Relay - Positive Peak Voltage

[Karipain99]

Hello everybody!
Please, who can explain me what is causing the various positive voltage peaks when energizing the relay! (please see photo attached) .


The power supply is +24Volt and a 42.2 V peak to peak is captured.
Thanks In Advance.


Karim.

 

[barry]

I assume this is an electromechanical relay. I would expect to see the peaks when the relay is de-energized, are you sure that's not what you're showing?

 

[jiripolivka]

It is the relay-coil inductance. When the current is interrupted, it causes so-called kick-back voltage that you see on your scope.
You do not show relay coil circuit. The kick-back is usually suppressed by connecting a diode in parallel to the coil (or to the transistor driver), so the coil voltage cannot exceed the DC power supply voltage.
Use e.g. 1N 4006 diode which can "swallow" the unwanted pulse.

 

[Karipain99]

Thanks guys for your quick answer!
I confirm that this happen when energizing the relay with 24 volt (you could notice how on the oscilloscope the pulse is set to high after the oscillating peaks).
I know about the kick back voltage but like I said here is when turning it on. (for info, this relay has the flywheel diode).
I also noticed this phenomenon on 3 different relays.
Thanks.

 

[barry]

Can you show us your circuit? HOW are you energizing the relay? It really shouldn't look like that at turn-on, unless you've got some contact bounce.

 

[arouse1973]

If this is being measured on the D.C power line then it looks like overshoot possibly caused by relay inrush current and supply line inductance. but as you haven't supplied any circuit diagram or pictures of your setup I can only suggest this as an observation.
Thanks
Adam

 

[Karipain99]

Hello Guys,
the circuit is very simple, here is the attached:

Thanks.

 

[KlausST]

HI,

where is your scope connected?

Klaus

 

[Karipain99]

Hi Klaus,
It is connected on the coil, between the positive of the coil and the ground (cathode - anode of the diode).
The ECM and the relay has the same ground.
Thanks.


Karim

 

[betwixt]

I don't suppose this is something as silly as a probe compensation setting. There is no reason why the voltage should spike unless the incoming voltage is higher and drops to 24V under load.

Brian.

 

[jiripolivka]

From your circuit I can see the probable problem source: no blocking capacitor across your 24 VDC line to GND. Connect as close to your relay coil to get the overvoltage suppressed. You may need ~10 uF plus 10 nF in parallel.

 

[arouse1973]

Yes the capacitor may fix the problem. But the OP asked what was causing this. The cause is probably as I mentioned in my last post the inrush current of the coil and externals inductances. When you first connect the power to the circuit the power supply will see a very small inductance in series with a very large inductance and a parallel capacitor. This capacitor forms a series tuned circuit with the input inductance of the power supply and any wiring you have. If you don't have any input inductance you will not get this issue of overshoot and ringing.
Thanks
Adam

 

[barry]

Um, as I said in post #5, if you've got some contact bounce JUST LIKE YOU WOULD GET FROM A SWITCH!!!, the coil would be energized and deenergized rapidly which would cause the inductive kickback that you are seeing.

 

[arouse1973]

Hi Barry
I tried this out with a small relay and I get the same. If you look at the scope pic in the first post you get this before the switch bounce starts. I also had this on mine, on the initial switch on pulse. Also if he is measuring from the top of the coil, how do you get a positive pulse, surely it's negative because one end is tied to 0 volts. The diode takes care of this across the coil. BTW you also get this effect even if you don't have a switch, no bounce but same overshoot.
Adam

 

[Karipain99]

Thanks Adam for the explanation,
I put a poralized 4.7uF parallel to the coil and now it is OK.
Thanks 2 to all of you.

Karim

 

[arouse1973]

NO PROBS GLAD WE COULD HELP!!!
Adam