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ThorsgaardFoundry
Novice
Joined: 03 Sep 2007
Posts: 30
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 Posted: Sat Jan 05, 2008 11:12 pm Post subject: oven didn't work |
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Well, i wired the elements exactly as described in TJ's diagram, and i kept popping breakers.. Unless I got something wrong? (Probably)
I got 5 lengths, each measure 6 ohms, and about 102" long. I wired up A,C, and E to one leg of a 120V line, and B and D to the other.. plugged it in and got insta-heat for 2-3 seconds, and popped a 20 Amp breaker./... Then, wired separate lines to two separate circuits to "jump" the circuits, and still popped breakers. If you wire the "Hot" to hot on one circuit, and the "Neutral" to neutral on a separate circuit, (Both being 20A circuits) can you effectively get double amperage circuit? (40 amps)? - or is it still only a 20A?
The wire I'm using is from clothes dryers, so it might be a little heavier, and designed for 220V, but still measured 6 ohms per leg...
I'm baffled - almost to the point of buying tube heaters from infraredheaters.com...
any ideas?
-Jeff |
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crashmann
Guru
Joined: 27 Sep 2005
Posts: 501
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| Posted: Sun Jan 06, 2008 1:06 am Post subject: |
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First we should look at how you've got the elements wired up. You said it's just like TJ's diagram, so that would mean:
| Code: |
First Circuit:
Hot Neutral
*-----A--6 ohm ----*
*-----C--6 ohm ----*
*-----E--6 ohm ----*
Second Circuit:
Hot Neutral
*-----B--6 ohm ----*
*-----D--6 ohm ----*
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When you have resistors in parallel, they actually reduce the resistance from Hot to Neutral. There's a shortcut when the resistors have the same value:
R / number of parallel links = total resistance
6 ohms / 3 parallel links = 2 ohms of resistance
Now we calculate the current flowing through the circuit:
Amps = Volts / Ohms
Amps = 120 VAC / 2 Ohms = 60 Amps (holy cow!)
For the B & D connections, it only comes out to 40 Amps, but that's still way too much current!
The trouble is that you need more resistance on each leg.
You have a max of 20 amps
The voltage is 120 Volts AC
R = 120 VAC / 20 amps = 6 ohms (TOTAL)
You have 5 legs, or resistors that you are connecting in parallel
5 parallel resistors * 6 ohms = 30 ohms of resistance per leg
If you're feeling adventurous, and the usual life endangering disclaimers apply here, connect two of the elements in series. Just tie them together using a screw and bolts. Then connect one end to Hot, and the other end to Neutral. Stand back, flip the switch, and you should be underwhelmed by 10 amps flowing through the elements.
| Code: |
Hot Neutral
*---6 ohm ---*---6 ohm---*
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120 VAC / 12 ohms = 10 amps
You should get some heat. Not necessarily a whole lot, but some.
Now cut it in half. Connect a single element between Hot and Neutral.
| Code: |
Hot Neutral
*---6 ohm ---*
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It should run very hot, and you'll probably pop the circuit breaker, and here's why:
120 VAC / 6 ohms = 20 amps (the limit of your circuit breaker)
If you wish to continue using the clothes dryer elements, you'll need to link two of them together using a bolt, or some technique that will not break under lots of heat. Then run two of these extended elements on the same circuit (which will again put your oven at 20 amps).
| Code: |
Hot Neutral
*---6 ohm ---*---6 ohm---*
*---6 ohm ---*---6 ohm---*
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Remember what I said about parallel resistors with the same value?
12 ohms / 2 = 6 ohms TOTAL resistance
120 VAC / 6 ohms = 20 amps (pop goes the breaker according to the math)
I found that my oven did not pop the breaker at the calculated 20 amp value. It was also not running warm enough. So I trimmed a little of the nichrome wire back an inch on each segment, reducing the resistance, until the breaker actually tripped. Then I knew how much resistance was required, and I installed new nichrome wire cut to that specification.
Give it a try with what you've got, and with 408" of wire, hopefully you can cover your oven base (and not blow yourself up  ).
Charlie (who ALWAYS unplugs his oven before touching the wires) |
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ThorsgaardFoundry
Novice
Joined: 03 Sep 2007
Posts: 30
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| Posted: Sun Jan 06, 2008 7:40 am Post subject: |
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What an extraordinarily complete reply! I am awed. (I am odd?) lol
Thanks so much, now I have something to go out to the shop for! (I got kinda depressed there for a second)  |
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crashmann
Guru
Joined: 27 Sep 2005
Posts: 501
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| Posted: Mon Jan 07, 2008 10:49 am Post subject: |
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One of the strangest concepts to understand is why parallel resistors actually reduce the resistance of a circuit. If you think of the circuit like a bridge crossing a river, it becomes clearer.
A one lane bridge (with traffic flowing in a single direction) has a certain amount of resistance. Only a certain amount of traffic can get through. If you add another lane, you've now increased the capacity of the bridge, or, reduced the resistance. Now more traffic can flow through.
Adding a third lane further reduces congestion (resistance).
For this analogy, we're not going to worry about traffic coming back across the bridge in the opposite direction.
Hopefully this helps to explain why parallel resistors reduce the resistance in a circuit.
If you wish to increase resistance, then you need to connect the resistors in series, one right after another.
Enjoy stretching your brain
Charlie |
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kayaker43
Expert
Joined: 13 Jun 2007
Posts: 175
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| Posted: Tue Jan 08, 2008 1:00 pm Post subject: |
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Yeah, I think crashman types faster than I think  I wish I had enough time to be that helpful.
Unless I'm doing it wrong, I find it extremely difficult to get good resistance readings. The readings are all over the place depending on how hard I try to make contact with the wire. I usually measure a long piece and divide by feet to average it,, but its still erratic?
lately I've been doing the math, then make the coil longer and keep shortening it until I get the amp draw I want. The calculations are sometimes close, and sometimes not  Amp draw is the bottom line.
As a final check I can pretty much recognize the correct reddish/orangey color to see whether its putting out the right infra-red.
_________________
Doug Walsh
www.build-stuff.com
Hobby-Vac and Proto-Form machine plans
Also other plans books and videos for people who like to build things |
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cod
Master
Joined: 05 Nov 2006
Posts: 322
Location: San Francisco
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| Posted: Wed Jan 09, 2008 1:28 pm Post subject: |
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| kayaker43 wrote: | Yeah, I think crashman types faster than I think I wish I had enough time to be that helpful.
Unless I'm doing it wrong, I find it extremely difficult to get good resistance readings. The readings are all over the place depending on how hard I try to make contact with the wire. I usually measure a long piece and divide by feet to average it,, but its still erratic?
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I put alligator clips on my volt meter probes , that seems to help
I also got one of these:
http://the-gadgeteer.com/review/kill_a_watt_electric_usage_monitor_review
which gives you inline volts,amps,watts and can tell you how much your machine is REALLY going to cost you. unfortunately for some people it oes to 15 amps. I think they have larger models, but this one is 25$ and is quite useful around the house.
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<.o'> |
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kayaker43
Expert
Joined: 13 Jun 2007
Posts: 175
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| Posted: Wed Jan 09, 2008 4:04 pm Post subject: |
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Unfortunately the Kill A Watt things don't come in larger sizes or 220 volts. Would be nice though!
_________________
Doug Walsh
www.build-stuff.com
Hobby-Vac and Proto-Form machine plans
Also other plans books and videos for people who like to build things |
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crashmann
Guru
Joined: 27 Sep 2005
Posts: 501
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| Posted: Wed Jan 09, 2008 4:43 pm Post subject: |
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Too bad the Kill-A-Watt is only rated for 15 amps. I'd hate to kill the Kill-A-Watt 
It would be interesting to see how much I spend running the vac table all day for an armor party.
Charlie |
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cod
Master
Joined: 05 Nov 2006
Posts: 322
Location: San Francisco
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| Posted: Wed Jan 09, 2008 5:23 pm Post subject: |
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hmmm, yeah. i coulda sworn there were higher models but i cant find one. I can use it because I have 2 parallel 15 amp circuits.
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