View Full Version : Help -- DCSSR Always On

09-12-2011, 11:24 AM
I'm working with some DCSSRs wjohn gave me with 24VDC. The 3rd channel of 4 automatically turns on after powering the SSR for a small period of time. It "ramps up".

I thought -- humm bad FQPF13N06L (MOSFET), but I replaced it and to my amazement the issue persists.

I've checked the resistance on the base for the MOSFET on all channels and they are to spec.
I've removed the opto and the issue exists -- this eliminates the 7805 and all but the MOSFET, 24VDC, and 1 resistor from the troubleshooting based on the schematic.
Again, I replaced the MOSFET (FQPF13N06L) and it persists.
I swapped out the light I was testing it with for another thinking it was shorted or something, and the same issue persists.
I disconnected the 3rd channel outputs expecting a different channel to drift on and the other 3 worked perfectly.
When I use my mulitmeter on the MOSFET base then + and - it turns on and off, but still drifts back on later.
All other channels work PROPERLY.

Only mod to the DCSSR is the 7805 is a SMT style, that I fit to the back of the board, but when I pulled the opto this part of the circuit should be inactive, so I do not think it is that.

Cannot send pics until I go home.

Very confused and in need of some ideas -- Jboyles.

P. Short
09-12-2011, 12:41 PM
That makes me think that you might have an open connection somewhere, or an incorrect resistor (resistance is too high).

09-12-2011, 05:50 PM
R3 did not have continuity on the gate side. I reflowed it and still could not get it, so I bridged it to R7's pad and now it works. I wish I could get a microscope under the solder to see that pad. The pads on the blanks seem fine. . .

Phil -- Thanks for the brain check. This cost me a spare PIC chip -- while I was testing I plugged the wrong ethernet cable into my REN64 -- It only had 24VDC on it. Whoops.