My diy dummy load...think it'll work?

[Steverino]

Hi fellas

I threw this rig together to use as a dummy load to test amps in the <50A ranges. It's a hodgepodge of parts. The resistors are an old bank of 50A (I think) Clarostat power resistors, an old cooling fan, a hobby box from RatShack and a wall wart. I even threw a pot in there for fan speed and a lighted toggle for it.

I haven't put any power to it yet, and am asking you guys what you think here. The resistors are in series, each is 2 ohms (except one on the top, it's unused and is 1 ohm). I have installed a switch to select 4 or 8 ohms, so the 4 ohm selection should be 100W and the 8 ohm 200W, right?

I'd appreciate any input, do you think it'll do the job? Thanks!

[drewl]

Wow! that's pretty....I just have a 4ohm resistor mounted in a metal box for testing!

[Ptron]

Pretty nifty. I think for the fan to be effective you're gonna need some more holes in that box. The Idea is to draw or push air across the resistors. The way it is there's nowhere for the fan to push air through (or pull it from, depending on which way it is facing). With the fan I would think the resistors could handle a good deal more power than their nominal rating.

Never mind. I just noticed that the box is on legs and you have holes on the bottom.

[TD_Madden]

if that box is plastic....dunno about longevity.

[Steverino]

if that box is plastic....dunno about longevity.

The box is plastic. The resistor decade is resting off the bottom by it's own lugs, about 1/4". We'll see.

But it should take 50W or so, you think?

Thanks for the replies, folks!

[Steverino]

Never mind. I just noticed that the box is on legs and you have holes on the bottom.

Yeah, sorry about that, good eyes though! I should have taken one more of the bottom, but it's not too pretty.

[Mars Amp Repair]

Your calculations are correct on the wattages & resistance...I only have concerns about the mounting screws for the rack of resistors...Seems like those points will melt through the plastic box if they get hot enough.

Agreed, it is very nice job.

I use 4 of the finned gold 50watt 8ohm resistors (from Mouser) that I mounted to a an old PV heatsink. One load is for 4ohms & the other for 8ohms...both 200watt capacity.

Pretty cheap to make & work well & the heatsink dissipates the heat well. I've found that for short periods of time they work well even driven in excess of 400watts for peak power checks.
Nice work...g

[bob p]

i'm surprised nobody's mentioned using one of these:

[Steverino]

[QUOTE=bob p;13658]i'm surprised nobody's mentioned using one of these:

*slaps forehead*

[drewl]

I'm guessing that's a heater?

We have a load box for testing high current DC telco power supplies at work which is basically a bunch of heater coils.....

[Steverino]

I'm guessing that's a heater?

Yep, now I've got two.

[bob p]

I'm guessing that's a heater?

Actually, its a vintage space heater -- A.K.A. a cooling unit for heated nichrome resistance wire.

1000 Watts / 120VAC = 8.3 ohms.

Just rewire the coil to act as a dummy load and use the AC line to power the fan.

[Mars Amp Repair]

Actually, its a vintage space heater -- A.K.A. a cooling unit for heated nichrome resistance wire.

1000 Watts / 120VAC = 8.3 ohms.

Just rewire the coil to act as a dummy load and use the AC line to power the fan.

This may sound a bit silly , but I always thought the resistance of heater coils like that change resistance as they start to luminesce...kinda like a light bulb...perhaps the change is not enough to make a difference. Am I wrongue on this one Sounds like a great idea...I'll have to find one of those...glen

[bob p]

This may sound a bit silly , but I always thought the resistance of heater coils like that change resistance as they start to luminesce...kinda like a light bulb...perhaps the change is not enough to make a difference. Am I wrongue on this one Sounds like a great idea...I'll have to find one of those...glen

Well, yeah, but...

If you look at your garden variety Ohmite vitreous enamel power resitor, you'll see a coiled stretch of nichrome resistance wire that's covered in vitreous enamel to make it flame retardant.

Both the space heater and the vitreous enamel power resistor are both using nichrome wire as the resistance element. The biggest differences are: a) how the wire is made fire-safe: vitreous enamel on one hand, ventilated enclosure on the other, and b) what subtype of wire (resistance per foot) is used in the coil.

Yes, the space-heater resistance wires are designed to operate red-hot because they're in a forced ventilation enclosure. If you've ever seen a vitreous enamel power resistor abused way beyond its specifications, they'll glow red-hot just like a space heater's heating coils (though that's not a fire-safe operating mode for an electronic circuit component). To be honest, I haven't ever done this myself, so I can't answer the linearity question. Still I doubt that any of us are going to pump 1000 Watts of power into the 8-ohm space heater element (enough to make it operate non-linearly).

[Ptron]

I don't know the answer to this myself, but I think his point is that the space heater wire may not have the "advertised" resistance until it heats up. IIRC, light bulbs have many times the resistance when hot than they have when cold.