Ballast Resistor

[rboltz]

Anyone know what the resistance is for the 24 volt coil ballast? My ignitor ballast is built in the coil so I can't measure it for comparison. I plan on using a 2920-00-358-6960 24 Volt Ignition Coil with my civvy distributor so need a resistor. My current 12 volt set-up also has a ballast resistor mounted on the firewall.

[amphi]

I'm thinking the 12v ballast resistor currently installed should be doubled in ohms --- or two of the same ones in series.

Your 24v coil primary resistance should be just about double that of the 12v coil.

Can you measure this with an ohmmeter and confirm?

If this is true and it is in fact doubled as it should be.

Your points should be seeing the same current on 24v as 12v.

Here is the theory:

Current(I) equals voltage(E) divided by resistance(R ) in ohms. I = E / R (Ohms law)

12v coil at about 3.2ohms plus a ballast resistor at 1.3ohms equals 4.5ohms total. This will produce about 2.6 amps thru the coil primary and thru the points at 12 volts.

12 / 4.5 = 2.6

12v divided by 4.5ohms equals 2.6amps.

24v coil at about 6.4ohms plus a ballast resistor at 2.6ohms equals 9ohms total. This will produce about 2.6 amps thru the coil primary and thru the points at 24 volts.

24 / 9 = 2.6

24v divided by 9ohms equals 2.6amps.

The above values are hypothetical but close to real world figures.

The actual current flow thru the points will be a little less than 2.6 amps due to the impedance in the coil from inductive reactance induced from pulsating DC current.

This is only a guess because I can't find a link confirming my theory. I usually include a source link in my posts to corroborate my thinking but just can't find one in this case.

Bottom line:

I would at least double the resistance of the ballast and check for excessive heat in the coil after idling the engine for a while. Also the condenser should be rated to handle 24v.

Maybe someone can chime in to either confirm or deny this theory.

[rboltz]

Your theory is correct. I am an electrical engineer so know how to compute the resistor but needed a starting point since there was nothing I could measure as I do not have a 24 volt set-up at present. I wanted to find out what the 24 volt "stock" resistor was so I could compare all the rest of the values that I know. From that, I could determine what I needed for the new coil, which may be different from the stock ignitor. The stock ignitors had a resistor under the coil but the one I have sitting on the bench does not so I can't measure it, therefore the missing link is computing the current across the points. As you mentioned, without some serious testing, the overall impedance of the coil is difficult to determine. Thanks for your help!

[amphi]

Glad to hear I am not too far off base.

The thing I really struggled with is P = I x E.

In the above scenario the power consumed at 24v (in a purely restive load) would be twice that of 12v since the current is the same. Maybe the impedance in the 24v coil, with more primary turns, will reduce the current to a point where the power consumed is close to what it is at 12v. Just a guess. The Power Factor should be negligible with the correct condenser. Again just thinking out loud with no back-up source to point to.

Is this the coil you are getting?

http://cgi.ebay.com/2920-00-358-6960...QQcmdZViewItem

[rboltz]

Yep, thats the coil. I was going to put the ignitor in and go completly stock but decided to keep the civvy distributor. That way, just change the coil and ballast and keep the vacuum advance!

[george walker]

The condensor is not voltage sensitive in application, the very same points /condenser would work in a 6, 12 or 24v system. Presumeably because the avialable voltage to them is about the same in all uses.

[george walker]

A 6v system doesn't have a resistor, when 12v (an 24v) came along insted of having different voltage applications of the same point/condensers they just started using a resistor (or resistor coil).

[ncjeepers]

I wish I knew the answer to that one, too.
Of all the umpteen ignitors I've pulled apart, not one had that ballast resistor under the coil and nobody seems to know where to get one.

[george walker]

In the TM section 6-58 electrical specs it lists: coil primary resistance-
4.26ohms-4.9ohms, and secondary resistance 12090-13910ohms, the separate ballast-resistance @75* 1.68 to 1.88 ohms.
Just in case- the manuals are avial right on this site to download for free just click the manuals link on the page header, only the manuals in the adobe format are avial (the left coulumn)

[rboltz]

Thanks George. I missed that in the manuals. I have hard copies and it is easy to miss something. PDF's, if created properly, make the job easier to search. Thanks again, this is what I need.