Driving a 180ma relay from PIC port

[Pic_User]

Tried a 4401 and some others but they won't latch on.

Hi Michael,

The PICs supply 25 mA per pin. That should be more than enough to hold those small signal transistor on. You may be looking at a different type of problem.
-Adam-

[skimask]

Hi Michael,

The PICs supply 25 mA per pin. That should be more than enough to hold those small signal transistor on. You may be looking at a different type of problem.
-Adam-

Do you have a ~1K-ish resistor on the base?

[tico]

use this circuit found towards the bottom of the page

http://rentron.com/PICX6.htm

[Michael]

I'm driving a 2N4401 with a 1K....even did a 560 ohm. An open collector config to turn on/off the ground circuit to the relay.

I'm actually using the input to the LM317 on my power supply (around 8 volts) for positive relay voltage. (5v relay)

Using pin D4 on the PIC, which works fine (goes high for 15 seconds) but using the transistor to the relay will just give a momentary pulse and won't engage.

However, if I tie the ground circuit directly and bypass the 4401, the relay works perfectly....so I know the power supply is fine.

Maybe the transistor on resistance is too high but like I say, surely there's a part someone has used to sink 180ma without a problem.

[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...