I need some advice. I have a 1932 Ford running a 350 with a 671 blower. I was working on the car and bumped a “power transistor “ on my firewall and it fell into my rocker panel and I was unable to retrieve it. I was on the road when this happened so not knowing what it was I cut the wiring from between the Mallory coil and the “power transistor” and the ballast resistor. This bypass got me back on the road but the car runs like crap. I was able to get the piece out of the rocker panel but don’t remember how to wire it back in. The resistor had 3 wires to it ground, positive, negative. Which one would go where? See pictures.
The resistor went to ignition switched power? If so I'm thinking middle connector goes to coil + Coil - goes to the right hand leg (looking at the transistor like you have in the photo) Middle transistor leg goes to ground Left transistor leg goes to the points Ballast transistor for the coil to drop the voltage to the coil The transistor acts as the points do by opening and closing conductivity. That open and close is triggered by the points, saving the contacts from wear. Phil
The middle can't be connected to coil + and also ground. Eliminate the ground in your explanation and I think you have it.
This is not a resistor, it is an NPN power transistor. It will (should) have a resistor connected to the Base ( left pin when looking from the front), which is probably taped into the wiring loom somewhere. The Collector (middle pin) would be connected to your coil negative. The last pin (Emitter) should go to ground (if you have a negative earth system). There should be something in that circuit to quench the back EMF from your coil, too. This is your most basic transistor ignition, hopefully the rest of it is there somewhere. There is usually a few more components than just the transistor in these systems. If strip the loom back enough, you could re-connect it , if you are careful to not over-heat the stubby pins that are left on the original, or get a new one. They are only a few bucks.
Does the heavy lifting of switching the coil on and off. The points are just used to switch the transistor on and off instead, which is a minuscule current, with no sparking to dirty up the points.
You're right, I read the datasheet for the NTE37 (PNP compliment). With the PNP you just dump through the transistor with maybe a resistor to limit the current through the base. He mentioned a 3 position ballast resistor, unless I misread, though.
Notice this is on the positive side of the coil. Normally the points are on the negative side of the coil. Connected like this the transistor will interrupt the flow of current through the coil, triggered by the points. When the points close the transistor will connect the collector and the emitter and current will flow. When the points open the connection between the collector and the emitter opens, current stops flowing. The points are shielded from the high voltage generated by the sudden collapse of the magnetic field, so no condenser is needed. ETA: While I was being verbose about it, PhilA cut out the b.s. and got to the point.
I would also assume that the switching is faster because you don't have to wait for the capacitor to charge. But if I'm reading this right, this also precludes you from using a ballast resistor so you need a coil that can operate on the full system voltage.
Can't see why you couldn't keep the ballast resistor. You just don't have to have it to keep the points healthy, but skipping it is an option.
Thank you for all of the information. I don’t have points in the car it was swapped over to a Mallory Unilite. Does that make a difference?
That is a pretty darned simple points-saver circuit. Continuous current rating of that transistor is 12 amps, so it should work a long time in an environment where coils are typically a couple ohms minimum and V8 point dwell time is 45°. @scott Hrncirik what was that transistor mounted to, some kind of an isolated heat sink? The diagram @squirrel posted says not to ground the mounting tab, which makes sense as it is typically connected to the collector lead and grounding it would render the circuit useless. EDIT DUE TO NEW INFO: If the igition system is a Unilite then it's "pointless" to even have this transistor wired in to the system.
Sorry for the lack of info. It was mounted to a steel member under the dash with a thick white glue (I assume to keep it from grounding).
If the heatsink is not meant to be grounded, there will be a tiny nylon insulator which keeps the mounting screw isolated from the tab. Make sure it goes back on, and , as said above, the white gunk is heatsink compound. Scrape some off the old one if you can, and use it to mount the new one.
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