Zener amplified AC Clipper w/ darlington pair

I'm trying to test my understanding of darlington pair use in a shunt voltage regulator. Attached is the circuit and datasheets for the transistors I'm using. This circuit is a Zener amplified AC Clipper. The frequency range would be 3000 Hz or less. The PNP side shunt regulates the positive alternation of the waveform. The NPN side shunt regulates the negative alternation of the waveform. I've added diodes to the base connections for protection. The collectors are all at ground since the cases will be against grounded heatsinks. The load is varying but resistive. I've included what I believe are max base currents. Voltages are included to determine the value of the zener.


What kind of problems would you see in this schematic?
Would crossover distortion be an issue since at crossover neither amplifier is conducting?
Does any more biasing need to be added to the zeners?
How can this circuit be improved?

Ed

The base-emitter junctions of the NTE 180 and NTE181 will break down when reverse biased, and the current will flow through the forward-biased C-E junction to ground. The desired clipping action may never occur. The only way I know to avoid this is to add a BF diode in series with each emitter.
What limits the input current?

The AC voltage input will drop down to 14VAC when the load and the shunt are pulling 20 Amps (this is worst case).

I was thinking that the reverse biased diode at the base of the NTE180/181 would prevent any reverse bias current. i.e. With the positive alternation at the emitter of the NTE181 and no current flow through the base there was no excessive reverse bias on the base emitter junction.

I've added the emitter diodes...

Do I still need the diodes at the base of the NTE180/181?

Of course the transistors should be on heat sinks..........

osuvstar said:

I was thinking that the reverse biased diode at the base of the NTE180/181 would prevent any reverse bias current. i.e. With the positive alternation at the emitter of the NTE181 and no current flow through the base there was no excessive reverse bias on the base emitter junction.

I've added the emitter diodes...

Do I still need the diodes at the base of the NTE180/181?

Click to expand...

See the attached drawing showing the B-E "zener" and the C-B diode. Note that a series base diode will not prevent B-E breakdown, nor will it prevent the breakdown current from flowing through the B-C junction if the collector is at a lower voltage than the base.

So no, you don't need the series base diodes.

Here is the circuit with the changes.....Am I ready for some fabrication and a mockup?

Thanks

Ed

You need to separate the zeners and bases, otherwise the C-B junctions short out the zeners. See attached.
I used transistors and diodes that I could find spice models for.
The resistors were added to raise the zener currents, and make them less susceptible to transistor betas. The values may have to change, depending on your transistors and your zeners.

Can I simply substitute a canned darlington instead of the separate transistors? This should reduce my component count. The MJ11028 looks interesting. The datasheet is attached.

I believe I should reduce the regulated voltage to 12V. With a possible 15 to 20 amps, I want to to make sure I'm staying within the SOA curve. Also the transistors will be mounted on a 6.5cm x 9cm 1/4" aluminum plate to distribute the heat.

Would resistor values to provide about 5mA of zener current be OK? If I'm reading the datasheet correctly the Vbe across the resistor would be 3V?

Thanks
Ed

Before we go farther, can I ask why you want to clip a high-power AC source? I want to make sure we're not wasting our time with a complicated solution to what might be a simple problem.