Why does this circuit need two voltage sources?

Question

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?

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

Answer 1

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.

Answer 2

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.

Answer 3

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

• 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.