Dummy making a dummy load box

[Tim88]

Well, a dummy compared to most here anyways  . Found this neat design at Milletts site http://www.pmillett.com/dummyload.html. Got the heatsink drilled, tapped and resistors mounted. Quite a bit of work, more than I figgered but the worst is over. Got the resistors and heatsink from the links on Millett's site. The fancy box was a tad too salty for me so got a $10 one on the fleabay. Cut the top of the box out with my chop saw then dremmel the corners. Made a template on MS paint, taped it to the heatsink and punched the drill holes for reference. Used my drill press to sink the holes which I highly recommend using to get them true. I used some hardened #6 screws I had laying around which on retrospect, wish I got bigger ones but I am cheap  . Used a hand tap, didn't have an end cutting tap in that size so ground the tip down. Working with aluminum can be tricky, gotta use tap oil so the tap doesn't bind. I am just going to set it up for 4Ω/8Ω and just use a double throw switch instead of Millet's fancy switches. Hopefully get her finished in the next day or two. Can post what materials and template if anyone wants.

[t1d]

Excellent work, in the true DIY sense. Keep posting.

[not1xor1]

this is dummier than yours   

[001]

how do You use it?

[Tim88]

Gettin there...

[Electro Detective]

Cadet Dummy likes this post   

Now where did I put that unloved old project box with tangled up power resistors and thick wire...?   

[Jwillis]

I'm to dumb for those fancy gadgets.This is mine.
If it doesn't work I can always club small rodents with it.

[lordvader88]

I soldered some 10W resistors to some perf.board. , it's a start.

I a few more metal can power resistors on the way, where the hell are they, I'm getting worried they were never sent.

[ledtester]

Here's an analysis of all of the possible resistances you can get using a suite of 1, 2, 4 and 8 ohm resistors (at t = 7:50):

[not1xor1]

how do You use it?

Are you referring to my power resistor box?
You just move the plugs and get 1Ω to 99Ω.
Of course it is useful just for AC (low frequency - I use it to test transformers) as electronic loads offer much better performance for DC.

[001]

how do You use it?

Are you referring to my power resistor box?
You just move the plugs and get 1Ω to 99Ω.
Of course it is useful just for AC (low frequency - I use it to test transformers) as electronic loads offer much better performance for DC.

Thanx
What experiments You do with transformers?

[unitedatoms]

nice boxes. There is an aluminum profile by Makersled.com looking exactly like for this purpose.

[not1xor1]

What experiments You do with transformers?

I have lots of salvaged unspecified transformers and want to get a rough idea of their power rating, etc.

But the real reason is that I had a lot of 10W power wound resistors and wanted to use them for something potentially useful. 

[vk6zgo]

I remember my old Boss making the point years ago, that it is a "Test" Load not a "Dummy" Load,  because it provides a real load for the DUT.
A "Dummy"Load would be a fancy looking empty box!

Transmitter manufacturers usually eschew the "Dummy" term, calling the load you test with, either a "Test Load" or an "Artificial Load".

[Tim88]

Found a place by me that is gonna laser engrave my faceplate for me. Figgered might as well make it pretty. Found a free version of CAD software so I could make a template with everything laid out already to lower the cost. Quite the learning curve to that software.

[tkamiya]

I just use regular incandescent lights.  Parallel a few and it can take quite a bit.

Only thing is, it's ohmic value changes depending on how much you put in it.  But if you want a dummy dummy load, it's cheap and parts are widely available.

[schmitt trigger]

this is dummier than yours   

Nice looking faceplate! Love the voltage/current labels, too.
 

I wish I had half your skill.

[Gyro]

It's worth keeping an eye out for 'old school', (school sometime literally), linear slider lab rheostats.  They make excellent resistive loads. One in my collection is a Zenith 3R3 @10A - that's 300W at 30V. Not only will it take an awful lot of power (even more with a small fan blowing across it) but it makes a great workbench heater in the winter.

Avoid the Chinese round potentiometer style ones, they don't have enough surface area or full current wiper rating.


Edit: My set comprises 3R3 @ 10A, 16R @ 4A and 135R @ 1.4A. Enough to cover most eventualities.

[battmyrne216]

You have inspired me to make a big power resistor. I plan on also making a 4" 6.5 ohm I like how this turned out.

[TERRA Operative]

I see you use the same power rating method I use.

[bdivi]

Another take on the topic :-)

https://www.eevblog.com/forum/projects/have-you-built-a-resistive-load-bank-ideas/msg2148061/#msg2148061

[Doctorandus_P]

Now this is a resistor:

https://www.aliexpress.com/item/Ceramic-2500W-Tube-Wirewound-Resistance-used-as-dump-load-resistors-braking-resistance/32831870690.html

In some of these tubular resistor there is a 3rd terminal in the middle with a screw and these can be adjusted...
Found an example on:
https://www.aliexpress.com/item/WaveTopSign-High-Power-Porcelain-Tube-Adjustable-Resistance-RX20-Ceramic-Resistor-Power-300-1500W-Resistance-Value-0/32966105005.html
And apparently these go upto 10kW, and they have a short time overload capability of 100x...

[H713]

Not really so much of a box, but it works pretty well. It's spaced off the back wall of the bench by about 1/2 inches to allow air ventilation (also I was too lazy to mill out pockets for the nuts on the back). Those are 8 ohm "100W non-inductive" resistors from China. I haven't manage to blow them up yet. If I want a 4-ohm dummy load, I connect the second resistor in parallel with clip leads. If I don't need to test two channels at once, I can go up to 32 ohm or down to 2 ohm.

Nobody ever said a dummy load had to be smart. When I get around to re-doing my bench, I'm going to make a rackmount dummy load that will have more options, including 8 ohms at 1500W for load testing big amps, and some higher values for load testing transformers and power supplies. And It's going to have a fan so I never have to stop and wait for it to cool down like I do now.

Another type I've seen for lower values in physics labs is a big spring-like coil of nichrome wire submerged in a tube of water. Supposedly they can take a tremendous amount of wattage due to the high specific heat of water. Just be sure to top it off every now and then.

[schmitt trigger]

A "What if" type of question to the cognoscenti in this board:

If you were to do a binary-weighted resistor load (1, 2, 4, 8 ohms and so forth) to be wired in series to obtain discrete arbitrary values.... how would you allocate the resistor power rating for each?

I would assign the highest power rating to the lowest valued resistor.
Let's say you want a load with a maximum 20 volt input voltage. Then the 1 ohm device would be 400 watts, the 2 ohm 200 watts, 4 ohm would be 100 watt and so forth.

Would this be a sensible approach?

[SiliconWizard]

If you were to do a binary-weighted resistor load (1, 2, 4, 8 ohms and so forth) to be wired in series to obtain discrete arbitrary values.... how would you allocate the resistor power rating for each?

I would assign the highest power rating to the lowest valued resistor.
Let's say you want a load with a maximum 20 volt input voltage. Then the 1 ohm device would be 400 watts, the 2 ohm 200 watts, 4 ohm would be 100 watt and so forth.

Would this be a sensible approach?

That sounds about right.

If you have a chain of resistors in series, the same current will go through all of them. For each of the resistors Rk, the worst case will be when it's the only one that is used, thus Pmax = Umax^2 / Rk.