# Igniton coil and NPN Transistor

#### vandaycalta

Joined Mar 22, 2016
53
Hello all.

I have an HEI ignition coil w/ primary resistance of .4 ohms (aftermarket coil, my stock gm measures 1.0 ohm), as measured the inductance on the primary is 9.47mH. I want to use a NPN darlington. If my Vcc is 12v, how do I know what the voltage will be when the mag field collapses so I know what npn darlington to use. I see the ST bu941zt w/ the zener diode that clamps at 300v min. but dont know how to know if it will work...how do I calculate that? I know it will be about 300-400 volts but dont know how to calculate this or find out how to do this? I know 12v/.4ohms = 30 amps but the ST bu941zt cant handle this for collector current.

Suggestions?

Val.

#### ScottWang

Joined Aug 23, 2012
7,179
Do you have the circuit or draw one?

#### crutschow

Joined Mar 14, 2008
29,520
The primary flyback voltage depends upon the output spark-gap voltage and the turns ratio. It's not a direct function of the primary supply voltage or the coil inductance.
What is the coil turns ratio?
The spark gap voltage can go as high as 40,000 volts. You divide that by the turns ratio to get the peak primary voltage.
Worst case is if you have an open circuit secondary where the voltage can go much higher. That's why the transistor type you posted has a built-in zener. It conducts to save the transistor (and possibly the coil) if the coil output is open circuited.

You will have to add a power resistor in series with the coil to limit the current to be below the maximum current rating of the transistor.

#### ian field

Joined Oct 27, 2012
6,536
The primary flyback voltage depends upon the output spark-gap voltage and the turns ratio. It's not a direct function of the primary supply voltage or the coil inductance.
What is the coil turns ratio?
The spark gap voltage can go as high as 40,000 volts. You divide that by the turns ratio to get the peak primary voltage.
Worst case is if you have an open circuit secondary where the voltage can go much higher. That's why the transistor type you posted has a built-in zener. It conducts to save the transistor (and possibly the coil) if the coil output is open circuited.

You will have to add a power resistor in series with the coil to limit the current to be below the maximum current rating of the transistor.
AFAIK: most car coils need a ballast resistor, its usually switched out for a bigger spark during cold cranking. Originally it was a points thing - if the engine stalled with the points closed, there could be enough current to burn out the LT winding.

Most motorcycle coils I've seen were PTC wound, they started off drawing about 8A at room temperature, the current sank to around 4A as it warmed up.

Years ago I got away with using a 9A 900V MOSFET for ignition. But it was a "wasted spark" twin - so there was no opportunity to run the engine with the HT winding unloaded.

#### vandaycalta

Joined Mar 22, 2016
53
The primary flyback voltage depends upon the output spark-gap voltage and the turns ratio. It's not a direct function of the primary supply voltage or the coil inductance.
What is the coil turns ratio?
The spark gap voltage can go as high as 40,000 volts. You divide that by the turns ratio to get the peak primary voltage.
Worst case is if you have an open circuit secondary where the voltage can go much higher. That's why the transistor type you posted has a built-in zener. It conducts to save the transistor (and possibly the coil) if the coil output is open circuited.

You will have to add a power resistor in series with the coil to limit the current to be below the maximum current rating of the transistor.
I did a little further research and found my turns ratio to be 1:133 (step up) (aftermarket coil).
Not quite familiar w/ the "spark gap voltage". Could you explain how I would go about finding that? Is it something that can be calculated or must it be done in a functional test.

Thanks again,
Val

#### shortbus

Joined Sep 30, 2009
9,328
AFAIK: most car coils need a ballast resistor
Here in the States, since HEI (high energy ignition) they get full voltage all the time. And they are now an E-coil configuration, not the old round style. The ballast resistor went by the wayside too, before the switch to HEI. It was replaced by a long length of stainless steel core wire, folded back on itself and in the engine harness.

#### crutschow

Joined Mar 14, 2008
29,520
I did a little further research and found my turns ratio to be 1:133 (step up) (aftermarket coil).
Not quite familiar w/ the "spark gap voltage". Could you explain how I would go about finding that? Is it something that can be calculated or must it be done in a functional test.
Here is a summary of sum published spark plug voltages.
They generally have to be measured, since the voltage depends upon a number of variables, such as the gap distance of the plug and the compression pressure.

For your purposes you want to used the highest value observed.
So, for example, with your 1:133 coil, 35kV at the secondary would generate 35kV/133 = 263V at the primary.
So your transistor with a 300V clamp should be fine.

#### ian field

Joined Oct 27, 2012
6,536
I did a little further research and found my turns ratio to be 1:133 (step up) (aftermarket coil).
Not quite familiar w/ the "spark gap voltage". Could you explain how I would go about finding that? Is it something that can be calculated or must it be done in a functional test.

Thanks again,
Val
Car accessory shops used to sell a kind of calliper gage that opens a spark gap and reads the voltage off a dial at the point the spark gets intermittent.

Haven't seen one for years.