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Forum Index : Windmills : 12V Dump Load Question.
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wallablack Senior Member  Joined: 10/08/2011 Location: AustraliaPosts: 164 |
I have the following dump loads on hand and want to build a dump load for a new 750W 12V turbine charging a 12V battery bank to 14 volts then dumping. Correct me if I am wrong but: 900W max / 14 Volts = 64.28 Max Possible Amps. I have the following resistors: 5 x 300W 3R 3 x 300W 1R & 8 x 100W 10R I can figure out a good configuration of these to achieve a suitable dump. Any ideas? Foolproof systems do not take into account the ingenuity of fools. |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Using the 5 3R resistors in parallel gives us 0.6R at 1500 watts. 12v into 0.6R is 20 amps. 20amps times 12v is 240 watts, will under the 1500 watts rating. Using the 3 1R resistors in parallel gives us 0.33R at 900 watts. 12v into 0.33 is 36.4 amps. 36.4amps times 12v is 436 watts, well under out 900 watt rating. So far we have 56.4 amps, 676 watts. Lastly the 8 10R resistors in parallel is 1.25R at 800 watts. 12v into 1.25R is 9.6amps. 9.6 amps times 12v is 115 watts, again well under our rating. So if you connect all up in parallel, you end up with a 791watt load at 12v, or 922watts at 14v I think your on the money, just parallel them all up. See http://www.thebackshed.com/Windmill/Docs/ElectricFormular.as p Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Warpspeed Guru  Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
It will also depend on the safe individual power capability of these resistors. What kind of resistors are they ? Cheers, Tony. |
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Don B Senior Member Joined: 27/09/2008 Location: AustraliaPosts: 190 |
Certainly as Glenn suggests, paralleling them all up should do the trick, although the 1 R resistors will get the hottest, followed by the 3R ones, and finally the 10R ones. If you build a dummy load you should try and keep the resistors horizontal and separated so that they have the best possible natural air circulation. The whole enclosure (and the space that it is in) also needs good ventilation as it will be exactly like having a 1 kW bar heater running. Maybe you can also mount one or several 12V fans in the load enclosure to help ventilate it and also help use up some of the dumped load. It would also be a good idea to set the bank up if possible so that individual resistors can be removed or unbridged if experience suggests that you need to alter the resistance value of the bank. Regards Don B |
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| wallablack Senior Member Joined: 10/08/2011 Location: AustraliaPosts: 164 |
Thanks guys. I did mount them all together, horizontally opposed as with my others and it works a treat. I don't know why these things confuse the sh*# out of me.... Everything has an "online calculator", everything......except configuring dump loads. Question. To keep it simple without going into values and calculations and it is nothing to do with the above. Just a hypothetical. Assuming the dump load is of correct size. What would happen "IF" I had a dump load that was twice the size of the required dump load. Foolproof systems do not take into account the ingenuity of fools. |
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| wallablack Senior Member Joined: 10/08/2011 Location: AustraliaPosts: 164 |
I have to admit I am a dummy when it comes to dummy loads. So it worked out well and has been going really well, thanks to Gizmo's mathematical help. But the thing has it's own bloody postcode and takes up a fair amount of wall space. So, I have seen dump loads on Jaycar a long time ago and when I brought 2 earlier this year I only got one as Jaycar said that they have no more stock. I contacted them and they stated they were no longer getting any more in as their supplier had stopped making them. Farting around on the internet today trying to find some a link sent me to Jaycar to the above mentioned dump load resistors. I rang them and they had found three so I snaffled them. They are 1.3R 1000W - Green wire wound ones they sold for their 500W 24V mills. How many of these three will I need for my mill? 750W, max 900W output, 12V and charge controller diverts at 14.1V. Foolproof systems do not take into account the ingenuity of fools. |
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Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
If im correct it would be 14.1 / 1.3 = 10.8 amp per resistor 900w / 14.1 = 64 amp 64 / 10.8 = 5.9 resistors So you would need 6 resistors in parallel for 900w at 14.1 volts Or 8 parallel resistors for 12 volt at 900w ............................................................ or using ohms formular for resistors in parallel 1 / 1.3 = 0.769 0.769 x 6 resistors = 4.614 1 / 4.614 = 0.217 ohms for 6 resistors in parallel Voltage / resistance in ohms give amps 14.1 / 0.217 = 65 amp Volts X amps = watts 14.1 X 65 = 916 watts Clear as mud Ha? Pete. Sometimes it just works |
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| wallablack Senior Member Joined: 10/08/2011 Location: AustraliaPosts: 164 |
Excellent work there Pete. 
That calc put it all into perspective, simple once you SEE it in front of you... Where is a good place to go to buy dump loads and get them quick? In AU would be best. I like the ceramic tube wire wound... Australian Resisotrs will make 1 for $123 + Freight and there is a really good bloke named Bill @ Hurtle Websters here in Vic who will make them...and they come in racey red. Both have a 2 week wait. Anyone know anywhere else I could look. Foolproof systems do not take into account the ingenuity of fools. |
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| wallablack Senior Member Joined: 10/08/2011 Location: AustraliaPosts: 164 |
So, the wattage rating means jack sh!t...yeah? Or Nah? Or is it a yeah nah. If I had a 50W o.22R Resistor the following would apply????????? 14.1V / 0.22 = 64 amp per resistor. 900w / 14.1 = 64 amps 64 / 64 = 1 This is saying that I can use 1 resistor that is a 50W 0.22ohm. This makes no sense.....to me anyway.  Foolproof systems do not take into account the ingenuity of fools. |
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| Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
Its a yeah nah. Your math is correct but the thought is wrong. The wattage rating of a resistor is the maximum power it can handle before failure. You can go under the resistor rated watage but not over. 900w into a 50w resistor, hmm? that is a little over its rating dont you think???? So you want at least a 1000w 0.22R resistor to do it with just one. Although it would be bloody hot at 900w. Or you can spread the load across several resistors and reduce the amount of heat emitted by each resistor, its still the same total amount of heat but spread over a larger area. Think of it as a heating element, at full power it glows red hot, at half power its a dull colour, so if you had 2 elements at half power its still the same watts and amount of heat than 1 element at full power. I like to spread the heat over several resistors so each just gets warm, rather than 1 that glows red hot. Its up to you, i think red hot things can start a fire easier than warm things, so safety is important to me. Sometimes it just works |
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| Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
[quote]Where is a good place to go to buy dump loads and get them quick? In AU would be best. [/quote] Well your own back shed of course, and im sure there is a good bloke there to make it. At the end of the day a dump resistor is nothing more than some wire wound around a core. A quick and dirty way is to wind some 1.5 to 3mm steel fencing wire around a broom handle then slide the wound wire onto an old glass fluro tube for the core. The longer the wire used the greater the resistance. Pete. Sometimes it just works |
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| wallablack Senior Member Joined: 10/08/2011 Location: AustraliaPosts: 164 |
I could see that going bad, very bad......My guesstimation would be the downfall... Thanks Pete, I thought the wattage thing had a BIG part to play...  Foolproof systems do not take into account the ingenuity of fools. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Check out some of the secondhand machinery graveyards that deal in heavy industrial power equipment. They have big electric motors, old diesel gen sets, electrical switchboards, and very often big resistive load banks. These resistive load banks are usually made up of either individual cast iron or stainless steel plates bolted together in series to act as loads for testing standby diesel generators or discharging testing very large battery banks. 
As the resistive element plates bolt together, you can very easily dismantle these loads and reassemble them to provide whatever resistance you require to suit your voltage and power requirements. This is the type of thing : http://www.irescoindia.com/wire_grids.htm I have some cast iron resistive elements here you can have for free if you wish to pick them up. These would each be around 0.06 ohms and should carry maybe 100 amps easily, even more with fan cooling. I have eight, which connected in series would be 0.48 ohms or 29 amps at 14v Seven in series 0.42 ohms, 33 amps at 14v Six in series 0.36 ohms, 39 amps at 14v Five in series 0.30 ohms, 47 amps at 14v Four in series 0.24 ohms, 58 amps at 14v Three in series 0.18 ohms, 78 amps at 14v These elements are each a lump of cast iron, cast in a zig zag pattern with very large mounting holes at either end. Very roughly 300mm long and 130mm wide. You can have these free if you wish to pick them up. Or you should be able to get some rusty old ones very very cheap indeed from an electrical junk yard. They are absolutely ideal for the job, because that is exactly what they were originally made to do. Cheers, Tony. |
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| Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
[quote]I could see that going bad, very bad......My guesstimation would be the downfall... [/quote] Why? just wind as much wire as you can to fit onto a 4 foot fluro tube, then if its too high of resistance cut the coils back, if its not enough make another and parallel them. Then you might be richer than me, at $123++ a resistor i would be winding some wire. Sometimes it just works |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
All this is why I originally asked this question (third post in this thread). Wire wound resistors commonly have a power rating arrived at for a 350C surface temperature rise. That is dangerously hot, the resistor will be smoking, and burn anything placed nearby. Something like a 60C rise would still be quite hot, like 100C surface temperature on a 40C day. If you limit the rise to 60C, that means a 100 watt resistor MUST only be run at 60/350 x 100 watts = 17 watts. That seems outrageously little power for a 100w rated resistor, but it is a fact. If you don't believe this, look up the specifications for your favorite wire wound resistor. It will say somewhere in very small print "at 350C temperature rise". That power rating at 350C rise figure is used only as a reference temperature for de-rating it to suit a particular design. So the published power rating is really a "theoretical" figure, not something you can use as a typical practical everyday running power rating in free air. Cheers, Tony. |
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| wallablack Senior Member Joined: 10/08/2011 Location: AustraliaPosts: 164 |
Sorry Warpspeed, your post was not ignored but taken into account as the point is a VERY valid point. What kind of resistors are they....Chinese compliments of Evilbay...no data sheets of course. BUT, on contact with them they replied with a data sheet and the ratings are @ 250 deg celcius... Those plates seem to be the go but I live in the middle of bumf*ck west so you can hold on to them for the next taker... I do appreciate the offer a lot,  Foolproof systems do not take into account the ingenuity of fools. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
O/k fine. I just thought I would raise the awareness of how hot these wire wound resistors can become. The resistors themselves will be fine, but the high temperatures will create hot smells, even smoke, and the resistors will probably go brown or black, and stay that way after they cool back down. Try connecting one of your one ohm 300 watt resistors across your car battery. With the engine running there should be about 14 volts. The resistor will draw 14 amps. 14 x 14 = 196 watts. That should give you a pretty fair idea of what to expect. It is just a case of mounting these resistors and wiring them up in such a way that nothing is going to melt, burn, or scorch. A suitable cooling fan might be a good idea to help carry away some of the heat. Cheers, Tony. |
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Wombat Regular Member  Joined: 27/05/2011 Location: AustraliaPosts: 72 |
Reviving an old thread I was searching for as I've acquired some very large wire wound resistors. There the biggest things I have ever seen!
Unfortunately, I don't have a camera to take a photo of them for you. Description as best I can.. Ceramic tube core, 460mm long, OD 62mm, ID 37mm Wire wound with 1.8-2.0mm, 0.4 ohms. Manufactured by Hurtle Webster (VIC) P/L There is a number '35214' They have two winding of 30 turns each. Now, the question... Can anyone give me a guestamate of there wattage? 
Oh, they are red in color.. Thanks Russ  |
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| Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
Ohms law is the calculation required here. You dont quote at what voltage you want a wattage for, so lets assume 12v. 12v / 0.4 ohm = 30 amp 30A x 12v = 360W Or 24V / 0.4 ohm = 60 amp 60A x 24v = 1440W. So you see it all depends on the voltage the resistors are used at. Pete PS... they will change colour from red when they get hot, im not sure what colour they will change to, but green ones change to a brown/red colour when hot. Sometimes it just works |
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| Wombat Regular Member Joined: 27/05/2011 Location: AustraliaPosts: 72 |
These came out a huge old UPS system. I quite sure they were in a 3 phase setup, 2 in series, 3 times. I don't know how they were actually used. Although.. The old wires on them are approx 3mm dia. 3.26mm dia = 8 AWG 8 AWG = 60A @ 60 deg C I would love to throw one across the 24V bank to see how warm it gets, but I'm to scared it might hurt the batterys or go bang! I put an 80 or 100A fuse in line and give it a go when I feel brave enough
Russ |
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