Open Collector Relay Control

[Hank Brandenburg]

I have assembled an electronic timer kit, #148 from Hobby Engineering
(http://info.hobbyengineering.com/specs/DIY-k148.pdf). I plan to
control a 20-amp 120VAC circuit with this timer. The relay I've chosen
to do this with is part # 653-G7L-2AB-DC12 from Mouser.

The relay specs say 1.9W is consumed by the relay for 12VDC, so I'm
expecting to need at least 160 mA to energize the relay.

The timer circuit uses open-collector output with a BC547 transistor
for control, and I'm sure this transistor will not handle the current
load required by the relay.

I'm looking for suggestions on how to accomplish this - perhaps cascade
the control circuit with a larger transistor?

Any ideas are much appreciated.

 

[default]

I have assembled an electronic timer kit, #148 from Hobby Engineering
(http://info.hobbyengineering.com/specs/DIY-k148.pdf). I plan to
control a 20-amp 120VAC circuit with this timer. The relay I've chosen
to do this with is part # 653-G7L-2AB-DC12 from Mouser.

The relay specs say 1.9W is consumed by the relay for 12VDC, so I'm
expecting to need at least 160 mA to energize the relay.

The timer circuit uses open-collector output with a BC547 transistor
for control, and I'm sure this transistor will not handle the current
load required by the relay.

I'm looking for suggestions on how to accomplish this - perhaps cascade
the control circuit with a larger transistor?

Any ideas are much appreciated.

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

 

[Chris]

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

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:

http://www.onsemi.com/pub/Collateral/BC546-D.PDF
http://www.onsemi.com/pub/Collateral/MPSA13-D.PDF

Good luck
Chris

 

[Jamie]

I have assembled an electronic timer kit, #148 from Hobby Engineering
(http://info.hobbyengineering.com/specs/DIY-k148.pdf). I plan to
control a 20-amp 120VAC circuit with this timer. The relay I've chosen
to do this with is part # 653-G7L-2AB-DC12 from Mouser.

The relay specs say 1.9W is consumed by the relay for 12VDC, so I'm
expecting to need at least 160 mA to energize the relay.

The timer circuit uses open-collector output with a BC547 transistor
for control, and I'm sure this transistor will not handle the current
load required by the relay.

I'm looking for suggestions on how to accomplish this - perhaps cascade
the control circuit with a larger transistor?

Any ideas are much appreciated.

change the transistor to a 2N2222 and put a little add on heat shink..
the 2N2222 will do 800 ma. it's 1.8 watts how ever, i think the
transistor is most likely in saturation and that means you will not
be seeing that heat at the transistor.

 

[ehsjr]

I have assembled an electronic timer kit, #148 from Hobby Engineering
(http://info.hobbyengineering.com/specs/DIY-k148.pdf). I plan to
control a 20-amp 120VAC circuit with this timer. The relay I've chosen
to do this with is part # 653-G7L-2AB-DC12 from Mouser.

The relay specs say 1.9W is consumed by the relay for 12VDC, so I'm
expecting to need at least 160 mA to energize the relay.

The timer circuit uses open-collector output with a BC547 transistor
for control, and I'm sure this transistor will not handle the current
load required by the relay.

I'm looking for suggestions on how to accomplish this - perhaps cascade
the control circuit with a larger transistor?

Any ideas are much appreciated.

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

 

[Chris]

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

 

[default]

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.

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 . . .

 

[ehsjr]

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

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

 

[Hank Brandenburg]

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 helping out a novice!

 

[ehsjr]

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 helping out a novice!

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

 

[Hank Brandenburg]

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!

 

[Chris]

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!

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?"

http://groups.google.com/support/bin/answer.py?answer=12348&topic=250

Good luck
Chris