1.5 Ohm load discounting any lead resistance. You are also looking at dissipating about 96 Watts. You can buy a pair of 3.0 Ohm (LM YN 100 Watt 3 Ohm 5% Wirewound Resistor) for about$8.00 on Amazon. I have no clue how good these resistors are, likely from China or go with Ohmite for about \$13.00 each. Load the power supply with one for about 4 Amps and then add a second one in parallel making for about a 1.5 Ohm load and 8.0 Amps of current draw.

Pretty much a matter of what one wants to spend on a one time simple test of a power supply. Personally I would try a cheap lamp first but the possibilities are endless along with the cost.

I also agree with:

Ron
What about going with a bulb but using a soft start with something like this?

#### Reloadron

Joined Jan 15, 2015
5,781
What about going with a bulb but using a soft start with something like this?
No shortage of ways to test a power supply under load, just a matter of how one wishes to do it and what they want to invest for a one time test. Yes, you could soft start or just bring the load up slowly.

Ron

#### RichardO

Joined May 4, 2013
2,271
No shortage of ways to test a power supply under load, just a matter of how one wishes to do it and what they want to invest for a one time test. Yes, you could soft start or just bring the load up slowly.
Maybe a compromise would be to use several small bulbs connected one at a time until the full load is applied.

A trip to the junk yard to get a bunch of tail lights might be in order.

#### Reloadron

Joined Jan 15, 2015
5,781
Maybe a compromise would be to use several small bulbs connected one at a time until the full load is applied.

A trip to the junk yard to get a bunch of tail lights might be in order.
That also would work fine.

Ron

#### Tonyr1084

Joined Sep 24, 2015
5,707
Know anyone who uses water filters? Carbon filters? I have an RO (Revers Osmosis) drinking water system and periodically change the carbon filters. Knowing as I do that some resistors are made out of carbon I wondered if I could use one of these very large carbon filters as a variable resistor. Actually there's more behind the story, but I bet you might be able to use something like this to determine the efficiency of your supply using an old filter. Check out one of my older videos:

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Thread Starter

#### shezza

Joined Mar 19, 2014
54
Thank you for all the replies! I took a 100W globe and connected it up, nothing! The power supply is putting out 12.13 which means it should be able to supply 97W. Only 3W short, it should still be fairly lit, yes? Thoughts?

I just hooked it up to a 55W globe and nothing happened for the first 3 seconds, then it started pulsating and finally, it lit. Though it only reached 2.73A which is well short of the globes full draw as 12V.

Seems pretty clear its capable of no more than 2.73A.

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#### crutschow

Joined Mar 14, 2008
26,413
Thread Starter

#### shezza

Joined Mar 19, 2014
54
What is its voltage rating?
I added some more information to the post. It is all 12V.

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