surge protection for the PIC

Nick · Guest

This is more of an EE question than a compiler question, so please direct me to a good forum like this one, but for EE questions.

Here is my question, I will be installing a pic and running off 120 vac fromt he wall, I was told by the head engineer that I should install some MOV to the system. After looking up the MOV I'm still not sure what side to get.

I know I should get atleast one rate for 150volts. Atleast thats what I think. All the help would be great!

Nick

newguy · Joined: 24 Jun 2004 Posts: 1907

If you put an MOV rated for 150Vrms on the primary, it should be fine. If you're really paranoid about surges, then you can also put another MOV somewhere downstream of the mains, likely at the input to whatever regulator you're using.

Nick · Guest

the pic is going to turn a solid state relay on which start the motors. Should I place any MOV on those lines? could I put a diode on the pin going out relay, shouldnt that only let the voltage go in one direction?

Nick

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

If you are running the 120VAC through a transformer to say 12VAC and then rectifying and regulating to 5VDC for the PIC I would put a MOV on the 12VAC line. The transformer is designed to cope with the crap on the 120VAC line. They have been building transformers for over 100 years and they know how to do it well. However the transformer will pass the crap through to the 12V rectifier and the rectifier may be the first part to fail. Putting the MOV on the 12VAC line will protect the recitifier and everything downstream of it.
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Nick · Guest

If I used a AC to DC transformer to power the pic would it still let crap in? would I have to place the MOV before the transformer kinda like a surge power strip?

Nick

rnielsen · Joined: 23 Sep 2003 Posts: 852 Location: Utah

If it was me, I would put the MOV on the primary side of the transformer. If you think about it, the power strips, that you use on your computers, usually have surge protection built into them. They are, pretty much, on the primary side of your electronic equipment's power supplies.

newguy · Joined: 24 Jun 2004 Posts: 1907

All the suggestions you've been getting regarding where to put the MOV (primary, secondary, etc.) are all good. You can pretty much place it at any one of these spots, or both if you want. What will determine where you place the MOV is probably cost, though. See what's available, and how much they cost. If a 150V MOV is cheaper than an equivalent (energy-absorption wise) 15V MOV, go for the 150V device on the primary.

You mentioned that you'll be driving a relay(s) with the PIC - you'll definitely need a flyback diode.

e.g.

+12 V
|
|
|-----------------
| |
| ---------
| ^
Relay coil / \ flyback diode
| -----
| |
|-----------------
|
Transistor -----------PIC
|
|
Ground

Sorry for the crappy ascii art. The flyback diode is to prevent the relay's back emf from damaging the transistor or the PIC.

One more suggestion: I can't remember where I found this information, but an application note I came across some time ago recommended that instead of a flyback diode, a TVS (transient voltage suppressor) works better at eliminating relay chatter and thus extending the life of the relay's contacts. A bidirectional TVS is really just two zener diodes head-to-head. For instance, if your relay is driven by 12 V, the app note suggested that a 24 V TVS would do the trick very nicely. It's something that I've adopted for my products, and it works quite well. For TVS's, go to digikey.com and look up the SMBJ family (i.e. 24V - SMBJ24).

While I'm at it, also be very careful where you route signal lines on the pcb. Keep signal lines away from the high voltage side, as transients created by the motor starting & stopping will induce some pretty ugly stuff on signal lines if you're not careful. One of the most frustrating problems I had with a controller was it constantly resetting every time the motor it was controlling kicked in & out. Turns out that I had inadvertently routed the line from the ICSP header to /MCLR parallel to one of the main in lines (I think it was the neutral). Get this - they were only parallel for about 1/4", and were on opposite faces of the board, but the motor noise was enough to reset the PIC.

edit: no matter what I do, I can't get the crappy ascii image to show up properly. You'll have to use your imagination.

Ttelmah · Guest

Nick · Guest

thanks for all the great information! This gives me a great head start in the problem.

Nick