Building a Dummy Load

[garytoosweet]

Planning on building a water heater element dummy load in a 5 gallon tank filled with mineral oil. Inspired by https://www.prosoundtraining.com/201...ge-amplifiers/

I want to have it switchable between 2-4-8-16 ohms and have adequate power handling that I shouldn't need to worry about it ever, from guitar to high power solid state PA amps.

I've sourced 5 elements, 4x 120V 1650W (8.727 ohms) and 1x 120V 2000W (7.2 ohms).

The best I could come up with is 4 toggle switches (4SPST, 1DPST) which will need to be set in various configurations to give me the setting I'm looking for.

This switching system seems a bit crude to me. This is my first time trying to come up with any type of circuit on my own, also first time ever drawing a schematic.

I will also parallel a Scope out and maybe a set of banana jacks with the amplifier in jack.

Any input on a possible better way to wire this up? Anything I'm missing that I should include? Would there be a way to get 2 of the 8R elements in series so I could eliminate the 5th 7.2R?

EDIT: Update with revised schematic.

[Jon Snell]

If you place a 24volt 3Watt lorry vehicle bulb in series with the amplifier in and the loud speaker (J2 - T and Sw-1 - 1), you will be able to monitor at a maximum level of 3Watts at roughly whatever your amplifier delivers. When the signal increases the light bulb will act as a PTV and light up instead of overloading your test speaker.
I use a 4Ohm and an 8Ohm fixed resistive load, secured to an aluminium foot plate 1/2" thick and 18" square as a heat sink, as amplifiers usually work into reactive loads and seemed little point in selecting various load levels.
SW-4 doesn't appear to do anything.
Odd component choices, I can't think where you will get those made.

[garytoosweet]

Right SW4 is meant to connect R5 in series with R4 but it is wired incorrectly. I will revise.

Thank you for the tip on the bulb in series, I'll look into that. That would be useful.

The components are off the shelf pieces selected due to economy. There are 120V 1800W elements which would be a nice even 8ohm load but they are around $100CAD each, the 8.727's are $10CAD/piece where I am.

[Helmholtz]

Just a side remark as you're building a kW load. If you expect load currents of 10A and more do avoid phone jacks. At 10A even a small contact resistance of e.g. 0.1R will be heated by 10W. Guess what that does with the contact points.

[Enzo]

In my shop, I used to fix everything. PA gear, instrument amps, FX, lighting, keyboards. You name it. I was a QSC warranty station among others, and was ready to repair 3000 watt PA amps. But I will tell ya, I fixed a hell of a lot more guitar amps than I ever did 3000w QSCs. Or Crowns or Mackies or even Behringers. SO it is only my opinion, but coming up with a huge high power dummy and using it for everything seems to me like buying a huge dump truck and driving it around town when a pickup truck would cover 99% of my needs.

And if I want my 3000w PA amp to cook for a while, I still likely need a dummy to work on that stack of guitar amps.

If you use vegetable oil instead of mineral oil, you can cook your Thanksgiving turkey in it.

I know I don't get a vote, but if I were doing this, I'd make the thing with high power needs in mind, put it on wheels like a shop-vac, and roll it in a corner, to bring out when you need 3000w capability. Then make a second dummy load for your bench. For lower power loads, 50 watt aluminum resistors are like $5 or less and can be series/parallel for any load you want. For of them is 200 watts.

[Chuck H]

I admit to not checking the link, but...

Why mineral spirits? Is there some temperature coefficient that's supposed to keep the ohmage in spec by using this medium? These are heated elements and mineral spirits are FLAMMABLE!!! Why not water? These are, after all, WATER HEATER ELEMENTS!!! Why not a water bath?

[Enzo]

Only reason I can think of is oil won't likely boil. but if water leaks, it is easily cleaned up.

[The Dude]

From the article:

A Cooling Bath


The loads would driven very hard for very long, which makes them get very hot. The elements needed a fluid bath to sink the heat. Water is cheap and plentiful, but it evaporates and would cause rust in a steel tank. Motor oil can withstand high temperatures, but it burns if it gets hot enough, as evidenced by the smoke that is produced by my lawn mower each weekend. Mineral oil is non-flammable and eco-friendly, but much more expensive than the other choices. Looking at the big picture, I decided on the mineral oil. I only want to do this once. The typical pharmacy size is 16 oz., and I would need 20 gallons. The Internet can be our friend.

[Enzo]

I sometimes watch that show where they compete to make knives from raw steel. Forge SOmething. ANyway, they get the metal red hot, and in part of the process, they quench the hot steel in a drum of oil. What sort of oil is that?

Forged in Fire. is the show. on History Channel.

[Jon Snell]

Right SW4 is meant to connect R5 in series with R4 but it is wired incorrectly. I will revise.

Thank you for the tip on the bulb in series, I'll look into that. That would be useful.

The components are off the shelf pieces selected due to economy. There are 120V 1800W elements which would be a nice even 8ohm load but they are around $100CAD each, the 8.727's are $10CAD/piece where I am.

If using tungsten electric fire elements, as they heat up, the resistance increases. Not a lot of use.
My dummy load has four 4 Ohm 50W plate mounted ceramic coated wire wound resistors. Two in series for 8 Ohm load.
Used to have Neutrik jack connectors but they melted and got upgraded to SpeakOn connectors some years ago and has a 24v 3W bulb in series with a pair of Apple Mac 2" long throw loudspeakers, rescued from a scrap Note Book many years ago.
It is now 40 years old and never let me down. Even the bulbs only stopped working only as I dropped a Fender chassis on it and they broke!
Even a 10W 8 Ohm WW resistor will stand 100Watts for a minute or so if mounted on a heat sink.

[Chuck H]

From the article:

A Cooling Bath


The loads would driven very hard for very long, which makes them get very hot. The elements needed a fluid bath to sink the heat. Water is cheap and plentiful, but it evaporates and would cause rust in a steel tank. Motor oil can withstand high temperatures, but it burns if it gets hot enough, as evidenced by the smoke that is produced by my lawn mower each weekend. Mineral oil is non-flammable and eco-friendly, but much more expensive than the other choices. Looking at the big picture, I decided on the mineral oil. I only want to do this once. The typical pharmacy size is 16 oz., and I would need 20 gallons. The Internet can be our friend.

Ah! thank you for the clarification. I have no trouble understanding the mineral oil alternative. The opening post (having not read the actual link at that time) stated "mineral spirits" and my warning light started flashing. Mineral spirits is a highly flammable aromatic solvent.

So I'll just state here, to be vigilant and for my own peace of mind, Never fill containers with paint thinner and put heater elements into it.

[garytoosweet]

Thanks for all input. I do have a 'daily driver' load of 4x 50W 16ohm Arcol resistors that I've been connecting with alligator clips for the past year. Finally mounted them to a 1/2" aluminum heat sink and put them in an enclosure with a fan and small monitor speaker as Jon Snell suggested, switchable between 8 and 4 ohms with binding posts and BNC. Much more useful and my alligator test leads will be happy they no longer need to keep melting.

I've found that in reality the 120V 1650W elements measured closer to 9ohms, while the 120V 2000W elements measured at 7.9ohms. I've revised the switching so this will be switchable between 2-4-8-16 with only 4 elements. Plan on having the switches and inputs in a small 'control' enclosure on the bench, the 5 gallon oil filled reservoir will sit under the bench with a short 13/8 speak-on cable connecting them. I'll post a photo here when it's all done.