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Do I need a non-inductive resistor for a dummy load?

M

Matt Giarra

Member
I need to build an 8ohm dummy load. The Arcol/Ohmite product sheet indicates their resistors come in “standard” and “non-inductive” models. Do I need non-inductive? I can’t find non-inductive 8ohm 50W from this company in stock anywhere, so if I do need non-inductive, I’ll need to adjust my search criteria.
 
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The inductance of a 50 watt 8 ohm (wire wound) resistor would be so low, it will not have insignificant effect at audio frequencies. Heck, if you are using it for a speaker load, it will have orders of magnitude less inductance than a speaker does. Actual speakers present both highly inductive loads and highly capacitive loads depending on frequency.
 
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People get hung up on 8ohms when speakers that are rated for 8ohms have a nominal ohm range of 8ohms but will vary from 5ohms to 12 ohms with frequency. Unless you are charting specs for the FTC its not critical. I found a great deal on a 5.2ohm 250watt non inductive, the 8ohm versions go for 5x's what I paid. Look for a standard value resistor like 4.7, 5.6, 6.8 or 8.2 as those are usually much cheaper. Using two 15 ohms in parallel, three 22 ohm or four 30ohm resistors in parallel will equal close enough to 8ohms to not cause any concern. Another benefit is two 15ohm 50watt resistors would turn into a 7.5ohm 100 watt and four 30ohm 50watt resistors in parallel would turn into a 7.5ohm 200 watt load. Check out a resistor in parallel calculator and save yourself some $$. New Dale NH resistors can be found on ebay on a weekly basis.
 
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petes'picks said:
People get hung up on 8ohms when speakers that are rated for 8ohms have a nominal ohm range of 8ohms but will vary from 5ohms to 12 ohms with frequency. Unless you are charting specs for the FTC its not critical. I found a great deal on a 5.2ohm 250watt non inductive, the 8ohm versions go for 5x's what I paid. Look for a standard value resistor like 4.7, 5.6, 6.8 or 8.2 as those are usually much cheaper. Using two 15 ohms in parallel, three 22 ohm or four 30ohm resistors in parallel will equal close enough to 8ohms to not cause any concern. Another benefit is two 15ohm 50watt resistors would turn into a 7.5ohm 100 watt and four 30ohm 50watt resistors in parallel would turn into a 7.5ohm 200 watt load. Check out a resistor in parallel calculator and save yourself some $$. New Dale NH resistors can be found on ebay on a weekly basis.
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Makes sense. The 8 ohm 50W standard resistors are like $4.59 on Digi-Key, so price isn’t a big deal in this case, but I see your point generally.
 
Non inductive resistors have been the standard for dummy loads for over 50 years for a reason. If you are going to be pushing a 50 watt per channel amp at max power then do yourself a favor and get a non inductive resistor load. If you are just going to use it to look at a sinewave at low power a regular old resistor would probably do just fine but I wouldnt push the dummy load more than a couple of watts if you chose to go without a non inductive resistor load. If

You will be pushing AC signals through the resistor so it really should be non inductive, if you try to push a square wave or high power AC signal through a regular resistor it will likely cost you more than the cost of a non inductive resistor to replace the blown parts. The next time I hook up my distortion analyzer or looking at clipping on scope I may try to take measurements from both types of resistors to see how it effects the readings at different frequencies but I will not use a amp I care about.
 
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Inductance would be too low to worry about when used as a load at analog audio frequencies. Plus, the Q would be so low that it would minimize the effect of the residual inductance of the resistor.
 
From what I have been able to research, an average 8 ohm 100 watt wire wound resistor has substantially less than 1 microhenry of inductance. At 20 kilohertz 1 microhenry would give a maximum added impedance of only 0.13 ohms to the load. Heck even at 100 kilohertz it would only add 0.63 ohms to the load impedance.
 
When I built my dummy load I used non inductive surface mount resistors. I also added two BNC connectors and a 10:1 switch.
 
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