A little rusty on my transistor theory...

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
228
I'm just trying to get back in the saddle so to speak when it comes to semiconductor devices. I haven't touched board-level stuff in the better half of a decade. I'd now like to tackle a couple of projects which require that knowledge once again.

At the moment I'm trying to regain my grasp on BJT transistors. NPNs & PNPs. An NPN where current flows into the collector and base and out of the emitter... and a PNP where current flows into the emitter and out of the base and collector... right? Maybe?:p
Now what's boggling me at this moment is trying to understand whether an NPN would conduct and/or sustain damage if a reverse polarity of say... exactly 100% of it's forward voltage rating were to be applied from emitter to collector. Would this harm anything? What exactly would occur in this sort of situation? Would a diode be necessary to protect the transistor from this reverse polarity or would the second N-P junction serve the function of a diode?

IMG_20210612_014256809.jpg

Thanks.
 

Ian0

Joined Aug 7, 2020
3,754
The two PN junctions do indeed act as diodes.
As it won't work as a transistor reverse biassed, just consider the two diodes.
So the collector-base junction would be forward biassed (and would drop 0.6V)
This would leave the almost the entire voltage across the base-emitter junction which would now be reverse biassed.
Base-emitter junctions typically withstand only about 7V - it won't like it!
 

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
228
What would be a good way of protecting against reverse potential situations like this without introducing undue forward voltage drop losses?

Would a field-effect transistor in conjunction with a diode be an ideal solution? Are FETs adversely impacted by reverse voltage in similar fashion?
 

Ian0

Joined Aug 7, 2020
3,754
MOSFETs have a diode between source and drain. It would appear as a short circuit if reverse biassed. If the voltage source had low enough output impedance it might destroy the MOSFET due to over-current. The gate-source and gate-drain connections appear as capacitances so they would be OK (provided that the MOSFET survived the possible over-current).
JFETs don't have a source-drain diode - they would fail in a similar way to the bipolar transistor.
 

Ian0

Joined Aug 7, 2020
3,754
What would be a good way of protecting against reverse potential situations like this without introducing undue forward voltage drop losses?
That's a tricky question - I suppose it depends on the circumstances in which it could become completely reversed. Are you thinking of power-supply reversal going into the circuit?
 

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
228
More along the lines of a regular operating occurance. E.g. a transistor in series with a freewheel diode placed across an inductor. The purpose of said transistor being to dynamically control the dissipation of energy stored in the inductor's field.

Fully conductive - low energy dissipation. The inductor can be driven normally via PWM.

Partially or fully resistive (taking care not to exceed the device's max power dissipation or forward voltage ratings) - high energy dissipation. The transistor acts as a 'brake' via a suitable control circuit to bring the inductor current down rapidly.
 

Ian0

Joined Aug 7, 2020
3,754
The base-emitter circuit would generally remain correctly biassed - it is just the collector that could reverse. A MOSFET with its reverse diode may be a better choice than a bipolar.
I think the most common problem in that type of circuit would be overvoltage which could generally be dealt with by the usual flyback/freewheeling diode or transorbs.
 

sparky 1

Joined Nov 3, 2018
607
Safety first with that stuff, Video shows sizzling bacon sound of AC without freewheeling action that you described. A flyback driver can use a diode for handling back EMF to cause the current to flow in one direction rather than AC. I tried to find a video that shows significant output, seeing rather than just explaining because in these circuits we might start out with an approximation based on a video demonstration the length of the arc and the sound. In this case the frequency comes from the 555 timer circuit. Because the circuit can be broken down, so that way you could simulate the driver on LTspice and take the schematic to PCB or protoboard having an idea about transistors that work or you may want to modify or start from scratch.
Also I'm not sure that the circuit can actually do what is being shown closeup even though the transistors do look promising.

 
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