View Full Version : load resistor

08-24-2004, 03:10 PM
Am almost finished building a load resistor bank for testing high power amps. I'm using stainless steel wire for the resistance element (could have used nichrome, but at much greater cost). The bank of resistors drops into an aluminum box which is finned. I intend to fill the box with fluid of some kind to conduct the heat from the wire to the box, and am thinking of using mineral oil. I have more than these metals involved, another being the copper leads from the resistor elements, and others from the silver solder. Am I ok to use mineral oil, or is there a better fluid to use? ATF, or bar oil? I want to avoid any water based solution because of the possible conductivity.

By the way, in case anyone is interested, stainless wire has 42 times the resistance of copper, and about two thirds the resistance of nichrome. There will be less heat per surface area using the stainless than with nichrome. I used more wire, but get greater heat dissipation with the stainless. I'm sure there are drawbacks, among them being the change in resistance with temperature increase, but it won't amount to more than 10 % in this case, acceptable and compensatable. These resistors are non-inductively wound, and in use I am expecting a temperature rise on the box itself to about 150 F. It will be fan cooled if continued high power operation is needed, and I can easily monitor it's temperature using an immersed diode and a simple meter circuit.

Mike W
08-24-2004, 03:17 PM
I think the old Heathkit dummy load (resister in a one gallon paint can) used mineral oil. Maybe you could borrow some oil from the power company? http://bbs.homeshopmachinist.net//biggrin.gif

08-24-2004, 04:06 PM
In the 1988 ARRL Handbook (American Radio Relay League), (page 34-22), there is a few paragraphs on suitable coolants for Dummy Loads.

They show construction for a 1500 watt Dummy RF Load and decided upon a product sold by Texaco distributers as Regal Oil R&O No. 46, and is known as "Turbine Oil".

Better Coolants are some of the "Transformer Oils", but, are typically not available in small quantities.
(Find where Mike lives and borrow some from his power transformer)

Tom M.

[This message has been edited by mayfieldtm (edited 08-24-2004).]

[This message has been edited by mayfieldtm (edited 08-24-2004).]

08-24-2004, 04:11 PM
Esso make an oil called "VoltEsso" for that type of application. Sold in 20L pails. Can't remember the price tho.

08-24-2004, 04:21 PM
Just out of curiosity, where can you purchase mineral oil?

08-24-2004, 04:25 PM
Use sunflower oil or canola oil. The industry is moving to vegetable oils for transformers as it poses no environmental risk.

"It appears as though the lifetime of transformers can be significantly enhanced if you use a vegetable-based transformer oil relative to a mineral-based transformer oil..."


[This message has been edited by Evan (edited 08-24-2004).]

08-24-2004, 04:44 PM
I'm curious, is there any fire hazard associated with these oils?


08-24-2004, 04:56 PM
Vegetable oil poses a much lower fire hazard than mineral oils.

Forrest Addy
08-24-2004, 04:58 PM
Since it's a load resistor for audio amps (am I right?) the load voltages will be relatively low (less than 40 volts. I suggest distilled water as a heat aink. Practically infinite resistance and has a heat absorption beyond peer. You benefit from having about 10 times the heat absorption per degree change in the heat sink fluid and your correction for resistance change with temperature will be greatly reduced too.

If all your immersed parts are stainless I wouldn't think there'd be any problem with electrolytic corrosion at all. Just be sure the two load conductors enter on opposite sides of the load bank.

J Tiers
08-24-2004, 04:59 PM
In inverse order, sort-of.

Yes there is a fire hazard, of course. The idea is not to let them be hot AND in contact with oxygen at the same time. And not to let them vaporize.

Vegetable oils are OK, except many/most will polymerize with heat (as per seasoning a frying pan) if any oxygen is present. The mineral oils will in general not polymerize without help. Heat usually breaks them down (cracks them) into shorter chain hydrocarbons. But in a closed can, might be fine. You could purge with CO2 or nitrogen.

As far as the change in resistance, it is significant, and will affect any power measurements you do. A 10% change in resistance will change your power by a similar amount, assuming constant voltage output.

If you can keep the wire at one temp, obviously there is no change. This presumably will be assisted a lot by liquid cooling. "Real" resistors use alloy elements with a very low tempco of resistance.

Ther are better temperature monitors than a diode. Below 100C, the LM35 is pretty decent. You can set it up to read out temp directly on a DVM, on any scale. We have used them for years.

08-24-2004, 05:11 PM
At 150° F there is no worry of polymerization. If using vegetable oil it would be a good idea to add some tocepherol (vitamin E oil) as an antioxidant. The oil should last for many years. Distilled water will work also as long as there is absolutely nothing that will dissolve in it and make it conductive (traces of flux?). Water is an excellent solvent.

[This message has been edited by Evan (edited 08-24-2004).]

08-24-2004, 05:29 PM
Thanks for all the replies, fellas. This is an audio load bank, as surmised, but there are four separate resistors involved, and there are eight stainless/copper/silver soldered junctions that will be immersed. This is four 4ohm resistors that can be configured as a single 1 ohm load, or two 8 ohms loads, in addition to 4 independent 4 ohms. Some of the amps I will be testing have + and - 120 volt supply rails, so there's considerable power available there. I would be happiest using water for it's best thermal conductivity as per Forrest. I'm considering painting the entire insides, including the resistance coils with something like barbeque paint, but I wonder if the paint would affect the heat dissipation more than would leaving things uncoated and using an oil as opposed to water. If I spray painted everything, I shouldn't have any corrosion problems. I would use distilled water anyway.

I didn't think of using a vegetable oil of some type. As I consider that, I wonder if I could use this thing to cook breakfast while a mondo amp is under test http://bbs.homeshopmachinist.net//smile.gif I'll consider using one of these oils.

By the way, mineral oil can be found at the grocery store, at least here it is. I'm surprized that mineral oil would shorten the lifetime of a transformer as opposed to using veggie oil. I'm off to check the link that Evan posted.

08-24-2004, 05:57 PM
The last time I work with a load resistor it was for business band radios (late 70s. My load resistor was a coild of wire in a 5 gallon bucket of cooking oil.

It worked.


08-24-2004, 06:23 PM
Hmmm interesting reading.

J Tiers, I agree that a 10% in resistance is going to throw calculations off, but from my experience in the electronics field, I know that speakers reflect a range of impedances to an amp that doesn't look anything like a fixed resistance. A 4 ohm speaker typically will measure differently, maybe around 3 ohms, but a test with frequencies will show a range possibly going down to 1 ohm, and as high as 15 ohms. Same for any speaker/cabinet combination, with the better ones having a tighter range of reflected impedances, but still nowhere near a fixed resistance. My testing will be to see that an amp can actually output according to it's power supply voltages and limiting circuitry, and have undistorted sound at those levels. Less important is knowing the precise level of output, more important to know that it's working properly. On a dummy load,it won't be possible anyway to see how an amp behaves with the complex load that a speaker usually presents. I'm sure you know all this, anyway. I usually tap the load resistor at about 5% of it's value or less, to get a signal to feed to a small speaker. This way I can actually hear some sound without having it blaring so loud, while the amp is delivering 95% or more of it's power to the load. A good ear can tell much more quickly if there's a problem, and then scoping the output will help to show what's going on.

I am aware of the alloy used in 'real' resistors, but I used what I had and that will be acceptable. This load will see it's first use testing a car amp that is supposedly capable of a kilowatt output into 1 ohm. I don't believe it, but we'll see.

08-24-2004, 06:56 PM
A kilowatt rms power? I don't believe it either. It best have a bank of batteries to hook to. It will be sucking about 100 amps.

08-24-2004, 07:06 PM
Even: so if they use vegatable oil in transformers, does that mean you can cook french fries in one?

Forrest Addy
08-24-2004, 07:37 PM
BZZZZZZ! Rant time!

At Kw sound levels who cares if it distorts? Are your amps intended for those "thump mobiles" driven by rude-nik post adolescents who play their music so loudly that at 1/4 mile away in heavy traffic makes it impossible for me to enjoy my own music - in my own car - with the windows rolled up?

If so, how could any responsible adult contribute to a wide spread culture of rudeness by making sound equipment clearly audible and intrusive for 4 miles on a quiet night? I wouldn't think the profit motive would be enough to over-ride guilt for contributing to an active nuisance.

One person's right to make music stops when it become a nuisance to others. Just becuase no-one complains doesn't mean that loud "rap" isn't objectionable to those forced by circumstance to hear.

Consider: what sedentary older citizen is willing to complain to a half dozen young and fit guys in a car? They pose the appearance of a threat.

It's up to the individual making the music to consider his potential for intruding on another's peace and tranquility. That's plain consideration for others, an attribute not only of adulthood but a Darwinian survival trait.

Just becuase it's possible to make music equipment installible in an automobile capable of 130 decibles doesn't mean it's reasonable, safe (ever see hearing acuity stats on the younger generation), or necessary (what can you hear as 130 Db you can't hear at 80?).

OK Rant off. Me feel better, now.

[This message has been edited by Forrest Addy (edited 08-24-2004).]

08-24-2004, 08:58 PM
Forrest, I echo your sentiments. It makes me sick, physically at times, to have someone's car stereo thump it's way into my brain, only to realize that the car is still a quarter mile away. I have always felt that it's better to have a good to great sound in your vehicle than raw power. Assuming of course that one is a 'mobile audiophile'. Some of us, me included, just want to have a decent sounding system, and that can be done with less than 100 watts total. Most are happy with the factory stereo, or the drugstore deck, but would like to be able to hear it without interference from the 'louder than a jet engine during takeoff' crowd. Nobody needs a kilowatt just for the subwoofer, and another kilowatt for the rest of the speakers. All this in a tiny cubicle- where's the sense? I believe, in fact, that it should be illegal, and that it's a hazard to other motorists. The trend, and in fact an entire industry, is now based on 'how loud can it get', or SPL. I may just turn this guy away anyway, as I don't believe in supporting this harmful activity. I do need the income, and there is a very real dilemma for ne there. I do repairs to electronic equipment, and by definition, that includes car stereo stuff. I have never, and will never, instal one of these brain pulverizing systems. I have to admit though, I'm a bit intrigued by the idea that one car stereo amp can output a thousand watts, and I want to see if it can do it.

The first use of this load bank will be to test this car amp, if I don't send the guy away first, but after that I could be working with professional audio gear. Only the promise of some work in this field makes the thing worth building in the first place. There's a local fellow who needs a tech to test and maintain his gear, so my main reason for building this load bank is to be ready when the time comes.

08-24-2004, 09:45 PM
Darryl: what do they use for voltage? Ohms law says it must be more than 12 volts to the one ohm load.

08-24-2004, 10:21 PM
I'm with Evan, at 100amps (or more) at 12 volts, it needs more batteries and an additional alternator.

They're probably those fake "music watts", a throwback to amp ratings of the '70s http://bbs.homeshopmachinist.net//wink.gif

Just curious, do you need to watch the inductance of the wirewound load with new amps these days ?


08-24-2004, 10:37 PM
When I have a Boom Car come by me, I crank up Glen Campbell or John Denver. I have a really nice sound system in my Corolla, but I don't crank it up to the point the speakers blow out.

I am still looking for a old Peter, Paul and Mary CD, with Blowing in the Wind on it.

Fight fire with culture.


Paul Alciatore
08-24-2004, 11:00 PM
I've worked with a lot of high power load resistors in my career, mostly for TV and radio transmitters. The big ones (30KW to 100KW) were water cooled. Distilled water flowing directly around and through the element. The elements were ceramic tubes with resistive film deposited on them.

At 120 volts and 4 ohms you are talking about 3.5 KW. That's not a particularly large load. I think you are going to a lot of unnecessary trouble. Commercial air cooled resistors can be had for reasonable cost in the 500W and 1KW range. Check out Ohmite:


It would only take 4 1K resistors or 8 500W ones to make your load and they are air cooled. No oil or other fluid. And don't start about their inductance. The inductance of a wire wound resistor is almost completely negligible at audio frequencies. Yea, I know the audio bufs will argue but they are the ones who will spend money (get ripped off) for gold cable and connectors for speakers. But, if you are really worried about it, there are non inductive, wire wound resistors available in the series.

Be sure to leave lots of room around them for air to circulate and/or add a small fan.

Paul A.

08-24-2004, 11:14 PM
Power in the thunder rides?I see them all the time,the slackers come in looking for odds and ends.
I see racks and I mean racks of batteries,24,36 and 48volts and 200amp delco marine aternators crammed under the hood.

But all that is changing,now the sport is to make them hop,hopping is not so bad,its even funny watching them do it in a new car for the first time.
Since G-loading often times isn't thought about,gas tanks,pumps and batteries hit the ground and fly through the air http://bbs.homeshopmachinist.net//biggrin.gif I really love laughing at them http://bbs.homeshopmachinist.net//biggrin.gif

I agree that b.s. music they play at that volume bass is below the range of human hearing,but elephants are on the mating rampage at the lcal zoo http://bbs.homeshopmachinist.net//biggrin.gif

Worse problem we have now are the kids on the Jap crotch rockets.Seen some(actually almost ran up my but)the other day,blew past me doing 65 like I was standing still all while weaving in and out of oncoming traffic.

The last bike in the pack was a guy with his girlfriend on the back.She was wearing a helment,but had on short shorts.Wonder if she knows what will happen to her pretty little legs if he dumps the bike over?Stupid a--holes all.
I usually play games with them,they go to pass I ride the centerline and block either side,really pisses them off http://bbs.homeshopmachinist.net//biggrin.gif

J Tiers
08-24-2004, 11:22 PM
If they STAY 150 deg, no polymerization in any hurry. It will happen even tehre, but slowly.

Inductance IS a problem at 20,000 Hz. it throws the power measurement off. Winding a flat coil resistor is easy, however. If fussy, wind it bifilar using a loop, so current goes down and comes back up adjacent wires.

For the power at 12V? No problem. I have a 12V photovoltaic system and a 2500 watt inverter. It runs my air compressor in teh garage, plus anything else I want to run.

Better have bigger wires, goo thing the audio duty cycle is low, averages 1/8 power or so.

And, all the car amplifiers cheat and lie as much as possible. Your setup should be fine.

And distilled works great. I'd just use it, 99 cents a gallon at the store. Most heat capacity per deg C rise of any substance you can get. Leakage at 1kW levels won't be your major problem.

08-24-2004, 11:50 PM
Paul, I may be going to a lot of trouble, well , I went to a lot of trouble, it's a done deal. I do need at this point to choose what fluid I use, and in the interest of non-corrosion, I would use the oil. From my limited testing so far, I can keep my fingers on the wire without burning them at the 140 watt level. I may not need any fluid at all, except to minimize the resistance change with heat, and for long term tests. Maybe if preconditioning the amp, 1 hour at 35% of rated output, something like that. Not that anyone ever does that, but maybe. Or for very high power operation, audio amp wise.

The inductance of the coil at audio frequencies is inconsequential, as you have stated. I did wind it bifilar anyway.

Doc, almost every car amplifier made has it's own power supply that takes the 11-14 volts fed to it by the vehicle and turns it into a higher + and - voltage to operate the actual amp part of it. They're generally made to feed 4 ohm speakers, so if the power supply makes + and - 40 volts, for instance, that can theoretically push 10 amps through the speaker at the peak. That equals 400 watts at the peak. That's for each channel. This is a very simplified explanation, but it serves to show how the lower voltage of a vehicle can pump enormous power levels through a resistance such as 4 ohms. If the load was 1 ohm, as is the case with the guy I'm dealing with, that would pump 40 amps out. To do that, the amplifier's power supply would have to draw more than 150 amps from the car. In reality, I've seen car stereo systems draw more than 100 amps from the vehicle, and cook the alternator in the process. One guy (idiot- or Forrest, insert your favorite name for these guys here) with engine idling, ran the battery flat in less than five minutes just with the stereo. The next day two of them went to a show in the states somewhere, and on the way they fried the 190 amp alternator that had just been hot pink anodized. I can just imagine the pully on that alternator being red hot, not to mention the diodes. That was a powerful system then, but it wouldn't win any SPL contests nowadays.

You guys are right, more than one battery would be prudent to test something as powerful at this, and a pretty hefty charger as well. I'm not going to worry about it since I doubt I'll be checking many of these amps out. I can have the battery fully charged and have the charger still going while I test the thing. Many amps have a protection circuit where it shuts off if the input voltage drops below 11 or so. I suspect that's what's happening in this case.

Ragarsed Raglan
08-25-2004, 10:03 AM

Have you considered using a 'Zambezzi Pile' - no need for coolant and very effective. I have a ZP 500amp - 12V load tester for checking batteries at engine cranking (i.e. starter) loadings.

And you can always BB a nice piece of steak whilst listening to your music!


08-25-2004, 01:13 PM
Daryl: Sounds to me as though you may have a bet of some sort going on with the owner. Vendetta time?

Mike W
08-25-2004, 02:26 PM
I made a load for testing alternators. I filled a metal barrel with headlamps. Each one had a switch to connect it to a busbar. A fan cooled them from a hole in the bottom. I think I had enough lamps to draw 80 amps.

08-25-2004, 02:56 PM
No vendetta. I don't have a problem with anyone in particular, but I don't approve of the practice of operating a car stereo system to the point where it can blast the paint off the inside of a freezer from a quarter mile away. I wonder about the sanity of those who do so, and am dismayed by the apparent lack of concern for others shown by those who insist on cranking it up so much that I can't hear my own stereo with the windows rolled up, as Forrest put it. I do feel for the person who comes to my door with the problem that he can't get enough sound pressure in the vehicle. It would seem that there's something missing in their masculinity that just has to be made up for with the loudest stereo system ever assembled by man. I wonder, are they trying to use volume to press the 'f--- you' attitude music (?) into their heads, when something in their subconscious is trying to keep it out?

It's always a good course of action to try making something good out of something bad, so I'm going to propose that owners of vehicles with these ear-damaging stereo systems rest themselves out to machinists who need to clean parts. Just put them in the car and crank it up! You could always experiment with different types of music, who knows, maybe some Dolly Parton tune when played at 160 db will clean the old paint off a lathe in need of refinishing http://bbs.homeshopmachinist.net//smile.gif

08-25-2004, 08:40 PM
Darryl: I did not mean a quarrelsome/feud type vendetta. A while back we discussed getting involved in a job and getting stubborn. One of those "I will make this thing work again, it only cost five dollars and but I will spend fifty hours just to be able to say I whipped it" -most of us admitted doing this type thing- some one summed it up- "vendetta time".

that kind of vendetta.

08-25-2004, 08:56 PM
Loud stereos,4x4's,street rods,motorcycles,etc,etc,all have the same thing in common-puberty!!!!! http://bbs.homeshopmachinist.net//biggrin.gif

Just think,its what every girl wants now-to go deaf at an early age http://bbs.homeshopmachinist.net//biggrin.gif

I blame the show pro-moters and the magazines,open the cover page and what do you see,a bunch of bikini clad chicks laying all over the hood of a vehicle.Remembering that sex sells brings us back to the puberty argument.I always remembered that those girls are getting paid and not from the guy the car belongs to,hes broke,after all he spent all his money on that foot gas pedal and chrome alternator fan http://bbs.homeshopmachinist.net//biggrin.gif

I got a buddy who tries(read not sucessfully)to impress women with his drinking ability,I don't think it has worked yet http://bbs.homeshopmachinist.net//biggrin.gif

[This message has been edited by wierdscience (edited 08-25-2004).]

08-25-2004, 09:10 PM
Ok, doc, I see what you mean. Well, maybe I have gotten a little overboard on this thing. I don't like to waste time that I won't be getting paid for, but sometimes I put the time in just to see if---.

I just finished repairing the amp, ( I did find a fault) and testing it. As some here surmised, there isn't enough juice from one car battery to avoid the voltage drop with the large current draw, even with the charger attached. Best I could get out of it was about 300 watts peak into a 2 ohm load. To do this, the amp wanted about 40 amps from the battery, and the voltage dropped to 11, where the amplifier protected itself. I think my battery is faded also, since there shouldn't be that much drop from a full charge at only 40 amps.

This particular amplifier makes + and - 70 volts to power itself. If that voltage under load dropped to 50, that would still translate to 2500 watts peak into a 1 ohm load. It would seem that a kilowatt rms is a realistic output level for this monster.

Well, ok, I guess I've satisfied my curiosity over this, and now I can put it to rest. A bonus is that I now have a dummy load that can easily handle the power. It never enen got warm during the testing. I filled it with filtered tap water for now, and I intend to scrub the silver-soldered connections one more time before I make the permanent fill.

J Tiers
08-25-2004, 11:52 PM
<font face="Verdana, Arial" size="2">Originally posted by darryl:

This particular amplifier makes + and - 70 volts to power itself. If that voltage under load dropped to 50, that would still translate to 2500 watts peak into a 1 ohm load. It would seem that a kilowatt rms is a realistic output level for this monster.


This sure is OT!

One thing to remember.There are several things that drop extra voltage.

The output stage does not have zero voltage drop. For bipolar, it is often at least 5 to 6 volts. For mosfets, anything from 4 to 10, depending on circuit. For class-D, I figure around 8% to 10% of the rail voltage, but that tends to be mostly "lossless" reactive drops.

So the 50V rail would likely get no more than 43 to 45 peak volts out under load. That reduces power a bit right there.

Fun....I have designed around 20 high power amplifiers that went into production, from 200 to 1600 watts, and I am now doing class-D and SMPS work. Never used a water-cooled resistor....we use the Dale NH-250 parts, non-inductive 250W resistors.

08-26-2004, 12:25 AM
Hmmm. I just did a resistance calulation. If you have a 150' roll of 14-2 wire you could short the hot and neutral together at one end and hook it at the other end to the amp. Bifilar wound and 0.9 ohms.

08-26-2004, 01:24 AM
The old 'roll of wire as resistor trick' . Done that many times, and sometimes had a hard time convincing people that it works.

I'm another homemade amp builder, though I haven't had any in production. The first one was a whopping 5 watts per channel. Man, was that powerful- at least it was for me then. I built a pair of 1 watt class a amps that got me kicked out of the apartment I lived in at that time. I currently have a pair of homemades in my stereo system, in addition to a Hitachi mosfet amp, which is a nice unit. I did have to work metal to make the chassis, so this isn't entirely off topic http://bbs.homeshopmachinist.net//smile.gif

Forrest Addy
08-26-2004, 02:14 AM
Darryl. Try the water first. It's cleaner and safer. Your runs will be so short that corrosion attack will be negligable especially in distilled water.

If you don't like the results you can alwayys change to oil.

As a matter of example. Whenever we tested the 750 and 1500 KW ship's service generators the grid was immersed in SALTWATER from a float alongside the pier. We're talking 460 3 phase. Some of the full output tests ran for hours. No problems. Nada.

08-26-2004, 02:21 AM
A few dead fish?

Paul Alciatore
08-26-2004, 01:10 PM
I've seen the salt water type loads for testing generators. They pass the current through the water itself. Visited a used generator guy once and he was testing a 50 or 60 foot long rig with a monster generator on it. I forget the KW rating but enough to power a small town. He had a salt water tank about big enough to put my truck in and the water boiled in a few minutes. Impressive. And in use, all you have to do is add water to keep the tank full.

Problem with using salt water is it's hard to control the impedance. So not so good for audio amps or any work that needs a bit of precision.

Paul A.

<font face="Verdana, Arial" size="2">Originally posted by Forrest Addy:
Darryl. Try the water first. It's cleaner and safer. Your runs will be so short that corrosion attack will be negligable especially in distilled water.

If you don't like the results you can alwayys change to oil.

As a matter of example. Whenever we tested the 750 and 1500 KW ship's service generators the grid was immersed in SALTWATER from a float alongside the pier. We're talking 460 3 phase. Some of the full output tests ran for hours. No problems. Nada. </font>

08-26-2004, 06:40 PM
Ok, let me get this straight. You guys ran the juice through the grid immersed in salt water- then collected the oxygen and hydrogen to run the generator. Then steamed towards the battle site, sifting the spent seawater for doping agents for modern semiconductors, while stunning all the fish enroute. Once you had shelled the enemy into submission, you returned, following the trail of oxides and picking up the fish on your way back to port. Once there, sold the fish and the semiconductor doping agents for a handsome profit. I imagine the long term plan was to create sea lanes of strangely electrified vegetation to guide future seafarers. Am I close?

Distilled water it is for the load tank.