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Old 15th Jul 2020, 10:55 am   #1
indiosse
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Default Power supply test under load with power resistor.

Hi all
I need to test a 9v power supply under load, and need a check of maths and my generally poor knowledge of electronics.

Anyway i have a power supply that is rated for 2a at 9v
I managed to hook the power supply up to a lamp and check that the voltage didnt fall

I am wondering if i can use one of these
https://www.ebay.co.uk/itm/5W-10W-25...cAAOSwpCBcliwN

I worked out (hopefully or probably i didnt) that if i buy the 10w (they dont have 8.1 one) at 10ohms it will drag approx 1amps out of the 9v supply where i can measure the voltage drop (or not) on the supply with my multimeter

This is how i calculated it:
if using a 10 ohm resistor power going through will be 0.9 amps:
0.9amps
9v / 10ohms

P (watts) 8.1w
= 0.9a * 9v

Do i need 10ohm resistor at 8.1w ?

Please help, because im probably not right :/
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Old 15th Jul 2020, 11:09 am   #2
Station X
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Default Re: Power supply test under load with power resistor.

Hello and welcome to the forums.

9 Volts at 1 amp is 9 Watts ie V X I or 9 X 1. So your resistor must be rated at at least 9 Watts.

The resistor value required is 9 Ohms ie V/I or 9/1.

If you use a 10R resistor The current will be 0.9A. ie V/R or 9/10.

The power will be 8.1 Watts. ie I X I X R or 0.9 X 0.9 X 10.

So your calculations are correct.

A 10 Ohm 10 Watt resistor will suffice.
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Old 15th Jul 2020, 12:54 pm   #3
Chrispy57
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Default Re: Power supply test under load with power resistor.

Hi,
you will be reassured by Graham's reply confirming that your calculations were spot on and the 10R 10W resistor would be OK. But let's take a step back and think about what you are trying to achieve: do you just want to test the PSU at half its rated load (as per your 10R) or get a more comprehensive idea of its capability?

Looking at the offerings from the supplier in your link you would get the most "bangs per buck" from buying two 10W resistors, 10R and an 8R. These could be used singly, wired in series and wired in parallel to check the PSU's performance at 0.5A, 0.9A, 1.1A and 2A while observing how the output voltage holds up with your DVM. So checking at 1/4 load, 1/2 load and full load. Food for thought?

Cheers
Chris
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Old 15th Jul 2020, 4:39 pm   #4
indiosse
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Default Re: Power supply test under load with power resistor.

Quote:
Originally Posted by Chrispy57 View Post
Hi,
you will be reassured by Graham's reply confirming that your calculations were spot on and the 10R 10W resistor would be OK. But let's take a step back and think about what you are trying to achieve: do you just want to test the PSU at half its rated load (as per your 10R) or get a more comprehensive idea of its capability?
thanks for the replies there gents, when i connect one of these devices, will it instantly start taking the power once connected to the positive rail or do i need to do something else to complete the circuit. ie connect the +ve to one side of resistor and -ve to other side of power resistor ?

re: further load testing, ideally yes i would like to to a test up to the limit, but i was just initially asking for my calculations to be confirmed.

im not too up on series and parallel differences, could you elaborate on what the difference is, if i read through your reply right, would series half the amperage and parallel wiring double it?
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Old 15th Jul 2020, 4:49 pm   #5
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Default Re: Power supply test under load with power resistor.

There are lots of online calculators available to help you with this stuff if you google.
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Old 15th Jul 2020, 4:55 pm   #6
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Default Re: Power supply test under load with power resistor.

Quote:
Originally Posted by indiosse View Post
when i connect one of these devices, will it instantly start taking the power once connected to the positive rail or do i need to do something else to complete the circuit. ie connect the +ve to one side of resistor and -ve to other side of power resistor ?
You'll need to connect the positive side of you PSU to one side of the resistor and the negative side of your PSU to the other side of the resistor before any current will flow.

Note that the resistor will get hot. That's what resistors do. They turn electrical energy into heat.
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Old 15th Jul 2020, 6:04 pm   #7
indiosse
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Default Re: Power supply test under load with power resistor.

right-o, i will order some and see how i get on, thanks so much
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Old 15th Jul 2020, 7:54 pm   #8
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Default Re: Power supply test under load with power resistor.

Yes, you've pretty much got the idea there. Hope the attached sketch will clarify things for you.

Cheers
Chris
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Old 16th Jul 2020, 10:23 am   #9
indiosse
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Default Re: Power supply test under load with power resistor.

Thanks this is great!
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Old 16th Jul 2020, 12:22 pm   #10
David Simpson
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Default Re: Power supply test under load with power resistor.

Why Aye Geordie man, all the info is spot - on. But, just a thought - does your 9V PSU have a safety cut-out, trip, or fuse ? Just, just in case something goes wrong whilst testing. Maybe it has a wee digital display indicating Volts & Amps, or an older metering set-up. Fuse inside the 13A plug - fit the smallest common rated 25mm fuse - - 1A.
Hey - show us a picture.

Regards, David
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Old 10th Nov 2020, 12:31 pm   #11
indiosse
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Default Re: Power supply test under load with power resistor.

everything working A ok, went for a 15ohm 10w resistor in end and im testing 9v power supplies, however i want to add at to the 15ohm, a 12ohm (10w) resistor and a 20ohm resistor (10w) as well all in parallel, my maths gives me a test resistance of 1.8a if i test at 9v with following maths :

1/15 + 1/12 + 1/20 = 0.2 then divided by 1 then answer 1/A = 1.8a

the 12 and 20 ohm, would be bought simply because of the stock of the supplier, he is missing some resistance values so i make the 1.8a with the 12 and 20

hope this makes sense!
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Old 10th Nov 2020, 3:26 pm   #12
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Default Re: Power supply test under load with power resistor.

A point to bear in mind is that for continuous use, metalclad resistors such as the ones in your link in post #1 are meant to be bolted down to an adequate heat sink to be run at their rated value. In free air, they need to be de-rated by 50%. However, if you are only going to run it for a few minutes that wouldn't be a problem, but if you wanted to run it for longer, maybe the 25 Watt one from the same supplier (at the same price) would be a better choice.
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Old 10th Nov 2020, 3:42 pm   #13
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Default Re: Power supply test under load with power resistor.

Hi Indiosse, If you contact me by PM you can probably come over and I can find you suitable resistors.

Ed
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Old 10th Nov 2020, 10:23 pm   #14
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Default Re: Power supply test under load with power resistor.

It's often useful to use readily available devices such as car headlamp bulbs as power supply loads, although their resistance changes with temperature. A 2 kW kettle is essentially a resistor of around 26 ohms with a power rating which will meet many requirements. The simple ones, with no electronics, will work on AC or DC equally well.

Kettles can be bought so cheaply that this could be less expensive than high power resistors - just don't let the water boil away.

PMM
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Old 10th Nov 2020, 10:49 pm   #15
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Default Re: Power supply test under load with power resistor.

Quote:
Originally Posted by pmmunro View Post
A 2 kW kettle ...will meet many requirements. The simple ones, with no electronics, will work on AC or DC equally well.

Kettles can be bought so cheaply that this could be less expensive than high power resistors - just don't let the water boil away.

Useful and thanks, I was on the point of looking around for a way to stress test a system and this will do nicely, with the advantage that I can measure the power output precisely by tracking the time taken to raise the temperature of a known volume of water by a certain amount... Cheers!
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Old 11th Nov 2020, 8:17 am   #16
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Default Re: Power supply test under load with power resistor.

When testing PSU's as outlined I monitor current, voltage and ripple with a scope, it's a good idea too to see how a PSU behaves when over loaded, if only temporarily by about 10% as well as shorted. One can also test with capacitive loads to see how they behave and the idea of using bulbs as loads is interesting as they place a difficult load on a PSU's like valve heaters they have a lower cold resistance and cause heavy inrush current often blowing fuses so if the intended load has a low R when cold you have to overate the fuse a bit and make it a slo blo.

When testing a DIY PSU it's a good idea to monitor the temperature of series pass elements etc over a long period of time too.

Andy.
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