Homebrew high-power resistor(s) ??

[Cliff Matthews]

I saw a cheap DC load (old MOSFET's??) in an AvE video recently (in screen cap below) and had a good laugh, but are there any merits to this, or is that just his unique style?

[RoGeorge]

The best ever! (starts from minute 2:00)

[tron9000]

That's genius!

[Cliff Matthews]

It looked like nothing more than adding RDS drop's and clipping on to whichever one you need (prior to dunking in water).

[T3sl4co1l]

Those are thick film power resistors in a TO-220 body.

Note that they are very small, and can ONLY dissipate rated power with an attached heatsink.  Quite a good heatsink is needed, at that!

AvE is also fond of simply stringing a bit of wire underwater (which if I'm not mistaken, is also shown in the same video?).  This has the downside of not being insulated from the water, so chemical reactions occur, corroding the terminals.

Those metal-body resistors also need heatsinking.  They are constructed exactly the same as the TO-220 kind: a metal resistor element, glued onto a metal heatsink connection.  Without heatsink, they are only good for 20-50% of full ratings.

The cheapest, most reliable resistor you can get, is still the ceramic body kind.  They will gladly operate up to red hot (>400C), though they won't last long under such abuse.  Max service temp. is 100-150C higher than anything that contains plastic (like the above resistor types), so they can dissipate much more heat in the same size -- you don't need to use them with a heatsink.  And they're priced nearly the same, so there's no reason not to use them!

If you find you need an oddball, low-ohm resistor, say for testing a high-current power supply: the best solution is to buy some stainless steel or nichrome strip, and string it up between bolted connections.  Stainless steel makes an okay resistor material, and can glow red hot without getting more than tarnished.  Plain steel makes a poor resistor because its resistance increases dramatically as it heats up.  (Nichrome is best, having almost the same resistance between room temperature and red heat.  Stainless is so-so, but a lot better than plain steel.)  Definitely avoid galvanized steel, which will release zinc fumes if it gets orange-hot!

Tim

[Mephitus]

RE: T3sl4co1l

I had some ceramic body resistors literally blow up on me the other month when I tried to use them for ballasting a transformer. It wasn't even that big of a transformer either. Maybe only 1.2K/VA rating. Ideas?

[T3sl4co1l]

They're robust, but not indestructible... thermal shock is probably the fastest way to kill them.

Okay, so 1.2kVA (read: thousands of volt-amps, not thousands per volt-amp!) transformer, but how big were the resistors?  Not 1200W I take it..?

Tim

[Mephitus]

To be more specific, it was a smallish MOT that (without proper tools to measure) for its size seems to be about 1k-1.2k VA rating give or take. The resistors where both 10? 20W. I was trying to measure the output voltage after a rewind. Even when ballasting with a 100W lightbulb I got some really bizarre testing results. I have since completely rebuilt the entire transformer as I found it had a good amount of oxidation between the laminations. I am waiting on a shipment of 18AWG wire to do a proper rewinding. tl;dr did the math and found that for full coupling it needs about 700-750.

[james_s]

Modern microwave oven transformers are cost engineered down to the last penny. The cores are no larger than they need to be and with little or no load on the secondary they can start to saturate, that will result in huge current peaks.

[Vtile]

Well as in Finland we have that sauna thing, which at the modern days heats with electric heater called kiuas.. 6kW 3-phase aren't too rare. Get one element and you have 26.45 Ohm resistor  rated to 2000kW/230Vrms (The figures are different obviously if Delta connection is used and element is rated to that.). One older EE told that it were pretty typical solution for dummy load in the older days, when electricity were still safe to handle for professionals.

 So take that generic electric 2000kW radiator and use it as resistor.. Why not?   

ps. 1kVA is more than 1000 W when powerfactor is something else (in case of transformer it is) than 1.

The ones who use these should remember the hazards of not only electricity itself, but also fire (including the cables between load and source) and if water is included then you have potentially both electric shock (ie. container tips etc.) and fire hazard in hand if not properly done and caution taken to the setup and work habbits and work environment.

Also to be noted that if you use such devices as dummy load, it is then a dummy load and not suitable to be used in original function as some household etc. device or repairing a such device. This is because of fire hazard and/or risk of electric shock because of deteriorated safety structures.

[Seekonk]

Assume you mean 2KW @230V.  I use a 2KW 120V water heater for my solar dump load.  That gives me all my hot water at the camp.

[Vtile]

Assume you mean 2KW @230V.  I use a 2KW 120V water heater for my solar dump load.  That gives me all my hot water at the camp.

Yes, you are correct. I should have written the nominal voltage 230 Vrms. Also to be noted that the ohm rate will be different if the power rating is calculated from delta or star connection (ie. 400Vrms vs. 230Vrms).

[David Hess]

I have had conformal silicon coated wire wound power resistors operate red hot without failing but for cheap dummy loads, I go with a fan blowing on an array of wire wound cement power resistors or nichrome wire.  Using enameled wire inside of a tub of water is a great idea though; I will have to try that.

[edpalmer42]

Depending on the value you need, electric stove elements make great loads.  I've got one that's rated for 2350W@240V (25R) and two that are rated 1325W@240V (43R).  Paralleling them gets me to 11R.  Overloading these is very unlikely!

[David Hess]

Depending on the value you need, electric stove elements make great loads.  I've got one that's rated for 2350W@240V (25R) and two that are rated 1325W@240V (43R).  Paralleling them gets me to 11R.  Overloading these is very unlikely!

Water heater elements work also and depending on your needs, come with an attached water tank.

[edpalmer42]

Depending on the value you need, electric stove elements make great loads.  I've got one that's rated for 2350W@240V (25R) and two that are rated 1325W@240V (43R).  Paralleling them gets me to 11R.  Overloading these is very unlikely!

Water heater elements work also and depending on your needs, come with an attached water tank.

True, but they're not high on the portability scale. 

[james_s]

I have a few water heater elements I got for a couple dollars each from a surplus place. They can dissipate a fair amount of power in free air, and when more is required I usually dunk it in a bucket of water.

[Jeroen3]

Depending on the value you need, electric stove elements make great loads.  I've got one that's rated for 2350W@240V (25R) and two that are rated 1325W@240V (43R).  Paralleling them gets me to 11R.  Overloading these is very unlikely!

Or electric outdoor heaters.

[Vtile]

The ones who use these should remember the hazards of not only electricity itself, but also fire (including the cables between load and source) and if water is included then you have potentially both electric shock (ie. container tips etc.) and fire hazard in hand if not properly done and caution taken to the setup and work habbits and work environment.

Also to be noted that if you use such devices as dummy load, it is then a dummy load and not suitable to be used in original function as some household etc. device or repairing a such device. This is because of fire hazard and/or risk of electric shock because of deteriorated safety structures.

[T3sl4co1l]

True, but they're not high on the portability scale. 

Where do they sit on the potability scale?    

Well, with the hard water around here, both potability and scale are high!

Tim

[SeanB]

I used a few spare TO3 transistors as dummy loads a few times. Old ceramic mug, fill with water, drop in device with wires soldered to the leads and a screw and nut to the case for collector, then use a resistance decade box to set the current desired. Would work fine and stable once you got it to boiling and would run till the water boiled off. Just used an ammeter in the one lead as a measuring device, and worked well. Of course you would occasionally leave it too long and have a red hot transistor and an audio warning ( sizzle) of low water level.

[james_s]

I think it would take some doing to deteriorate the safety structures of a Calrod type heating element. They're constructed similarly to pyro cable, there's not much you could do to hurt one using it as a dummy load aside from burning it out.

[T3sl4co1l]

The main downside to any kind of heating element is, the internal (resistance wire) element is insulated by some sort of media (I think usually powdered magnesium oxide?), and if you put too much power into that, the wire simply fuses open (or shorts to the enclosure).  The heat flux also depends on the water temperature and flow rate (critical heat flux before voiding), and surface condition (scale buildup?).

You definitely get way more power density with a water cooled heatsink, than anything else.

Tim

[Vtile]

Above is not 100% true, while might be so in typical electronics level of energy applications. With mains and above free air constantan resistors are not exotic at all. Even many toasters do have bare constantan (or similar) resistors in them wrapped around mica or similar high heat insulator.

What comes to water as heat absorber, it is a fact. It happens to be one of the do it all wonder substances in the engineering universe.

Those toasters would be actually rather interesting source of DIY high power resistor materials, even the insulator should be safe since it is on device in contact with food (well this doesn't assure anything of these days of made in PRC floating around in every corner and really old devices propably do have asbestos in them).

Still one of the best and most safe source (if you do your math and not drive them to explode) of highpower resistors are light bulbs as long as you can get them. Ie. vehicle headlight bulbs might be rated as 45W or more. If nominal voltage goes down and nominal watts stays same the resistance drops according the ohms law. Best of it is that they are self stabilizating devices so parallel connection is easy as long as the bulbs are same type. They also can withstand high overvoltages in short amounts of time (fraction of a second). Ie. https://www.amazon.de/Carpoint-0730016-Philips-12620B1-Abblendlampe/dp/B003NE1QJO