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Need to create a dummy load 
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#1
Jan1410, 08:45 AM

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I need to create a dummy load on a 120VAC circuit. The load must draw .3A. How can I do this?



#2
Jan1410, 09:06 AM

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P: 8,953

A resistor?
If you don't know Ohm's law to work out the resistance and how to work out the power in the resistor are you sure you should be playing with 120VAC? We aren't trying to be difficult but if you are an 8year old kid somewhere who has decided to learn electricity with potentially lethal line voltages because they don't have a spare AA battery we don't really want to be responsible 


#3
Jan1410, 09:07 AM

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You could get four 10 watt 100 ohms resistors and put them in series.
Total resistance = 4 * 100 = 400 ohms Current = 120 volts / 400 ohms = 0.3 amps Power dissipated in each resistor = 0.3 A * 0.3 A * 100 ohms = 9 watts Total power = 0.3 A * 0.3 A * 400 ohms = 36 watts 


#4
Jan1410, 09:26 AM

P: 3

Need to create a dummy load
Thank you. That is what I thought, 400 ohms. Just wanted to make sure. I tried a different resistor that was here, 1K  magic smoke.
Forgot to factor in wattage rating for resistor. 


#5
Jan1410, 09:54 AM

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A regular little resistor can dump about 1/4 watt of heat  any more than this and it will just melt A 10W resistor is a larger more expensive lump of metal it wil look something like this It's also going to need mounting to some sort of heatsink to dump the 40W of heat 


#6
Jan1410, 10:18 AM

P: 3

Thank you again. I forgot about the wattage values for resistors. Been quite a few years since I last dealt with resistors.



#7
Jan1410, 04:33 PM

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If 0.33 amps would be OK, you could use a 120 volt 40 watt lamp bulb.
Power = voltage * current so current (in amps) = power (in watts) / voltage Current = 40 watts / 120 volts = 0.33 amps. 


#8
Jan1510, 04:17 AM

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#9
Jan1510, 05:36 AM

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Thanks
PF Gold
P: 12,132

The only snag with using light bulbs is that you can't be certain of their resistance at anything other than their normal operating temperature. The resistance varies over a range of ten to one from hot to cold. You may need to experiment a bit and even use two in series (one high wattage and one low wattage) to get the resistance you want  if its value is critical. You can also buy very low wattage heaters which don't operate at white heat and will have more stable resistance values.



#10
Jan1510, 08:58 AM

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#11
Jan2710, 10:37 PM

PF Gold
P: 3,097




#12
Jan2810, 04:34 AM

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Thanks
PF Gold
P: 12,132

My comment really related to the possibility of the supply volts not always being the nominal value  the resistance varies right over the filament temperature range. Medium / High power resistors are not expensive are they? It all depends on the accuracy required, in any case.



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