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Coil Resistor?

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Jus*Jack View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jus*Jack Quote  Post ReplyReply Direct Link To This Post Posted: 07 Mar. 2008 at 4:49pm
I too, am reasonably well-acquainted with Mr. Ohm, Sam, but...<g>...WHAT CURRENT FIGURE to use?

Aye! And there's th' rub!
Seeya!

<Jack>

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Post Options Post Options   Thanks (0) Thanks(0)   Quote sean Quote  Post ReplyReply Direct Link To This Post Posted: 07 Mar. 2008 at 5:00pm
Originally posted by Jus*Jack Jus*Jack wrote:

While we're about it (calculating the resistance), anybody wanna take a shot at the wattage rating that the resistor should have?
30 watts or more, mentioned it earlier.

Glad you brought it up though, since this is the one issue in 6 to 12v conversions that everyone seems to ignore, or be unaware of: POWER DISSIPATION!

Let's assume our 6v coil has 1.2 ohms primary resistance, so it will draw 5 amps, and dissipate 30 watts (power = v x i).

At 12v, this same coil would draw 10 amps, and dissipate 120 watts!  Quadruple the power! ie. 4 times the heat!

By adding a 1.2 ohm ballast resistor, total circuit resistance is doubled, so now, at 12v, the circuit is back to 5 amps  (V = I x R, 12 = 5 x 2.4).

Total power dissipation is now 60 watts (P = 12 x 5), half is dissipated in the coil (30 watts, back to normal), and the other half (30 watts) is dissipated in the resistor.  This is wasted power.

You could actually get by w/a 15 watt resistor, since the current through the coil on a running engine is approx 2.5 amps (circuit is being interrupted at approximately 50% duty cycle).

But, it's best to design for worst case scenario - accidentally leaving the key "on" such that it's drawing the full 5 amps.

Sean
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Post Options Post Options   Thanks (0) Thanks(0)   Quote samcj2a Quote  Post ReplyReply Direct Link To This Post Posted: 07 Mar. 2008 at 5:13pm

Jack,

All you needed to do was solve the two simultaneous equations I gave you!  Geek  LOLLOL

Sam

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jus*Jack Quote  Post ReplyReply Direct Link To This Post Posted: 07 Mar. 2008 at 5:32pm
Sorry Sean...must missed that, while "sparring" with Sam! <g> And thanks...I'll have to dig out those resistors and check to see if I've got any that are that high in wattage.

And Sam...I DID solve the two equations, simultaneously...by asking both you AND Sean! <g>
Seeya!

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jus*Jack Quote  Post ReplyReply Direct Link To This Post Posted: 07 Mar. 2008 at 5:54pm
Yep...got a couple of 'em...50W, 1.5 Ohm, 1% (Mil-Spec), Aluminum resistor by CAL-R...looks like it would work real well. Looks like this, but 2" long:



Kinda pretty, aren't they?
Seeya!

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Post Options Post Options   Thanks (0) Thanks(0)   Quote samcj2a Quote  Post ReplyReply Direct Link To This Post Posted: 07 Mar. 2008 at 6:03pm
Jack,
 
I enjoyed your "solution". 
 
No sparring intended:  You need to check your coil to see what its resistance is.  If it is 1.5 ohm, that's fine, but if it is 1.2 as Sean has said the typical 2A coil is, the voltage at the coil will be under 5 volts if I did the math correctly.  That may or may not matter.  I don't know.
Sam

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Post Options Post Options   Thanks (0) Thanks(0)   Quote sean Quote  Post ReplyReply Direct Link To This Post Posted: 07 Mar. 2008 at 6:07pm
Jack, yes they're "purdy", but 1.5 ohm might be too much.

Using 1.2 ohm for the coil, this resistor would drop 6.7v, your coil would only get 5.3v.  The bigger the resistance disparity, the worse it gets.  It might work fine, no way of knowing w/out trying it, but lower resistance would be the safer bet.

Sean

PS, you say you got a couple of 'em?  You could put 'em in parallel, for .75 ohm, then your coil would get 7.4v.

The resistors could handle the power, but your coil would now be dissipating 45+ watts.  Might be OK, or might a bit much.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jus*Jack Quote  Post ReplyReply Direct Link To This Post Posted: 07 Mar. 2008 at 6:11pm
You're both right, of course...and I do believe that I've got one or two in the 30W range that are smaller...in fact I might have to series them to get UP to that resistance!

And Sam...I used the term "sparring" only in the FUN way....I don't take umbrage at your comeents, just as I would hope that you don't take any at mine! That's one of the things that I enjoy so much about this forum...that we "regulars" can kid each other while asking/answering the serious questions. I thnk that it helps to keep the "stuffiness level" down around here, and hopefully, encourages others to know that they can jump in at any time, and find themselves among friends!

Now...to go root around in that old parts bin some more! <g>
Seeya!

<Jack>

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Post Options Post Options   Thanks (0) Thanks(0)   Quote samcj2a Quote  Post ReplyReply Direct Link To This Post Posted: 07 Mar. 2008 at 6:14pm
And, as usual, Sean is right.  The voltage at the coil would be five and a third volts, just over 5 volts.  Embarrassed  Shoulda used a calculator!
Sam

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 46CJ2A Quote  Post ReplyReply Direct Link To This Post Posted: 07 Mar. 2008 at 10:04pm
Geese!!  And all I wanted to know was if I could use a 12 volt battery to do an initial startup on my newly rebuilt motor which has a 6 volt coil and distributor. Smile Smile Smile
 
Jack
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Post Options Post Options   Thanks (0) Thanks(0)   Quote samcj2a Quote  Post ReplyReply Direct Link To This Post Posted: 07 Mar. 2008 at 10:17pm
And, now we know that the short answer is that, yes, you can do that safely if you run over to the store and buy a 1.2 ohm coil resistor and wire it in series with your 6 volt coil.  LOL
 
That is about a $5-6 item, BTW!  
 
And, thank you for letting us hijack your thread for a lot of other discussion.
Sam

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jus*Jack Quote  Post ReplyReply Direct Link To This Post Posted: 07 Mar. 2008 at 11:06pm
Sorry about hijacking your thread Jack! <g> It's just that I've had these resistors for a long time, and suddenly thought that maybe I'd found a reason that I've saved them for all that time! <g>
Seeya!

<Jack>

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 46CJ2A Quote  Post ReplyReply Direct Link To This Post Posted: 08 Mar. 2008 at 1:50am
It's been a great discussion and I've learned a lot.
 
No need to stop now.
 
By the way, I turned the engine over without the spark plugs installed this morning and it did move faster, but still not fast enough.  So I put my charger on the NEW battery and found it wasn't fully charged.  I tested it earlier with a meter and thought it was full of juice.  I'll try it again tomorrow after it cooks a little..
 
Now, what was the wattage of that resistor again? Dead
 
Just kindin'.
 
Jack
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