I would like to add a transistor to my vw's ignition system to do the switching of the current to the coil. I have got two different possible circuits online
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(the first 2 pics) I have lots of NPN transistors from various power units, so would like to use NPN while keeping the points to do the triggering.
Came up with the 3rd circuit in the link above. I will add a resistor to limit current flowing through the points and a diode to prevent current induced in coil's primaries from messing up the transistor.
Anybody done something like this before? Any reason circuit 3 wouldn't work?
I would like to add a transistor to my vw's ignition system to do the switching of the current to the coil. I have got two different possible circuits online
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(the first 2 pics) I have lots of NPN transistors from various power units, so would like to use NPN while keeping the points to do the triggering.
Came up with the 3rd circuit in the link above. I will add a resistor to limit current flowing through the points and a diode to prevent current induced in coil's primaries from messing up the transistor.
Anybody done something like this before? Any reason circuit 3 wouldn't work?
To get a spark, you need first to "charge" the coil with the highest current it can handle. Charging coil with current takes some time, like charging a capacitor with voltage. When the max amount of current flows through the coil, you have a strong magnetic field in the coil's windings. When you break the primary circuit suddenly, the magnetic field colapses, and this change in magnetic field tries to induct current in both primary and secondary windings. But because both windings are open (secondary has a gap on the spark plugs / distributor rotor, and the primary on the points or transistor switch), the voltage will automaticaly rise until it can "jump" and close the circuit (spark occurs in either primary or seconday circuit) to discharge the coil. The voltage to the secodary will be couple of hunderd times higher (depending on thr ratio of the windings), so it will easier create a spark and discharge the coil.
If you create a path for the "discharging" current in the primary with a diode, the coil will discharge easily without creating the necessary spike to provide the spark on the secondary. So forget transistor protection with a diode.
The only thing that will limit the spike (and the spark voltage) is the capacitor in the primary, so a small enough capacitor must be used. Electronic points do not really need a capacitor. Points need it to limit the spike so that less sparking occurs on the points and so that less of the coil's energy is lost from sparking on the points (and the points will live longer with less sparking).
You will thus need a transistor with a high Vce voltage, around 300-400V. It would probably work with a Vce > 150V but you need a safety margin so that you can use a larger gap on the plugs, or remove the capacitor completely to get a stronger spark.
Circuit 3 would be much better, if the hot side of the points was connected directly to +12V, and not to the coils output. As you said a resistance is needed to limit transistor's base current and a ballast resistor is needed to limit coil current (if not already included in the coil).
Thanks for taking the time to look at the circuits and give your valuable input. I apologize for the double post, I got an error the first time and thought the message hadn't been transmitted.
The diode I had in mind would have been placed to prevent current flowing any other way, other than the right way through the transistor.
Is there any other way of protecting the transistor, other than limiting base current.?
An NPN / PNP transistor may be damaged (if correctly connected) in the following ways.
1) Over current through CE.
2) Over current through BE.
3) Over voltage to CE.
4) Over voltage to BE.
5) Over-temperature, or exceeding the power capabilities of the transistor and not using correct cooling.
- Case 1 is covered by the ballast resistor (resistor in series with the coil, some times incorporated inside the coil) and by using a transistor capable of the current.
- Case 2 is covered by a resistor driving the base of the transistor. You have to calculate the value of this resistor, because the current capability through CE, is the base current multiplied by the b constant (current amplification constant) of the transistor.
- Case 3 is covered by using a transistor with capability of withstanding high Vce, and by conditioning the shape of the spike with a capacitor between C,E.
- Case 4 is covered by feeding the points directly from 12V and not from the coil output.
- Case 5 is covered by selecting a transistor with the appropriate wattage, and using a big cooler for it.
Note that usually common NPN transistors you may have in your box from power supply applications or power amplifiers, do not have a high enough Vce voltage. You should go shopping for such a transistor, or maybe find an old TV and see if what they are using for driving the primary of the High Voltage Transformer suits your needs.
This is the schematic I am referring to (use a constant width font to display properly).
+12V -------+---------------------------------+ | | | | [ C ] | [ O ] | [ I ] | [ L ] | [ ] | [ P ] | [ R ] | [ I ] | [ M ] | [ A ] o | [ R ] | [ Y ] |- Points or electronic switch. | | | o | | | \ | / | \ | / Rl (Ballast resistor) | \ | / | \ | / | | | +-----+ | | | | | |C | | \ | Cf ===== \|B Rb (Base resistor) | ===== |-------/\/\/\/\/\--------------+ | /| | / NPN | |E +-----+ | | _____ ///// Ground
I currently use this system in my '71 Super. It is fantastic. No need for dwell adjustment as it maintains a 9mS spark at all revs.
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As an electronics tech by trade with 25 years experince, Bill is right, the transistors are very likely to self destruck with the evil back emf generated by the coil. I have a circuit for the above kit. Let me know if you would like it..
received from various members of this group (thank you to all who wrote) and my own testing in the laboratory, I can safely say circuit
3 will work BUT it wouldn't last long. A bit of protection has to be added to safeguard the transistor from effects of the coil's inductance, heat, too much current and voltage.
Anyways, I will believe this thread will point people searching for similar information in the right direction.
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