Cheap DIY Dummy Loads
Moderators: pompeiisneaks, Colossal
Cheap DIY Dummy Loads
Amp builders and repairers should have some kind of dummy load available. The usual practice, finding very high power low-ohms resistors, can be expensive. But there is another way. You can make arrays of low ohms resistors and get the same power and more flexibility.
Making higher power resistors from arrays of lower powered resistors is a trick that I was shown many years ago, and it works well in some cases.
What brought this to mind is that I got my regular email ad from Marlin P. Jones' surplus place, and they have 1 ohm, 10W resistors on sale for $0.19 each. Eight of these in series makes an 8-ohm, 80 watt dummy load for $1.52. Not too shabby. Get 16 for $3.04 and you can make a 16 ohm, 160W resistor by putting two 8-ohm strings in series, or a 4 ohm 160W resistor by putting the same 8-ohm strings in parallel. Go really crazy and buy 32 of them for $6.08 and you can make up series/parallel combinations to get 2 ohms, 4 ohms, 8 ohms, and 16 ohms at well over 100W.
That's nice, but how to mount them so they're usable? One easy way is to make a breadboard - the old fashioned kind, made with a real piece of wooden board (or plywood, for stability) and screw in short screws in an array on the board, then soldering the resistors to the screws, which hold them up off the burnable wood and ensure lots of air circulation to let the resistors get HOT without damaging anything. Add a terminal block at one end for input/output and perhaps even patching for different load ratings, and you get a very flexible bench accessory.
For extra points
If you want better modelling, most 8 ohm speakers are actually about 6 ohms resistive and 6mh more or less due to the voice coil inductance. You can buy 6.8mH/4A inductors at Mouser for about $6.50 each. Use one inductor and six 1 ohm / 10W resistors for each "8-ohm" dummy resistor and you get much closer to a real speaker - if this is what you want to do.
Making higher power resistors from arrays of lower powered resistors is a trick that I was shown many years ago, and it works well in some cases.
What brought this to mind is that I got my regular email ad from Marlin P. Jones' surplus place, and they have 1 ohm, 10W resistors on sale for $0.19 each. Eight of these in series makes an 8-ohm, 80 watt dummy load for $1.52. Not too shabby. Get 16 for $3.04 and you can make a 16 ohm, 160W resistor by putting two 8-ohm strings in series, or a 4 ohm 160W resistor by putting the same 8-ohm strings in parallel. Go really crazy and buy 32 of them for $6.08 and you can make up series/parallel combinations to get 2 ohms, 4 ohms, 8 ohms, and 16 ohms at well over 100W.
That's nice, but how to mount them so they're usable? One easy way is to make a breadboard - the old fashioned kind, made with a real piece of wooden board (or plywood, for stability) and screw in short screws in an array on the board, then soldering the resistors to the screws, which hold them up off the burnable wood and ensure lots of air circulation to let the resistors get HOT without damaging anything. Add a terminal block at one end for input/output and perhaps even patching for different load ratings, and you get a very flexible bench accessory.
For extra points
If you want better modelling, most 8 ohm speakers are actually about 6 ohms resistive and 6mh more or less due to the voice coil inductance. You can buy 6.8mH/4A inductors at Mouser for about $6.50 each. Use one inductor and six 1 ohm / 10W resistors for each "8-ohm" dummy resistor and you get much closer to a real speaker - if this is what you want to do.
I don't "believe" in science. I trust science. Science works, whether I believe in it or not.
Re: Cheap DIY Dummy Loads
Nice, R.G.!
And for folks who don't mind spending a little more... I got four 50w, 4R resistors, and a 4P3T rotary switch and made Aiken's 2/4/8/16R load box. Very nice. About $50 in parts. I added the line out tap.
And for folks who don't mind spending a little more... I got four 50w, 4R resistors, and a 4P3T rotary switch and made Aiken's 2/4/8/16R load box. Very nice. About $50 in parts. I added the line out tap.
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Re: Cheap DIY Dummy Loads
Add a line out and you have $25 solutions to $900 problems.
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Re: Cheap DIY Dummy Loads
Yeah I've built one of aiken's too, love it. Use it all the time when scoping signal so I don't have to hear to a 1k sine wave killing me
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Re: Cheap DIY Dummy Loads
I have an old Marshall Power Brake attenuator that will run in a silent mode.
Is there any reason that this would not be advisable to use as a dummy load?
Is there any reason that this would not be advisable to use as a dummy load?
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Re: Cheap DIY Dummy Loads
As long as it matches R and V
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Re: Cheap DIY Dummy Loads
Thanks for the tip, R.G.! Ordered!
Cheers!
Cheers!
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Re: Cheap DIY Dummy Loads
I took a similar approach. I built each load resistance out of series/parallel combinations of six 50W resistors a piece. I was able to source all of those resistors for under $2 a piece searching Newark's overstock selection. I chose resistance values in order to distribute the load as evenly as possible in order take advantage of each resistors rated power. Each load setting of 4, 8, and 16Ω will just shy of 300W (heat sink'd). The resistors are still available at Newark (Links for the resistors below the photos).
Here's a couple of photos of the build and a schematic: http://www.newark.com/cgs-te-connectivi ... t=bargains
http://www.newark.com/cgs-te-connectivi ... t=bargains
http://www.newark.com/cgs-te-connectivi ... t=bargains
http://www.newark.com/welwyn/wh50-5r6jb ... t=bargains
http://www.newark.com/cgs-te-connectivi ... t=bargains
http://www.newark.com/cgs-te-connectivi ... t=bargains
Here's a couple of photos of the build and a schematic: http://www.newark.com/cgs-te-connectivi ... t=bargains
http://www.newark.com/cgs-te-connectivi ... t=bargains
http://www.newark.com/cgs-te-connectivi ... t=bargains
http://www.newark.com/welwyn/wh50-5r6jb ... t=bargains
http://www.newark.com/cgs-te-connectivi ... t=bargains
http://www.newark.com/cgs-te-connectivi ... t=bargains
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Re: Cheap DIY Dummy Loads
I used an old power supply chassis... Two channels switchable from 4 to 8Ω. The "Power" switch parallels both together to go down to 2Ω. 1/4" and BNC line out for each side. BNC out is nice for scoping the output, 1/4" can go out to signal tracer. Someone gave me the Dale 250W resistors and I paid about $40 for all other parts.
Gonna buy some of those 1Ω resistors tough... Great price!!
Gonna buy some of those 1Ω resistors tough... Great price!!
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Re: Cheap DIY Dummy Loads
oooh, those 250W resistors are sweet. I'm gonna' get me some of those someday when I grow up.
Re: Cheap DIY Dummy Loads
Are the rotary switchers safe enough on the long term, or is better to let the female jack do the job, with a dedicated inlet for each impedance?
What about those resistors? https://eu.mouser.com/datasheet/2/427/hlnhl-101681.pdf
They are very very cheap.
What about those resistors? https://eu.mouser.com/datasheet/2/427/hlnhl-101681.pdf
They are very very cheap.
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Re: Cheap DIY Dummy Loads
A high quality, moderately heavy duty rotary switch would be safe and reliable for years. A cheaply made one probably won't be. But individual input jacks seem fine as well.
Re: Cheap DIY Dummy Loads
Same: 8 x 8R Ohmite '100W' resistors, 2 x e-bay chinese heatsinks and an enclosure, 1 SPST switch, I jack , 16 nuts and bolts and heatsink compound. Does 4R or 8R loads. I think it came to about $20 altogether. The surface area for the vanes on each of the sinks is 111"sq (not counting the base of the metal box enclosure. So it probably doesn't have enough heatsink to allow for 100W on each resistor, but I figure each resistor is good for about 25W in this case, which gives a load capable of 8R @100W or 4R @200W give or take. Probably more if I blow it with a fan. (I figure the case will help contain any blow-outs - either that or it'll become a shrapnel bomb.SoulFetish wrote: ↑Wed Aug 08, 2018 10:35 pm I took a similar approach. I built each load resistance out of series/parallel combinations of six 50W resistors a piece. I was able to source all of those resistors for under $2 a piece searching
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Re: Cheap DIY Dummy Loads
I fired Geico and saved 92% in 12 minutes.
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