Driving a 180ma relay from PIC port

[Acetronics2]

Hi, Michael

Could ever your Pic have a RESET when power applied to your relay ... mmmmh ???

That could be a very good explanation ...

do try a LED instead of the coil ... just to see what's happening ...

sooo ...

1) do you have a BIG tank capacitor on your pic supply ( say 1000µF + )???
2) do you have the .1µF capacitor between vdd and vss ( the closest to the Pic )
3) did you think to the reverse diode ( 1n400x) // on the relay coil ???

Alain

PS ... once more: a scheme IS necessary to the troubleshooting !!!

[Michael]

Well....I changed the DC+ from the LM317 raw input to the 5v regulated output and all is fine.

Drawing 120ma as compared to 180.

??

I didn't want to draw current through the regulator, hence my using the input side.....it should have worked that way....are you guys sure the 2N2222 can do 180ma with a pic port?

Works fine now....another one of those "twilight zone" moments in electronics.

[Michael]

I could have been keeping the 317 from doing it's job and therefore affecting the 5v supply to the PIC....the transformer was only rated for 300ma.

Probably the case.

[Pic_User]

PS ... once more: a scheme IS necessary to the troubleshooting !!!

Hi Michael,

I agree that a schematic can say 1000 words (in all languages too!).

Eight volts input for five volts out, is cutting it close. The relay is probably pulling the (regulator input) voltage below “drop-out” level, during the valleys, in the ripple, of your supply. Causing the PIC to reset.

Yes, the transistor should hold the relay on.

Just take the base resistor off the PIC pin and connect it to the 5 volt supply. This will tell you if the transistor can do the job...

-Adam-

[RunningMan]

The other thing you could so is use a N-Channel FET to low-side drive the Relay. In otherwords, keep your driving + to the relay and let the FET close ground on the Relay... My circuits generally use this method where we are driving a relay.

NDS7002 is one N Channel MOS you could use, and put a pull down between it and the micro for insurance. this particular FET is rated at 280mA on the Source to Drain. The gate drive is well below the 25mA.

As for a flyback diode, any general purpose Diode will also work...

[mister_e]

and the Rds is interesting too... interesting current limiting/protection device in some case.

[Pic_User]

2N4401
40 Volts collector voltage
600 mA collector current
DC current gain with 180 mA collector current is about 100 minimum

With a minimum gain of 100 it should be able to switch 180 mA collector current with 1.8 mA from the PIC.

Less than 2 mA.

You may be looking at a different type of problem.
-Adam-

[RunningMan]

MMBD4148SE would be a good general purpose diode, you could also put a 5.5v varistor between the IO on the pic and the gate on the FET if you wanted maximum protection, although I think it's generally not needed. In my opinion if you were to put an over-voltage device on the circuit the Varistor would work better because it acts faster than a zener. Any reasonable package (assuming surface mount)...like 0805 would work.

Incidentally, I was looking at your requirement to not draw more current thru the regulator and in my mind this low-side drive would ensure you are not sourcing thru your power supply for the pic. I am assuming this is an external relay or something you don't want mixing in with your signals on the board.

This particular circuit i have suggested has been tested to well beyond 1Million cycles on a bench. we finally decided we'd beat it to death and took it off the supply during testing. This circuit would also appear to reduce or at least simplify your drive circuit design. BJTs are cool, but the FET is a bit less mysterious for driving a circuit like this.

[Michael]

Thanks.

I need to stock up on a good general purpose FET so I'll grab some NDS7002.

I would like to keep my relay from drawing through the regulator, so I'll breadboard something.

As they say in Indiana....Cheers.