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In Reply to: RE: Semiconductor ID help needed posted by Michael Samra on April 26, 2015 at 14:55:22
Hey Michael,
I was hoping you would respond. It is a Sears (probably made by Chamberlain), but this is actually a Touch Code Lock Model 139.53716 by Sears (keyless entry). All it does is short across a 28 VAC control line and trigger the garage door control, which is a 1974 unit. We upgraded the radio and keypad much later. I think this FET just shorts out the 28 VAC, same as a switch would do. This is old timey stuff, but it works great (or did). Here is a photo of the board. The touch control keyboard plugs in on the left. You can see the blank space where I unsoldered Q4 and also the legend G D S on the terminals
Also a link to the manual.
Thanks for the assist.
John
Follow Ups:
G/D/S sure seems like a FET, to me. The $64 question is whether it is depletion or enhancement. I suspect that once the FET's mode of operation is known, just about any old hefty TO220 case part of adequate voltage rating should be OK, in this switching application.
The part is "fried" and simple tests are hardly conclusive. However, try the ohms setting of a multi-meter to see if continuity exists between source and drain. If continuity exists between S & D, you are likely dealing with a depletion mode FET. OTOH, if an open circuit is found between D & S, an enhancement mode part is likely.
Eli D.
Eli,Thanks for your response. The board has two TO-220 devices. The one I posted before, Q4, which I am calling bad, and Q3, which I think is good. I unsoldered Q3 so I could turn it over and it does have the same top number on it. So I think I have a good part of the same number. The "good" part, Q3, measures infinite from D to S in either direction. It also shows a forward voltage on the diode test of 0.56 volt with the -lead on D and +lead on S. The other way shows infinite or open. So this would look like an enhancement part? Is it P or N?
Additional tests:
Between G and D, another diode junction is detected. With +lead on G and minus lead on D, I measure 0.58 volt forward voltage. The other way is open circuit. Resistance test shows infinite either way.
Also between G and S, I measure about 42 ohms either polarity and forward voltage is low either polarity (no diode junction detected in this way)Tests made with BK Precision Test Bench 388-HD.
Did I give you enough info to ballpark the type?
Thanks,
John
Edits: 04/27/15
Just a WAG, a 100 V./50 W. rated "N" channel enhancement mode FET could be tried. Mouser carries all sorts of MOSFETs. Check their website out.
Eli D.
Thanks, Eli. I appreciate your looking at this.
John
Also see if there is a replacement board available. Yeah - like they would provide that!
Also test parts associated with that SCR.
This is vintage! Meaning-no boards available new, and not even a schematic available.
I have checked the bipolars and diodes and all seem OK.
Thanks for the suggestions.
John
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