Built Kit, resistance problem – SOLVED

[Peter]

Excellent kit, good instructions BUT needs an additional step before the Vin and Vcc resistance measurements.  After building very carefully (I’ve built kits since Eico and Heathkit many years ago, yes I’m an old man).

I fired up the ohmmeter and found 130 ohms to ground on Vin (Vcc was 981 ohms).  Looking over my soldering and traces found nothing so I started removing components to isolate the problem, the inductor, the USB  connector (which I destroyed while removing) etc.  Nothing seemed to work, but then all of a sudden I had 8K ohms…. huh, then back to the 130 ohms…

The problem is that the transistor driving the relay is a FET with a very high input impedance, I think it was being turned on with me touching the board.  After figuring this out, I went ahead and powered the board with 24Vac and its working fine.

I think the solution, since the resistance readings are done with out the chips plugged in (that would have kept the FET’s gate low) would be to ground pin 20 on the 40 pin chip.

Hope to save anyone else a day’s frustration and distrust of his/her soldering abilities and save my 12 hours of frustration.  Great product.

Pete

 

 

[Ray]

Hi Pete, sorry to hear that you had to go through such trouble to figure out the problem. From your description, I just checked and realized that it’s partly our fault because the MOSFET is supposed to be a MMBT3904 transistor but we substituted it with a 2N7002 MOSFET without realizing that this particular version doesn’t have a pull-down resistor. We did test all VIN-GND and VCC-GND resistances of the partially assembled boards and did not find any issue. But given that now the weather is dry and static electricity is high, it may have caused yours to trigger the MOSFET. Anyways, it’s great that you find out the cause. Thanks.

[Peter]

Ray thanks for your response, I felt stupid when I finally found the problem, schematic helped a lot!

You have a great product and I bought it based on a lot of the support I saw you post on this forum.

[Pierre Constantineau]

This is a great kit. Easiest one I have done. Completed in less than an evening. Too bad it’s now discontinued. I guess I got mine just in time.

I had the same 130 ohm reading on Vin and 1.8k on Vcc. Considering that I had tested them the evening before and that they both read fine,
I made sure I didn’t have any bridges or shorts and based on this entry, checked the voltages. They read about 4.5V and 2.8V instead of the 5V and 3.3V I was expecting. I assumed that this would be within the tolerance, I then inserted the chips and powered it on without any issue. I haven’t checked the Volts with the chips in place but considering it all works fine, I flashed to the latest firmware and installed near my distribution panel.

I just need winter to be over to see the programs do their thing…

Works great!

[Ray]

@Pierre, glad to hear it worked. Well, 4.5V is not unusual, but I thought 2.8V is a bit on the low side. Because that voltage comes from the 3.3V linear regulator, unless if the circuit is drawing a lot of current, it should be pretty close to 3.3V. If you want to test further, I suggest you unplug 24VAC, but instead power the circuit with a USB cable, and retest the VCC-GND voltage. See if it’s close to 3.3V. It’s just my precaution — I am pretty sensitive to these numbers, and I feel 2.8V is a bit unusual.

[Author]

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