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From seeing a few schematics where the flyback or snubber diode has been placed across the transistor C-E terminals (Right Configuration), instead of what I typically seen as the the flyback being placed across the coil terminals (Left Configuration).

Which of these are "correct"? Or does each have a separate purpose?

As a note, the diodes are normally listed as external 1N400x type diodes (on TIP120 Darlingtons), not the internal body diode of the BJT or Mosfet.

Final note, I have seen a few schematics that have both diodes, one across the coil and another across the CE terminals. I assume that one is just redundant without really affecting the circuit in that case, is that a wrong assumption?


simulate this circuit – Schematic created using CircuitLab

The answer to When/why would you use a Zener diode as a flywheel diode (on the coil of a relay)? touches on this slightly, by showing a regular Diode in the above left configuration, while showing a Zener Diode in the right configuration. It doesn't say that the opposite isn't true (or why) So as a second part, can a Zener work in the left configuration, and a regular diode in the right configuration? If so, how does it change how it operates?

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Woo Gold Badge 10k views. – Passerby Nov 9 '15 at 22:17
up vote 9 down vote accepted

Consider the operation of the circuit.

When the transistor is on current is flowing in the coil from top to bottom as the circuit is drawn we now switch the transistor off. The current in the coil still wants to flow.

For the circuit on the left this current can now flow back to Vcc via the diode the voltage across the coil has reversed direction and is limited by the diode the current can decay to zero safely.

For the circuit on the right the diode does not help. The current flowing in the coil will force the voltage on the collector to rise to the point where the transistor (or possibly the diode) breaks down and starts to conduct. At this point the current can start to decay in the coil but the energy in the broke down transistor (or less likely diode) will be excessive and may well result in the transistors death. Note a zener diode here will work because you allow the voltage on the coil to reverse so the current can decay to zero while limiting the voltage across the transistor to a safe value.

It should be noted the allowing the voltage across the coil to reverse to an higher voltage means the current can decay more quickly which is why you sometimes see a zener in the right hand circuit or more than one diode in series in the left hand one.

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A zener can work in both but a diode wouldn't

A zener.

For the left it would just function a diode (with some supply clamping..) For the right it will rapidly discharge the coil (if rated correctly - tvs)

A diode

For the left it will be a normal chopper with a free wheel path. For the right you have a dead transistor

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The latter can't possibly be correct. The induced current flows in the same direction that the original current did, and a reverse-biased junction diode won't help. The voltage that builds up from such a current across the now near-infinite resistance is what damages the transistor in the first place (the Zener works by allowing the current to flow past once the voltage reaches a given maximum). That the transistor is still operational after switch-off in such a configuration is dumb luck.

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The inductor is causing a high voltage peak because it the current path is interrupted. The current will try to find the a new path and until it does, it increases its voltage. Best alternative

The left circuit is the best of the two, it suppresses the voltage spike at the source. If the voltage across the inductor rises, the diode starts conducting until the energy is all dissipated in the circuit.

The right circuit attempts to do the same thing, but relies on power supply having a low impedance path. This is not always true and some voltage regulators don't like reverse current to be fed into its output. Bad alternative.

The zener or MOV alternative suffers from the same issue as the right circuit, it relies on a low impedance path through the power supply. Bad alternative.

I personally don't like 1N400x for this use because it is rather slow. For small currents (<100mA) I prefer a 1N4148 which is much faster. For larger currents I would check one of the various selection guides on Internet.

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Can you elaborate a bit more why a zener rated below the Vce breakdown placed across the collector and emitter is not a good approach ? When the relay turns on, current passes through the collector, and when the relay turns off, the collector voltage builds up until the zener breaks down and current passes through the zener to ground. Problem ? – efox29 Jul 14 '15 at 16:21
@efox29: Using a diode across the coil will cause flyback energy to be dissipated slowly, largely in the resistance of the coil windings. Using a zener will cause flyback energy to be dissipated more quickly, but most of it will go into the zener. Whether this is a good or a bad thing depends upon the thermal characteristics of the coil and the zener. – supercat Nov 11 '15 at 21:04

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