cascode switching

Yes, the top diagram is completely wrong.

You don't show any ignition condenser in your schematic. Did you remove the condenser from the points (you should)? I think the stresses on the transistor would be reduced quite a bit if you reconnected the condenser to the collector.

There are 2 problems with this circuit. The first is it doesnt make sense. The tr does nothing of any use. Lose the tr, connect coil direct, and it'll work fine. The 2nd is the b/e jn will see massive ac voltages as the coil LC rings, so no wonder it died.

Thats hard to imagine, connecting c to b is generally regarded as a Bad Idea.

No, yes, and in both I think.

NT

Check this out

Let me show you and example of cascode switching. Look at figure two in the datasheet in this link.

Let me show you and example of cascode switching. Look at figure two in the datasheet in this link.

The schematic is not all that bad. The transistor has a current gain less than 1, in that the points have to carry both the coil current and the base current, but when the points open, the points have to interrupt way less voltage than when they are connected directly to the coil. So the transistor provides voltage gain.

At that moment the points open, all the coil current is detoured through the base as reverse current, turning the transistor off very quickly. This is great for fast switch off, but may be so fast that the voltage will peak extremely high, because not all the coil primary inductance is coupled to the secondary, so the collector voltage will be proportional to the secondary voltage, but will have an additional leakage inductance component. Slowing the rise of the coil voltage with some capacitance (possibly a little less than the original point condenser) will help the transistor get fully switched off before full voltage is reached (lowering its switching losses), and lowering the peak voltage the collector-base junction has to withstand.

I removed the condenser from the points of course. I didn't put a cap across the transistor because I never had to do that on any of the other ignition circuits I built. I wonder if there's something about cascode that escapes me. First time using it.

Put the cap across the COIL.

...Jim Thompson

Thanks for reminding me I could do that. If anybody's interested, I went with this circuit because it has the lowest voltage drop of several circuits I tried. Coil driver IGBT's are robust but the ones I've experimented with seem to have a much higher voltage drop than bipolar transistors. The difference as much as a whole volt. And I didn't want to drop any voltage because this is going on a six volt vehicle. A volt is a terrible thing to waste!

Thanks for reminding me I could do that. If anybody's interested, I went with this circuit because it has the lowest voltage drop of several circuits I tried. Coil driver IGBT's are robust but the ones I've experimented with seem to have a much higher voltage drop than bipolar transistors. The difference as much as a whole volt. And I didn't want to drop any voltage because this is going on a six volt vehicle. A volt is a terrible thing to waste!

I remember Maplin Electronics publishing a book for about ten pounds. The circuit consisted of a BU205 tv line output transistor driven by BC461. The BC461 is in turn driven from the breaker points/ trigger head via a 555 timer wired as a scmitt trigger. Unfortunately this book is no longer in print so try see if local library has it if in UK. The circuit also contains RC/ varistor filtering to protect the driving components, notably the 555 (max +Vcc = 18v).

They're a bunch of nervous nellies if they think they have to use a Scmitt trigger for something like this.

debounce might be handy though.

No you're not ! Look at what the base is connected to.

Graham

John Popelish wrote in news:zsqdnamRk-_DXEreRVn- snipped-for-privacy@adelphia.com:

Carrying your logic one step further, do we not come across a second mode that might have destroyed the transistor:

Once the voltage produced by the inductive current collapse peaks, it starts back down through zero on the negative swing of the ringing wave form. At the point where the voltage at the collector drops 0.7 volts negative (referenced to its other end at B+), the collector-base junction of the transistor becomes forward biased through the 10 Ohm resistor. The collector-base junction becomes a 0.7 volt voltage clamp for all the stored energy with only a 10 Ohm resistor for a load. It would explain why this particular topology failed where the others didn't.

I'd be interested to hear your response.

Rick

I understand the situation you describe, so let's think about the magnitude of the current pushed through the base by this mechanism. The maximum forward biased emitter to base junction current was (6 volts - ~0.7 volts)/10 ohms = 530 mA. But the collector current peak is (6 volts - Vcesat)/ primary resistance, which is almost certainly quite a bit higher than 530 mA. The current peak on the first rebound will be lowered from the original peak, by the energy that went into the spark, so it is a bit of a guess for me to know how much current we are talking about.

Hooking a scope probe to the base lead would be instructive.

I still think the worst thing the circuit does to the transistor is to try to force it off while the primary is applying maximum voltage to the collector. The load line shaping provided by a bit of parallel capacitance really unloads this power dump into the collector each pulse by getting the transistor almost completely turned off while there is just a modest voltage on the collector, and then limiting the peak voltage as the energy in the leakage inductance charges the cap, instead of being consumed as heat by breaking down the transistor with over voltage.

That the points are likely to bounce and wiring i.e. inductance.

A cascode is FAST, when the emitter opens the collector current is directed through the collector-base junction until the collector-base storage has been swept out. This takes a couple of usec. After that, the transistor will switch of in a few nsec causing the collector voltage to shoot up.

Antisaturation diodes helps with the stored charge and a decoupling capacitor on the base-ground will let the charge be dumped cleanly without "exciting" long wires with usec current pulses of many A and fast risetimes.

If he switch bounces, the transistor will switch a high voltage few dosen times every cycle - it might not like that.

An oscilloscope with decent probes would be useful.

yours looks like a misunderstanding of that one

NT