# Inductive Kick

#### MannaMan86

I am trying to regulate a 3.3V power supply from a 120V socket, which is connected to a GFCI. When I plug an electronic appliance in (an electric razor), I get large inductive kickback into my power supply. Any tips on how to get ride of the kick?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
What sort of power supply are you using?

Can you give details on the magnitude of this "inductive kick" and how you're measuring (or noticing) it.

#### MannaMan86

My switching power supply produces 5.4V DC, which then goes to a 3.3V linear regulator. An outlet is connected to the 120V, 60HZ power source, which is what is fed into my power supply. When I plug an electric razor in the outlet, I measure the voltage at the 3.3V regulator output, and I get 3.3V. When I turn the razor on, I still have 3.3V. When I turn the razor off, the oscilloscope triggers and I have a voltage spike from approx. +6V to -8V for about a split second, then I have 3.3V again.

I'm trying to get rid of this spike. Ideally I'd want to keep the voltage positive at all times. I have tried snubbing circuits, but I am not certain how to connect it.

Thanks.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Place a schottky diode in series with the output of the 5.4V adapter, then place across the 5.4V rail a 100uF and a 0.1uF capacitor (in parallel).

As an alternative to the diode, you could also place a choke (an inductor) in series with the +ve lead.

This should combat the problem if the spike is coming via the power line, and through your 5.4V regulator. It is also possible it is being picked up some other way. If this is the case, this solution may not work. (It is even possible it is being picked up by the oscilloscope and that without that, the spike would not exist)

#### MannaMan86

Thanks for the tips - will try it out. Always trying to learn new stuff with electronics.

#### MannaMan86

The problem may be coming from ground. The last try didn't work, with the Schottky and the snubber.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
How do you know that the spike is really on the 3.3 volt rail? Is the device connected to it resetting?

Has the diode and capacitor made *any* difference?

What is the input voltage to the regulator now?

Is there a capacitor on the output of the 3.3 volt regulator?

#### MannaMan86

Steve,

I appreciate your help on this! I found a solution that keeps my 5V and 3.3V regulators working, and also rids (rather, dampens significantly) the spike I was seeing before.

I put a Schottky diode in reverse-bias position in series with my 5.4V DC switching supply. In parallel with that, I put a snubber consisting of 100 Ohm and .1uF in parallel. Further, I got rid of a capacitor that was lingering from a previous IC I had taken out, and I reduced my input supply voltage using a 100 Ohm resistor (as opposed to the 22 Ohm resistor I was using previously). All is looking well now.

#### MannaMan86

Steve,

My circuit started having problems again, so I tried your method. I think, the first time I tried it, I used a 100 Ohm resistor, instead of a 100uF in parallel with .1uF capacitor. It seems to be working fine now. Thanks.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
It would be interesting if you can show us the circuit you're now using.

#### MannaMan86

Steve, I can't put the circuit up here for security reasons, but a simple inductor in series with the diode was able to do the trick just fine. No zeners, no parallel combinations.