I am restoring a car and I am trying to understand the igniter circuit in order to repair it. When TR2 is open (not grounding) then there is a spark.

Right now I don’t understand why TR2 has two voltage sources going to its base.

I have considered that the boxes on the right side of the igniter on IGT and IGF lines could be voltage regulators, but I am not sure. I have always just assumed that they were junction boxes. Does anyone know this symbol?

boxes in question

ignition circuit. Igniter has 2 boxes on IGT and IGF

slightly different model than the one I’m working with. Case was glued shut so I had to break it

I tested for continuity and the striped copper strip is chassis ground

Resistance from W-R (terminal 3) to TR2 base is measured at 130ohms

I was not able to get any continuity reading from IGT to TR2 base

No continuity from terminal 5 to IGF

  • \$\begingroup\$ That's not a standard symbol. I'm wondering if it's some kind of ferrite bead designed to provide some positive feedback to give the transistor an extra "kick" to get it to switch quickly. If that was true then wire 2 would pass through it without electrical contact. Can you provide a photo of the igniter's insides? \$\endgroup\$
    – Transistor
    Commented Jan 17, 2023 at 17:30
  • 2
    \$\begingroup\$ When TR2 is open (not grounding) then there is spark. ... no, the spark happens at the moment of opening \$\endgroup\$
    – jsotola
    Commented Jan 17, 2023 at 17:32
  • \$\begingroup\$ If those rectangles are junctions, then TR2 will always be ON, because the base will always have a positive voltage on it. \$\endgroup\$
    – SteveSh
    Commented Jan 17, 2023 at 19:30
  • \$\begingroup\$ Hummm. It looks like the ignition coil is really a transformer, with the primary being the smaller winding on the right. When TR2 opens up, that interrupts the current flow in the small winding. The collapsing magnetic field then induces a voltages in the bigger, secondary winding that fires the spark plug. \$\endgroup\$
    – SteveSh
    Commented Jan 17, 2023 at 20:59
  • 1
    \$\begingroup\$ @Bryan Good eye. That’s exactly what it is. The one in the picture is from a Supra, the one I’m working on is out of a Cressida, one of the most computer-heavy cars of the 80s \$\endgroup\$
    – Jayj48
    Commented Jan 18, 2023 at 13:59

3 Answers 3


The internals of the symbol called the igniter do not have a direct mapping to the circuit shown in the photo. The internal features in the schematic do not have any real meaning.

The photo shows the igniter as a "Thick-film hybrid" circuit where resistors and wiring are printed on an alumina substrate with discrete capacitors and a separate transistor die. The silver traces (overlaid in green in some areas) are the conductors, the black rectangles are resistors. The square in the middle is an IC with 16 pins.

The base of the transistor at the bottom of the photo is driven by the IC - it is not driven directly by any the input signals.

The input signals to the hybrid provide power and a trigger to IC that will provide a controlled drive to the transistor. It probably also checks for failure conditions by measuring the voltage at the collector. It may also regulate the base current depending upon the collector voltage.

The connections to the transistor at bottom of the photo will be:

  1. The collector is the heavy gauge copper colored wire.

  2. The emitter is the wire bonded to the larger of the arrays on the die.

  3. The base is the remaining smaller array connection. It can be seen how the base and emitter are interdigitated to provide a large active region with high current capability.

The collector is insulated from ground by being mounted on a small alumina pad to provide heat sinking but electrical insulation.

  • \$\begingroup\$ Nice piece of detective work. I've never seen a hybrid like that that wasn't hermetic. \$\endgroup\$
    – SteveSh
    Commented Jan 18, 2023 at 1:28
  • \$\begingroup\$ @SteveSh By saying ‘hermetic’ do you mean the circuit was sealed shut? There was a lid that was glued shut and the whole circuit is covered in some kind of gel. Just a note, the one in the photo is not the one I’m trying to test because I didn’t want to break the original. It is the same brand, age, and function, just a slightly different shape \$\endgroup\$
    – Jayj48
    Commented Jan 18, 2023 at 1:50
  • \$\begingroup\$ I would call that "hermetic light", or "mostly hermetic". It's not hermetic in the sense that mil/space hybrids are hermetic. \$\endgroup\$
    – SteveSh
    Commented Jan 18, 2023 at 2:11

IGT stands for "ignition trigger".

IGF stands for "ignition feedback".

The little block on the IGF line is likely to be voltage scaling and overvoltage protection, perhaps an active buffer.

The block on the IGT line is the same, including a driver stage. Those bits are implemented on the IC in the middle of the hybrid.

Why does this circuit need two voltage sources?

It doesn't really. ECU provides properly timed ignition pulses. The W-R line provides 12V when ignition is turned on. The driver inside the igniter needs the 12V supply and the ECU pulses to drive the transistor Tr2 properly. There's only one supply source for Tr2 driver: ignition voltage from W-R circuit. There's only one source of ignition timing pulses: W (IGT) line from ECU.

I don’t understand why TR2 has two voltage sources going to its base.

There is only one signal going to the base. The block it comes out of has a few more transistors that shape the pulses from the ECU and amplify them, and perhaps also protects the coil from a "stuck ON" condition should the ECU get disconnected or fail.

I have considered that the boxes on the right side of the igniter on IGT and IGF lines could be voltage regulators

They are not really voltage regulators, but they are signal conditioning.

If you'd connect B-R directly to ECU's IGF terminal, the ECU inputs would be destroyed probably.

The ECU also doesn't have enough drive strength to control the coil grounding transistor Tr2 - and its output is 5V open-emitter through a protection series resistor. One or two more transistors are needed to develop enough current and the waveform shape needed for coil grounding. Those are in the IC in the center of the hybrid substrate.

  • \$\begingroup\$ Ok I think I understand, tell me if this is wrong. The 12v is the voltage source that activates TR2 (thus grounding the coil). The 5v from the ecu is a signal that controls if the 12v goes to TR2 or not. \$\endgroup\$
    – Jayj48
    Commented Jan 18, 2023 at 19:13
  • \$\begingroup\$ @Jayj48 You got it! \$\endgroup\$ Commented Jan 19, 2023 at 13:33

I've never seen a PCB assembled quite like that. It's very unusual.

enter image description here

  • I'm guessing that Tr2 is the yellow square at the bottom of the image and that C, B and E are as labeled in the image below. That would make the striped copper strip the resistor to ground on the right.
  • C is connected to the terminal on the bottom left which must be terminal 5 on the schematic.
  • If that's 5 then the terminals might be 1 - 5 running anti-clockwise from the top.
  • That would make the mid-left terminal 3 which should be connected to the mystery symbol. The only thing I can think of is that the symbol represents the bonded wire jumper but what that's doing I don't know.
  • \$\begingroup\$ You said it looks unusual. If it helps, it’s Japanese and from 1986. And yes the terminal at the bottom left is 5. Should 5 have continuity to ground when IG s/w is closed? \$\endgroup\$
    – Jayj48
    Commented Jan 18, 2023 at 1:59
  • \$\begingroup\$ This is not a PCB, but a hybrid-hybrid assembly. The ceramic substrate with the IC in the middle (black square) is one hybrid. Functionally it’s like a single-sided PCB perhaps, but much more heat resistant. That is then hybridized again with a separate transistor. Both are on the common heatsink. These are found, even in test equipment, although there the hybrids usually have everything needed on them and are not wire-bonded to external components. The most common hybrids are resistor networks. Look at a SMT resistor array - it’s a tiny hybrid! \$\endgroup\$ Commented Jan 19, 2023 at 13:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.