Dummy Load Resistor - Building

[Ryteone]

I wanted to build one of the dummy load resistors (along with many other tools I think I will be needing and wanting) but wondered if anyone off the top of their head knew where/which to buy.  I've looked on Mouser's and DigiKey site but wanted to make sure I was getting the right part (being a nubee).  I know to get a power resistor but would I be better off just buying a 8 ohm 100W instead of a variable tap one?

[Geo]

I know to get a power resistor but would I be better off just buying a 8 ohm 100W instead of a variable tap one?

That would work. 100W might be overkill, I don't know. If you already have a speaker, I would just use that, unless you anticipate testing the amp while it's turned up or making weird noise and you don't want to hear it.

[HotBluePlates]

If you truly need an 8 ohm 100w resistive load, I'd suggest having a total power handling ability of more like 200w. You could use 2 16 ohm 100w aluminum-housed wirewounds in parallel. The aluminum-housed resistors should be bolted to a metal box, as they are rated assuming that they will be bolted to a chassis, which then aids in heatsinking. You might then have some wood supports or feet at the ends of this "chassis", because the metal underneath those resistors will be HOT, HOT, HOT!

I have seen a box made up to be a resistive load before that also had intake and exhaust fans to help keep the resistors cool while they were enclosed in the box. But that's an excessive bit of complication for you. Supposedly, you will be using the resistive load with a signal generator and a scope to make some tests on the amp, either looking at waveforms, or to make a careful power output test, which would then resuire you to have acess to the hot and ground terminals of the resistors.

[PRR]

For start-up testing of guitar amps: 10 ohms and plenty of watts.

There is no need to get a 8.00 ohm resistor to load an amp.

If you are going to do Power Testing, yes, you should get near the rated resistance so your numbers are "proper". This is more true on transistor amps than on tube amps.

FWIW, I used to do a LOT of testing and measuring with a 7.5 ohm 44 Watt resistor. I even used it for strain-testing 180 Watt amps by dropping it in a bucket of water. Added water every day as it steamed off. (When I strain-test an amp, I strain the sucker.) The resistor failed after only 20 or 25 years.

My current victim is 10W or 15W, and I'm using Radio Shed's 8 ohm 20 Watt 2-buck resistor. My gross-overload condition on this amp gives ~24 Watts of distorted heat, so I'm just testing for long enuff to see what comes out, not a marathon test.

I do not see any point in getting a variable resistor. If you "set" a 100 ohm 100W resistor to 8 ohms, only 8/100 of the resistor gets all the heat, and it is really an 8 or 10 watt resistor.

[birt]

when i was working on a trace elliot V8 (8 kt88 tubes) last week i built me a dummy load out of a pack of 10 watt resistors i had. i put it in an enclosure of a pc power supply that has a fan. it can handle more than 600watts (i want to use it for PA amps too) :p

[jjasilli]

PRR:  I'm using Radio Shed's 8 ohm 20 Watt 2-buck resistor

Me too.  I put it in a box that already had speaker hook-ups on it.  Sometimes it's more convenient for safety to the amp, than hooking up to a speaker.  

If you really want to go all out, check out this:

http://www.aikenamps.com/DummyLoad.pdf

[Sbluestubes]

https://amptechtools.powweb.com/truload.htm
200 watt resistors $15.00 each

[slidincharlie]

If you truly need an 8 ohm 100w resistive load, I'd suggest having a total power handling ability of more like 200w. You could use 2 16 ohm 100w aluminum-housed wirewounds in parallel.

Does the watt rating of resistors add  when they are wired in parallel? I bought some big resistors on purpose a while back but I haven't build the dummy load yet.

The aluminum-housed resistors should be bolted to a metal box, as they are rated assuming that they will be bolted to a chassis, which then aids in heatsinking. You might then have some wood supports or feet at the ends of this "chassis", because the metal underneath those resistors will be HOT, HOT, HOT!

Is there any risk to burn the wooden base?  :-/

[hydroboy40]

Here is a link to another dummy load project - http://www.terrydownsmusic.com/technotes/dummyload/dummy_load.htm
The only problem was the resistors used in it were quoted to me at a very expensive price. anyone know if these  resistors are available for affordable prices?   [smiley=cussing.gif]

[birt]

If you truly need an 8 ohm 100w resistive load, I'd suggest having a total power handling ability of more like 200w. You could use 2 16 ohm 100w aluminum-housed wirewounds in parallel.

Does the watt rating of resistors add  when they are wired in parallel? I bought some big resistors on purpose a while back but I haven't build the dummy load yet.

yes

The aluminum-housed resistors should be bolted to a metal box, as they are rated assuming that they will be bolted to a chassis, which then aids in heatsinking. You might then have some wood supports or feet at the ends of this "chassis", because the metal underneath those resistors will be HOT, HOT, HOT!

Is there any risk to burn the wooden base?  :-/

i don't really think so

[HotBluePlates]

The only problem was the resistors used in it were quoted to me at a very expensive price. anyone know if these  resistors are available for affordable prices?

How much is "very expensive"? Did you click the part name in the page you linked to? It opens Newark's page with these resistors, and has them for under $3. Mouser sells them for $5 and some change.

And these are 50w resistors after all... not tiny 1/2-watters.

[sluckey]

Here's a dummyload I built a couple years ago for less than $10. The two 8[ch937]/50W resistors are mounted on a scrounged heatsink. They are wired to a terminal block such that using a single jumper I can have a pair of 8[ch937]/50W loads (good for stereo checks), or one 16[ch937]/100W, or one 4[ch937]/100W load. The 6 foot long black cord terminates in a 1/4" phone plug. I've used it for power testing on all my amps. I've even used it on my Twin Reverb. It does get hot. I really recommend using a heatsink with these resistors if you're gonna pump much current thru them.

http://home.comcast.net/~seluckey/dummyload.jpg

[slidincharlie]

Wow, looks nice and professional!
I don't know if I will find such a heat sinker in the electronics shop though...

I have a few 8 ohm and 4 ohm 50W ceramic resistors that I bought on purpose from Ted Weber a while ago. Do you think that ceramic resistors are less sturdy (read: prone to burn) than metal housing resistors?

--Carlo

[hydroboy40]

The Dale resistors were qouted by allcomponent.com and although i dumped the email response from them(because of the cost) I recall they were asking around $18 each

[sluckey]

Do you think that ceramic resistors are less sturdy (read: prone to burn) than metal housing resistors?

No. They are meant to be air cooled so allow plenty of air space around each resistor. Resist the tempttation to stack'em up like bricks. I'd give at least 1" clearance on all 6 sides for air flow.

I almost went with some old style hollow tube ceramics until I saw how cheap the Dales were at Mouser.

[PRR]

> Do you think that ceramic resistors are less sturdy (read: prone to burn) than metal housing resistors?

"Burn" is just about how much power you pack in how little space.

One thing about the cute 50W metal-case jobs: they MUST be clamped to a LARGE heatsink. Otherwise 5W or 10W will kill them. Most dumb old ceramics are Free Air Cooling: they don't need heatsink.

In much guitarwork, big dumb cheap ceramic makes more sense than futzing with heatsinks.

Look in any Egyptian pyramid. The ceramic jars are in good shape, the non-Gold metal objects are not doing so well. How long do you need your dummy-load to last? My $0.50 surplus ceramic 44W lasted decades. If I'd mounted it well it would still be working.

> Is there any risk to burn the wooden base?

Sure. Use apple-wood, smells nice when it chars.

To keep costs down, most power resistor ratings allow the body to run HOT. -Usually- not hot enough to actually burn wood or phenolic or paper instantly. But if you have 100W in a "100W" resistor, and wood/paper might fall on it, don't walk away.

If you have 50W in a "100W" part, it will be only half-hot. It will loosen (blister) your skin but won't actually leave char marks, and probably won't char paper.

10 or 12 of Hoffman's 100 ohm 10W resistors in parallel on copper nails on a board ought to load-up most tube geetar amps, unless you are into Big Gear.

You can also take a mile of iron fence wire, rig it zig-zag on nails on a board. Use ceramic insulators, keep it dry and not red hot, such resistors last over a century, as seen in 1880s electric systems.

[slidincharlie]

Thank you PRR.
I have a few ceramic 50 watters, that I don't how to secure to a base (wooden or heatsink).
Advise is welcome. I attach a photo here.
Should I use the rectangular holes for a screw and the "fork" terminals for soldering, or the other way round?
--Carlo

[SoundmasterG]

I have lots of 50w, 100w and 200w power resistors at home....10 ohms, 5 ohms, etc. I made myself a load box with eight 16 ohm 200 watt resistors and some switches several years back. I can test pretty much anything with it so don't much need all these other resistors. I'd sell you a couple of the 10 ohm ones or whatever else I've got if you want some? If so, send me a PM. These are the large ones that many people say look like sex toys btw.

Greg

[slidincharlie]

As per birt's post above, the watt rating of resistors adds when they are linked in parallel, so the total watt rating increases.
What happens when they are in series? Does the total watt rating decrease or does it remain the same?

[birt]

the power rating also adds when you are useing them in series.


but i can imagin that if you put a 4ohm 100W in series with a 4ohm 10W and put 100W of power in there the smallest resistor will take half the power... and probably this will happen if you add them in parallel too.

[slidincharlie]

You're probably right... Anyone?
Anyway I'm thinking of series or parallel resistors with the same watt rating.
--Carlo

[RickRS]

You're probably right... Anyone?
Anyway I'm thinking of series or parallel resistors with the same watt rating.
--Carlo

birt's right.  If you use the formula for power as Power = Voltage x Current (P = E x I) and rewrite it to show resistance and current, it becomes P = I² x R.  So for that series example, the same current flows thro both resistors, therefore P is dependent on R.  

With both resistors being equal values, they will both have the same heat dissipated in them.  For birt's example of 100 watts input to our assembly of two 4 ohm series resistors, they have 50 watts dissipation in each.  And the 10 watt rated resistor is overheating big time.

If we use a parallel pair of equal ohmage values, we get the same results.  

Assume two 16 ohms resistors in parallel, one rated 100 watts, other 10 watts.

The Power formula can be rewritten again to some voltage and resistance as P = E² / R.  Now in a parallel configuration, the voltage will be equal on both resistors.  With both having the same R and the same E, Power is the same for both as well.  So, again, we have equal power dissipation in both.  Without doing the math, that means 100 watts in to our 2 resistor assembly results in 50 watts in each resistor.  And once again our 10 watt resistor is seriously overheated.

[RickRS]

You're probably right... Anyone?
Anyway I'm thinking of series or parallel resistors with the same watt rating.
--Carlo

So lets see what we can do.  I going to assume you have those 50 watt ceramics in 8 ohms and 16 ohms.

If you have two 16 ohms in parallel, your total wattage rating in 100 watts and resistance is 8 ohms.

If you take two 8 ohms in series, that is 16 ohms total.  (Ohm's Law: series resistance adds)  Take that string of two series 8 ohm and parallel it with another string on two 8 ohms in series.  So you have four equal resistors and the total resistance sums to 8 ohms.  With this configuration, power is distributed equally to all four resistors and you have a total power rating of 200 watts.

Tell me what ohmage and wattage resistors you have, how many of each, and I'm work you up a load assembly.

Now, a word of advice:  Power rating for load resistors assume you can supply the necessary cooling in the form of air flow and heatsinking.  The standard design approach is to derate the wattage rating by half so the resistor is running at a lower temperature and will last for a lot longer time.  So ideally, you would take that 4 resistor assembly in the example above, use it for no more than 100 watts, use a fan for cooling and it will last forever.

[capnjuan]

8 ohm / 200W dummy load; aluminum housing, sand-filled.  Puts end to multiple loads for different outputs - not cheap. John
http://www.amazon.com/200W-Non-Inductive-Dummy-Load-Resistor/dp/B0002KRE3E

[DummyLoad]

i use up to four of the 4 ohm in the link below...


clicky linky

[ITSALLINTHEWOOD]

I also bought some 100W ceramics resistors to do a project like this.
So...per RickRS, if I have one 3 OHM and one 1 OHM, both of which
are 100W, and I wire them in series, I end up with 4 OHMS at 200w ?
Is that correct ? And in the same configuration to make an 8 OHM
unit (3/3/1/1), I would end up with 400W ?  [smiley=rock_band.gif]

[PRR]

> if I have one 3 OHM and one 1 OHM, both of which are 100W, and I wire them in series, I end up with 4 OHMS at 200w ?

No. 4 ohms 133 watts.

But take your assumption and work backwards. 200 Watts in 4 ohms is, ummm, screw square-root formulas. 24 volts in 4 ohms is 6 Amps. 24 Volts times 6 Amps is 144 Watts, too low. 32 Volts in 4 ohms is 8 Amps, 32V*8A is 256 Watts, too high. 28V in 4 ohms is 7 Amps, 28V*7A= 196 Watts, close enough.

When you put resistors in series, the voltage divides by the resistor ratio. If you put 4V across 3+1 ohms, you find 3V across the 3 ohm and 1V across the 1 ohm.

So if we put 28V across 3+1 ohms we will get 21V across the 3 ohms and 7V across the 1 ohm. (We could instead have noted that 7 Amps times 3 Ohms is 21 Volts.)

And 21V in 3 ohms or 7 Amps gives 21V*7A= 147 Watts in the 3 ohm resistor. Which is significantly in excess of the 100W rating. The fact that the 1 ohm is loafing at 47 Watts, half of its rating, will not save the 3 Ohm working way over its rating.

And when the 3 Ohm is working AT 100 Watts, the 1 Ohm will take 1/3rd of that power, 33 Watts. The maximum safe value for the combination is 133.33 Watts.

There are very few tube amps which can cook a 133 Watt resistor.

Most tube amps also have a 8 ohm tap. Your 3+3+1+1 or 8 Ohm array can take 266.7 Watts. If you have a tube amp which'll cook a 250 Watt resistor, post pictures!

If you are doing solid-state: it is nice to find 16 ohm resistors in large Watts and at low prices. You parallel two for 8 and four for 16. A typical transistor amp may be rated 150W in 8, 100W in 16, 250 Watts in 4. If it had zero power supply sag, it would be 100W in 16, 200W in 8, 400W in 4; but sag happens. If you can find 16 ohms which can eat the 16-ohm power of the amplifier, two or four in parallel will be able to eat the 8 and 4 ohm powers.

[ITSALLINTHEWOOD]

Thanks for all the info. I understand pretty much all of it, although the
volt part is a little confusing. But I'm still learning. My highest watt amp
is a silverface Super Six at 100W, 4 OHMS. I have four other silverface, two at 4 OHMS, both 70W and two at 8 ohms, one at 45W
and one at 35W. And other stuff........way too much stuff. I need to play more. Thanks again. [smiley=dice.gif]