Discussion in 'General Electronics Chat' started by pillyg, Jan 22, 2014.

1. ### pillyg Thread Starter Member

Sep 19, 2011
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0
I need to artificially test a power supply up to around 1000W. I also need to know how much power the PSU is outputting. I have created a load using some electric toaster wire, but I have no idea exactly how much power is being used.

My question is how can I create a 1000W DC load that can be measured to ±10W, hopefully on the cheap side?

2. ### inwo Well-Known Member

Nov 7, 2013
2,435
315
Measure it.

A fixed wattage load is hard to do, and usually not necessary.

It will change with EIR and temperature.

Measure volts and amps till you get what you need.

3. ### MrChips Moderator

Oct 2, 2009
12,636
3,455
Use Ohm's Law.

I = V/R

There are three variables:

V = voltage
I = current
R = resistance

You have to know two of the three variables. Which of the two can you measure?
Once you have measured two variables you can calculate the third.
Then you can calculate power in one of three ways:

P = I x V
P = I x I x R
P = V x V / R

BTW, you cannot build a 1000W load. That is not how it works.
If a load is rated at 1000W it simply means that the load can handle 1000W and not destroy itself.

4. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
I have some convection air-cooled resistors rated at 0.18Ω at 1200W. I got them when I was building a 12V battery tester. They were originally made as "locomotive braking resistors".

For short term testing of power supplies, I have used wire-wound ceramic power resistors rated at ~50W but I immersed them in a tub of motor oil. Starting with the oil at room temperature, you have several minutes to do your tests where you can dissipate several hundred W in the resistor. Monitor the temperature of the oil bath during your testing. Stop when the oil reaches 200degC

Be very careful with the hot oil, as it can give you severe burns...

5. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,439
1,627
Since you provided just a scant piece of information ...

Power has two components to it's measurement: Voltage and Current.

Voltage can be measured very accurately with any DMM.

There is usually a limit to measuring current with a DMM, and it is oft inconvenient to break the circuit to insert the meter anyway. So do what professionals do here, use a current shunt. They are very accurate low resistance high current resistors that put out a small voltage, say .075 volts for 100A.

You can find current shunts at all the major component distributors.

6. ### alfacliff Well-Known Member

Dec 13, 2013
2,449
428
how much voltage is your supply rated? if its a 1000 volt supply, a 1000 ohm resistor would wouk, if 100 volt, a 10 ohm. power = voltage times current. resistance = voltage devided by current. make sure your connecting wires are heavy enough to not introduce stray resistance into the load.

7. ### Dave_UYZ New Member

Jan 16, 2014
29
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I wanted a 30A shunt for my PSU and found several at a variety of resistances (for 75 and 100mV) on E-Bay.
I also bought three of those neat little heaters for making tea or coffee in the car; they are about 120w each and cost only a couple of £.
Naturally, the down-side is that it can take a while to arrive from China. . .

Take a look HERE.

8. ### pidsmb New Member

Jun 12, 2011
1
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Use a water resistor. Dip the output of ur psu slowly into water as u monitor the output voltage and current. The use of water resistors is common in power measurements where passive resistive loads are not available.Observe safety though.