Singing the PNP Blues

[Willbe]

since the circuit will eventually go into an automotive environment.

...Vehicle electrical systems, when the engine is running, have a "load dump transient" of several hundred negative volts.
...Bosch [among others] may have specs on the expected envelope for this "dirty" power.
...You'll need power conditioning at a system level for your circuitry; diodes, chokes, capacitors, maybe an IC regulator.

 

[rezer]

If in Hi-Z, the I/O pin allows enough current to forward bias the E-B junction of the transistor, then I'd say you're right, but I doubt that it does.

As a precaution, I guess one could put a diode on the output of the I/O pin to prevent that from happening.

Allowing the transistor to turn on in high-Z state is not the issue. You don't want to exceed the max input voltage to the IC, which in this case would be 12V with a pull-up resistor.

 

[Analog]

Please tell us how this diode should be fitted, pointing to the I/O pin or pointing to the PNP base?

My poorly worded comment, I meant a breakdown (zener) diode from the input pin to ground to clamp the input voltage. This may be a circular argument, because if we believe the premise of the purpose of this type of clamp, then it would mean the transistor would never turn off. I would like to experiment some with this just to see what happens.

 

[Analog]

Allowing the transistor to turn on in high-Z state is not the issue. You don't want to exceed the max input voltage to the IC, which in this case would be 12V with a pull-up resistor.

Yes, I agree.

 

[Roff]

There are a couple of other two-transistor configurations that will work.

 

[picbits]

Look at the High Side power switchers from ST which have been designed for the car environment.

Makes life so much easier for switching larger currents.

If its only a small current you need to switch then go with the above suggestions

 

[piper91765]

In figure two, didn't the OP just make a mistake in his schematic, leaving the switch, which represents his uC output, high. In the verbage, he has the output being low, which would work, wouldn't it?

No, if the output is low, he is correct in saying that it will never turn off. The base is never more positive than the collector. What if you flipped the transistor in figure 2 upside down so that the emitter is connected to ground. then the base-emitter drop is dependant on the uC, not the source.

Try that.

 

[piper91765]

oops, I think that'd make it inverting.

Just follow this example from wikipedia if you want the transistor on the high side of the load.

https://en.wikipedia.org/wiki/Common_collector

Why switch to pnp?

 

[Roff]

oops, I think that'd make it inverting.

Just follow this example from wikipedia if you want the transistor on the high side of the load.

https://en.wikipedia.org/wiki/Common_collector

Why switch to pnp?

But the NPN emitter will always be about 0.7V lower than the base. With a 3.3V supply on the UC, the emitter will never go above about 2.6V, regardless of the voltage on the collector.

 

[kchriste]

If in Hi-Z, the I/O pin allows enough current to forward bias the E-B junction of the transistor, then I'd say you're right, but I doubt that it does.

They are MosFet Push-Pull outputs not bipolar transistors. When in Hi-Z mode the P and N channels MosFets are turned off but still connected between the Output, Vdd and Ground. MosFets have an intrinsic diode between the Drain and Source terminals that will conduct if the pin is driven above Vdd or below Vss by more than 0.7V.

 

[Atomsk102]

Just follow this example from wikipedia if you want the transistor on the high side of the load.

https://en.wikipedia.org/wiki/Common_collector

Why switch to pnp?

According to that article "a small voltage change on the input terminal will be replicated at the output".

I need current and voltage gain from this circuit. It looks like the Common Collector configuration will only provide current gain.

To use this circuit, I'd still have to put another NPN between the I/O pin and the base of this one.

 

[Atomsk102]

...Vehicle electrical systems, when the engine is running, have a "load dump transient" of several hundred negative volts.

So I'll need something like:

**broken link removed**

Thanks

 

[rezer]

So I'll need something like:

**broken link removed**

Thanks

Yes, to protect against transients.