Help redesigning simple ignition coil driver cct to use NPN transistor instead of PNP

[radioshack]

Here's the original circuit:

https://www.electro-tech-online.com/custompdfs/2011/06/TransIgn.pdf

It's simply designed to ensure that on an old engine with the spark controlled by points the points will not have to carry the current for the coil.

I have access to some BUV46 NPNs
http://www.datasheetcatalog.org/datasheet/SGSThomsonMicroelectronics/mXyzqqvu.pdf

Vce = 400V, Ic = 5A

So I think I can use two in parallel to allow the Vce = 320V, Ic = 10A needed for the original design.
I'm confused as to how to drive the NPN bases in this alternative circuit.

Any help much appreciated!

 

[ericgibbs]

hi,
Did you notice in the original circuit that the power PNP is a Germanium type and your datasheet is for Silicon.??

 

[ericgibbs]

hi
Datasheet for MP3731

 

[crutschow]

To use an NPN transistor you would have to use an added inverter stage to invert the signal from the points.

The data sheet for the BUV46 shows an "on" saturation voltage of 1.5V at 3.5A. This is much higher than the germanium transistor and will dissipate significantly more power as well as reducing the voltage available to the coil.

 

[radioshack]

Thanks all for your input.

 

[radioshack]

hi,
Did you notice in the original circuit that the power PNP is a Germanium type and your datasheet is for Silicon.??

Sorry - No, I hadn't noticed. As you I'm sure can gather I'm not at all well versed in this stuff.

To use an NPN transistor you would have to use an added inverter stage to invert the signal from the points.
The data sheet for the BUV46 shows an "on" saturation voltage of 1.5V at 3.5A. This is much higher than the germanium transistor and will dissipate significantly more power as well as reducing the voltage available to the coil.

So, if anything I should find a 300V+ NPN with a lower switch-on power & I'll still need an inverter stage - yes?


Can anyone explain to me the role of the 2.5mH Choke in this circuit?
Is this protecting the damaging the PNP from the voltage ramping up to violently?
There's clearly a usable energy store over the time the choke is in cct, but is it protecting the other components or supplying the stored energy to the coil?

Sorry if I've totally misunderstood!

 

[crutschow]

So, if anything I should find a 300V+ NPN with a lower switch-on power & I'll still need an inverter stage - yes?


Can anyone explain to me the role of the 2.5mH Choke in this circuit?
Is this protecting the damaging the PNP from the voltage ramping up to violently?
There's clearly a usable energy store over the time the choke is in cct, but is it protecting the other components or supplying the stored energy to the coil?

Yes.

I believe the 2.5mH choke is just to provide a path for the base leakage current (which is much higher with a Ge transistor than a Si device) when the transistor is turned off. But it's not clear to me why a choke is used instead of just a resistor since the time-constant due to the choke and its intrinsic resistance is short (about 200µs) with respect to the maximum circuit operating frequency.

I built a similar circuit way back in the 60's for a 56 Chevy I owned but I don't remember it having the choke. I also remember using a different coil optimized for use with a transistor circuit that had a turns ratio of about 400:1, I believe, rather than the standard 100:1. This reduced the voltage flyback spike on the transistor when it turned off. But in those days, high voltage transistors where not common and quite expensive.

Edit: Upon further reflection I have determined the purpose of the choke. When the points are closed, it conducts current based upon its resistance and the base-emitter voltage drop. When the points open this current continues for a short interval and generates a voltage across the two "D" diodes that reverse-biases the base-emitter junction. This sweeps out the carriers in the transistor base region, and reduces the storage and turn-off time of the transistor.

 

[radioshack]

Thanks Carl.

Another forum suggested, "I assume that the inductor's purpose is to achieve maximum fast transistor turn-off" - but I can't comment on whether this is true or not.

Yes - the description actually says that this is a straight-in swap for a non-electronic coil (I assume with many less turns) - so maybe that can go some way to explaining the role of the inductor too.

 

[crutschow]

Another forum suggested, "I assume that the inductor's purpose is to achieve maximum fast transistor turn-off" - but I can't comment on whether this is true or not.

Yes - the description actually says that this is a straight-in swap for a non-electronic coil (I assume with many less turns) - so maybe that can go some way to explaining the role of the inductor too.

Yes, as my edit states, the inductor's purpose is to reduce the turn-on time. That's especially needed for the old Ge power transistors which were very slow.

Don't think it has anything to do with the ignition coil.