Open Collector Relay Control

Replace Q7 with a MPSA13 or equivalent. Plenty of gain and power handling capability, and cheap.

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Yup. Replacing the BC547 with a darlingon NPN transistor like the MPSA13 is an easy, one component fix.

The pinouts of the two transistors are different, though. You might want to turn the flat side of the MPSA13 around, and install it "backwards". That will result in the correct pinout on the board holes.

If you install them with the flat of the TO-92 pointing the same way, it won't work. Look at the drawings on the data sheets and see for yourself:

Good luck Chris

"BobG" = Groper from Hell

( snip nauseating SHIT from this anencephalic)

you ASD f***ed TROLL !!!!

....... Phil

Use a BC337 or 338 as a direct replacement - it has the same pinout. It will handle up to .8 amps and can dissipate more power than the MPSA13 (which has the wrong pinout) recommended earlier. It also does not suffer from the > 1.2 v Vce that is unavoidable with darlingtons. It has about the same capability of the 2N2222 mentioned earlier, but the 2N2222 also has the wrong pinout.

Ed

Hi, Ed. Good choice -- that will be a closer replacement, and it's pin compatible.

I was wondering, though -- the 4.7K pullup will limit base current to a little less than 1mA. If the OP decides to go with your better choice, he might want to replace that resistor (R20) with a 470 ohm 1/4 watt resistor, to ensure enough base current to saturate the BC337. He is driving a 160mA load.

Cheers Chris

As long as the PIC can drive Q6 it should work. Since it is easy to just change the program - I figured limited drive from the processor is the reason two stages were used in the first place -and I'm too lazy to try to find out . . .

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Good point. I didn't look at the schematic, just the xsistor specs. The 470 you mention would be much better. It's not a good relay driver circuit as it stands, and on top of that the BC547 is too wimpy for many relays, so I hope he decides to replace it with one of the three options he's been given.

Ed

Thanks to all for great feedback on this. I'm going to try the BC337/338 with 470 ohm resistor; The parts should be here by the weekend and I'll let all know how it comes out.

Thanks for help> Chris wrote:

Good - getting rid of that wimpy 547 is the right thing to do. Don't forget to put a diode across the relay coil backwards (banded end connected to the + side).

Ed

Just a quick update. I replaced with a BC337 and 370 ohm resistor and the circuit power the relay as desired.

Thanks to all for the help with this!

ehsjr wrote:

OK, Hank. Glad to help.

Just as a simple reality check, try touching the case of your new transistor after the relay has been on for a few seconds -- make sure it's not more than a little warm.

By the way, please bottom post if you'd like to post again. Since you're a Google person, please read Google Groups Help Topic "What's good 'netiquette' when posting to Usenet?"

Good luck Chris