One of my projects has a pic turning an N-Chan MOSFET on which then connects a ground and turns a relay on.

I was told that I must have a relay with a diode in it. To prevent EMI from coming back and burning my transistor up. I can do that.

However I looked at my bag of relays and noticed somthing... They don't have a resistor symbol across the coil like others do, and they dont have the diode symbol... Instead the have a long rectangle. Um... What does THAT mean ?

There are also the words "Negative Spike Suppresion" printed on the bottom. They are Beuler relays.

I did some reading and found that De-Spiking can occur from a diode or a resistor... They say the diode is better, but the resistor can handle more abuse.

I am dumb and dont know how to use the diode check on my DMM (it says .033V when I measure across the coil) and the resistance is 66ohm. But I think if there were a larger resistor in there then that of the coil, I wouldnt be able to read it. no ?

Any way to tell whats going on ?

 

The rectangular symbol represents the relay coil..Some small relays come with the diode as part of the package, most don't.
Negative spike supression ??
A reverse diode will give you adequate supression, a resistor will add current draw to your output , it will be an additional load..
Diode check :Meter in diode check(arrowhead) place leads across diode, reading will either be open (ol) or show small voltage(voltage level varies from type to type,typically .7v) Reverse leads on the diode and you should get opposite of first reading..Reading open in both directions indicates open diode (no current flow) and a voltage in both directions indicates shorted diode (current flows both directions). Most of the time you can check diode in circuit, if you get a questionable reading, disconnect one lead of the diode from circuit and check again. Checking a diode in the relay coil circuit will give false readings since the coil acts as a conductor that is parallel with diode..
Hope this helps..

__________________
gerty

 

Yea... Negative Spike Suppresion. Sounds like a diode to me ... ?

Its a Beuler relay, seach on google... They say nothing about resistors or diodes etc. Just the words Negative Spike Suppresion.

I thought that was how diode check works, I'll have to crack on open and see, thats good advice.

Any idea on how many times that relay could trigger without hurting the transistor ? I'm getting some funny things going on with it right now, but its late and I keep changing my code so half the time I test somthing I'm really not even sure what it is I'm expecting (its reallyreally late)

Thanks!

 

Maybe the relay case contain a MOV or R-C component for spike suppressing (this solution not polarity depended). The N-FET most of case contain a diode, this is enough protection.

 

Hmmm.... your right. The Fet does have a diode in it.... I'm not sure if its going the right way or not....

I'm using the source connected to ground, and the drain connected to a wire I have comming in, when I want the relay to trigger I conenct the relay and it ground the drain wire. Relay clicks. It says the diode is on the source pointing to the drain (i think).

So... is "Negative Spike Suppresion" going to get me the same result as a diode? I guess it doesnt matter if my transistor has a diode in the right direction.

 

Is there any way to have this circuit connected so that the source and drain are connected and when I drive the gate low then the drain and source disconnect ??

Or would I need a P-Channel MOSFET for that ?

 

Nevermind. It isnt a P-Channel i need (i still dont know what they do)

But rather a depletion-type N-Channel.... Apparently the equivalent to a normaly open circuit

 

Do you mean you want to drive the gate voltage negative relative to the source in order to turn the MOSFET off? Why do you need that?

 

I wrote normally open, but I meant Normally Closed.

So I have a transistor that is normally closed (a link between source and drain), I output to the base pin and it opens up (no link). Easy.

That way if the pic on my board were broken, all the devices would be stuck ON instead of stuck OFF. Which I'de prefer as a sort of safety feature.

Problem is, I cant find a depletion type transistor... Anywhere (non-online). Let alone an N-Channel Depletion Type MOSFET...

 

A depletion-mode N-channel MOSFET conducts with zero volts between gate and source. A negative gate-to-source voltage must be applied to turn the transistor off. This is not what you want.
An N-channel enhancement mode MOSFET is off with zero volts applied from gate to source. The gate-to-source voltage must be positive to turn it on.
A P-channel enhancement mode MOSFET is also off with zero volts gate-to-source. The gate-to-source voltage must be negative to turn it on. The difference here is that the source is more positive than the drain. The source needs to be connected to the positive supply rail, and the load goes from the drain to GND.
BTW, if your PIC were to fail, there is no guarantee as to what the output voltage on a pin would be.