# back emf of relay from regulated supply

Started by December 3, 2012
```If a relay coil is powered by a regulated power supply, will the back emf g=
enerated by the coil cause any damage to the regulator as cathode of protec=
tion diode is at o/p of the regulator? From what I read about driving relay=
s using trnasistors the protection diode protects the transistor from the b=
ack emf genereated by the relay by diverting the back emf away from the trn=
sistor. Now it goes to the o/p of the regulator powering the relay coil.

thanks
bhav
```
```On Sunday, December 2, 2012 11:53:48 PM UTC-5, bhav...@gmail.com wrote:

The clamp diode allows the relay current to circulate in a loop from one relay terminal to the other until it decays to zero. It does not put current back into the regulated supply.
```
```On Sun, 02 Dec 2012 20:53:48 -0800, bhav.jnk wrote:

> If a relay coil is powered by a regulated power supply, will the back
> emf generated by the coil cause any damage to the regulator as cathode
> of protection diode is at o/p of the regulator? From what I read about
> driving relays using trnasistors the protection diode protects the
> transistor from the back emf genereated by the relay by diverting the
> back emf away from the trnsistor. Now it goes to the o/p of the
> regulator powering the relay coil.
>
> thanks bhav

The usual circuit puts the diode in parallel with the relay such that
when the transistor turns off the relay is dumping as much current into
the voltage supply via the diode as it is consuming via its other end.
So the net current from the relay/diode combination drops to zero, rather
than back-feeding the regulator.

(The usual circuit makes the relay slower to release, too -- because the
coil only sees one diode drop, the current decay is pretty much
determined by the relay coil's resistance.  Putting a resistor in series
with the diode, sized so that the transistor VCE spec isn't violated,
will speed things up.  So will putting in a zener instead of the
resistor, with the correct voltage and polarity).

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
```
```<bhav.jnk@gmail.com>
>
If a relay coil is powered by a regulated power supply, will the back emf
generated by the coil cause any damage to the regulator as cathode of
protection diode is at o/p of the regulator?

** Long as there is an electro cap at the output of the PSU, it will absorb
back emf energy from a relay - so there is no switch off voltage spike.

With a transistor and no diode or cap, such spikes will overvoltage the
collector.

....   Phil

```
```On Sun, 2 Dec 2012 20:53:48 -0800 (PST), the renowned
bhav.jnk@gmail.com wrote:

>If a relay coil is powered by a regulated power supply, will the back emf generated by the coil cause any damage to the regulator as cathode of protection diode is at o/p of the regulator? From what I read about driving relays using trnasistors the protection diode protects the transistor from the back emf genereated by the relay by diverting the back emf away from the trnsistor. Now it goes to the o/p of the regulator powering the relay coil.
>
>thanks
>bhav

Are you having problems?

You might want to have at least a small bypass capacitor on the output
of the regulator.. the distributed capacitance in the relay coil could
otherwise cause the regulator voltage to go up a bit.. I doubt it
would damage the regulator but it's good practice.. there are other
ways that stuff can get coupled in with a mechanical relay.

Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
```
```On 2012-12-03 05:53, bhav.jnk@gmail.com wrote:
> If a relay coil is powered by a regulated power supply, will the back
> emf generated by the coil cause any damage to the regulator as
> cathode of protection diode is at o/p of the regulator? [...]

Forget about back-EMF, for a moment. The first thing to consider
is that the relay coil is an inductor and will 'try' to keep
its current constant. When you switch off your relay, can this
current go somewhere without damaging anything? If so, you're
safe.

So, if you switch the relay with an emitter-follower, you don't
need a catch diode. Now, what's the power supply like and how
do you switch the relay?

Jeroen Belleman
```