sizing a dumpload for a newbie

[ZooT]

I've got a coiled nichrome element out of a 120 VAC space heater here.

I've found through tinkering that by probing the element with a postive lead connected to a deep cycle battery and to one end of the nichrome coil and a negative lead running through an amp meter that different lengths of this coil draw different amounts of amperage.

The whole 36 inch length draws one amp, half draws about 2 amps and onward and upward...

Given this, can I use this technique to size a dumpload via finding the proper length of coil to draw the desired amperage to dump and clipping it and putting some "uninsulated" crimp on eye connecters to both ends....

I'm talking within reason of course.....meaning no even faintly glowing coils....

[ghurd]

Capt Slog did a nice diary about the same thing.

http://www.fieldlines.com/story/2007/8/30/8538/81113

I like the idea of using the entire length, in pieces, so it doesn't get too hot.

Then use the pieces in groups, so each fet doesn't get overloaded.

It avoids the imbalance and timing issues too.

G-

[TomW]

Zoot;

You can combine the nichrome in parallel strands to spread the load rather than a real short hunk to get the amps you want.

example:

one length draws 2 amps.

split it in half and parallel them and it will draw 4 amps.

Indeed the nichrome will last much longer and be safer if you keep it from glowing.

Experimenting should yield a useful arrangement to dump a wide range of current without creating a load that glows like a light bulb.

Experimenting will give you knowledge you will not likely forget so that is often the best way to figure stuff out.

Be sure you size the dump load so it can dissipate the maximum amount of amps your system can supply plus a little headroom. It is usually not good to run things too close to the limits.

Connecting terminals to nichrome can be problematic. Solder is out and crimp connectors are marginal. The last heater i dismantled used spot welded male spade connectors and some cool male spade connectors with a pinch bolt setup to hold the wire. I have seen some tricks connecting them but cannot actually describe them.

Good luck with it.

Cheers.

TomW

[ghurd]

Hey Tom,

Bit of a minor math error there.

At 12V, a 12 ohm resistance uses 1A.

Cut in half for 6 ohms uses 2A.

Paralleled together uses 4A.

Like you said,

Something important...

A 12" 12V, 12 ohm resistor takes 1A, for 12W, meaning 1W per inch.

The same thing but 24" is 24 ohms and takes 0.5A, for 6W, meaning 0.25W per inch.

Cooler is always better.

G-

 

[Volvo farmer]

Ge dryer heating element, comes with a bunch of Nichrome wire and these neat copper coated bolts, nuts and lock washers. The connections hold a 4000W load at 240V in a dryer, should be fine for a dump load. Street price about twenty dollars shipped.

[Norm]

I get my Nichrome wire from old heating elements

....seems as tho they short out thru the little

round insulators in the stand offs ...my theory

is that a conducting oxide is deposited on the

insulators over time about every 10-15 years I've

had to replace the heating element..had to replace 3 times in the last 40 years on the same Maytag had to replace the plastic hub of the blower pulley with the metal hub of another

pulley ( the plastic pulley seems to wear out

about every 10 years...(never did have to ever

hire that lonely Maytag repairman) LOL !

[Opera House]

I got one of those kits in an auction lot of stuff.  Think my kit was 2KW,  Put it in my Maytag dryer and then the wife said the clothes weren't getting dry even at 60 minutes on the timer.  Put an interval on timer (5 minutes on, 5 minutes off) in series with the timer motor and that made the dryer run up to two hours.  

[ZooT]

Believe me when I said I'm not interested in glowing wires...at all....in fact I'd prefer them to be warm, but still cool enough to touch if needed....

I'm working on an "all in one box" combo dumpload controller/dumpload/low intensity space heater that I can mount to a wall on standoffs and not have to worry about.....

[electrondady1]

ok,

so without having one handy to measure the resistance,

a 1000 watt base board heater,

 will produce 50 watts with 12 volts?

[finnsawyer]

Presumably the 36 inch length has a resistance of 12 ohms.  When connected to the 120 volts ac you would get 10 amps of current for 1200 watts of power being dissipated or 33.3 watts per inch of wire.  This causes the wire to glow red hot, which you don't want.  So, you need to reduce the power.  Say you reduce it by half to 16.6 watts allowed per inch.  The resistance is 1/3 ohm per inch.  We will use N inches, so the resistance is N/3 ohms.  Connecting that to the battery, the power dissipated would be (12x12)/(N/3) = 432/N.  This power in turn must equal 16.6 times N, as that all the power dissipation we will tolerate.  So,

     432/N = 16.6xN,  N^2 = 26,  N = 5.1 inches,

So, each 5 inch section could dump 84.7 watts of power.  You could get 7 such sections out of the 36 inch piece for a total power dump of about 590 watts.  You can run these numbers for different values of allowed power dissipation per inch.  Good luck!    

[ZooT]

Mine is the lower power coil(1000 watts?)from a 1000/1500 watts cheapie space heater and the entire 36" section draws about 1 amp at the 14 volts that it starts dumping at, so I'm thinking that's about 15 watts(?) that it'll dissipate...

My HF 45 watt solar kit says it'll do 45 watts and max 3 amps....so 45watts/3amps=15watts

[Capt Slog]

As Ghurd says above, I made a dump load out of a heater in the same way.

Mine never glows but can get quite hot to the touch, I spread the load over several coils to acheive this, I'm not a fan of anything like this working to it's limits.

With what I had left over, I've since made a line resistance to get the speed of my windmill up a bit.

[ZooT]

Quote:

Limits are a man made creations and are there for a reason, and running something right to it's upper limits leaves no safety margin...

My little Honda accord will do 140mph.....but that doesn't mean either it or I like going that fast.

By the same token that's why I'm diddling with this dumpload and the controller I've built for it......My regular controller is a homebrewed bang-bang 30 amp controller, and it's dumping to a 30 amp semi tractor sleeper heater and when it dumps it dumps.....right now!

Hopefully this little dumpload and controller will take the place of that one, but I'll still leave that one connected for those "oh my gosh" times...

"Da' folks in da' know" here tell folks to start small and work their way up...... and that's what I'm doing.

I've only got a little 200 watt turbine and 45 watts of HF PV so we're talking at best on a sunny day with solar flares under an ozone hole in a full gale maybe 20 amps........but I don't think I'll ever seen anything like that ...so if I can make this thing do 10-15 amps and use the other one as backup and learn a little something in the proccess.....I'm doing well....

Like I've also said, I've got a xantrex C-35 and one of ghurds kits here.....but I want to learn how to make this stuff myself....because it's fun 8^P

[finnsawyer]

Why are you making it so hard on yourself?  you can dump all 45 watts with one section without it getting red hot.  

    45 = (14x14)R, R = 196/45 = 4.35 ohms.

The entire length of wire has 14 ohms resistance or 0.389 ohms per inch, so you need 4.35.389 = 11.2 inches of the wire to dump all 45 watts.  Of course, that assumes your numbers are correct to start with.  If you cut the wire in two each section would have 7 ohms resistance.  At 14 volts each section would dissipate 28 watts of power for a total of 56 watts, which would give you some margin of error.  In that case each section would run cooler than the single 11 inch piece.  You probably need to refine your numbers.  Why can't you measure the resistance of the wire cold?

[finnsawyer]

Damn!  That first equation should be 45 = (14x14)/R.