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Coil Resistor?

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    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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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 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 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
 
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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 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 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 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.
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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 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 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 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.

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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!

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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 4:41pm

Well, Jack, here are the formulas for DC applications which we have until the points open after whcih things get messier because the current is changing and, and Geek!

Power = Current squared times Resistance or  (I**2)*R 

Voltage = Current times Resistance or I*R
 
so Current = Voltage/Resistance or I = V/R
 
 
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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 4:33pm
While we're about it (calculating the resistance), anybody wanna take a shot at the wattage rating that the resistor should have? (That's usually the important issue with low-value resistors) Most that I have seen are the large ceramic "blocks", and are probably good to 5 or 10 watts (never actually looked), but I'm just curious...I happen to have some _very_ "flashy" gold-anodized resistors (with heat-sink fins) that I believe were surplus from some NASA projects in years past. Most are in the <5 ohm category, and I believe some even go as high as 10 watts! Pretty things, and it might be fun t' use one in my driver.

...and I'd bet that one would look really neat in Roc's CJ2APU! <g>
Seeya!

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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 4:22pm
Originally posted by samcj2a samcj2a wrote:

Is that a reasonable way to look at it, Sean?
That's it.

2 equal resistances will precisely divide the applied voltage. Unequal resistances will divide the voltage in proportion to their resistance ratios.

eg, w/12v applied , a 1.0 ohm resistor will see 5.7v, while the 1.1 ohm coil will see 6.3v

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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 4:02pm

My opinion is that almost any 12 volt coil with a matched internal resistor or any 6 volt coil with a roughly matched external resistor would work.   If the internal resistance of the 6 volt coil is 1.1 to 1.5 ohms, any resistor with close to the same resistance should work for this purpose as long as the resistor is equal to or slightly less than the coil resistance, right?   You just want to drop the voltage in approximately half so you don't burn up the the coil primary windings.  If these things work with 8 volt batteries, then close is okay as long as you don't have an external resistor with greater resistance than the coil resistance and th voltage at the coil goes below 6 volts.  It probably still works if you are close, but who knows what the lower threshold is get a decent spark.

Is that a reasonable way to look at it, Sean?
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 3:50pm
Jack:
Quote I was still hoping someone might have that 12 volt coil number though.
There's the rub.  There's no such thing as a 12v CJ2A coil, so most just find a suitable 12v coil that was meant for another vehicle.  There could be dozens of different makes & models of coil that would work.

Simplest might be to use a 12v setup from later model Williys:

Autolite original part numbers (have them cross-referenced):
  • CAF-4003
  • CAG-4002
  • 200562
  • 200574
Or ask for sales part "5-10"
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