I want to know about D1 diode principle.

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Thread Starter


Joined Nov 14, 2019

I'm job seeker about HW engineer.
I have some questions.
How does D1 diode work in this circuit?
Why is D1 used in this circuit?

Can somebody help about my questions?
Thank you for reading.
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Joined Aug 21, 2008
When current is flowing through the coil a magnetic field builds up related to the number of turns and the current through the coil. When the transistor turns off, the coil's magnetic field collapses. This changing magnetic field causes the voltage in the coil to reverse and if nothing is there to limit the voltage, the voltage can easily be high enough to damage a transistor.

That is what D1 is for.

When the transistor switches off and the magnetic field collapses, causing the voltage across the coil to reverse, D1 turns on and keeps the voltage at the collector of the transistor from going higher than about 0.7 volts above the power supply voltage.

In this schematic D1 is a 1N4007 which is rated at 1,000 volts reverse voltage -this circuit only has a 12 volt power supply so even a 1N4001 which is rated at 50 volts would be ok.

In some circuits the transistor can turn off very quickly and the relay coil might not have much capacitance and in that case, the voltage on the collector can rise very quickly. In such cases a diode like the 1N4007 with its 30 microsecond reverse recovery time (turn-on delay) might be too slow to protect the transistor. In that case, a faster diode should be used.
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Joined Sep 11, 2014
It's called a freewheeling diode (analogous to a bicycle freewheel).

A bicycle freewheel allows the pedals to drive the wheel but prevents the wheel from driving the pedals, during coasting.

The freewheeling diode allows the transistor to drive the relay but prevents the relay from driving the transistor to destruction, at turn-off.

- Nandu.


Joined Jul 11, 2016
If you don't know the role of D1 in that circuit, then you are not yet ready to work as a hardware engineer.
what if despite the 1 missing fact he can write the control system for the local nuclear plant . . . ? everyone left-handed should be killed ? (they won't manage anyway)


Joined Aug 21, 2008
I think this thread has run its course and can't see anything improving if left open because we have drifted too far off topic. I apologise to all involved for the diversion.
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