W1209 thermostat module
I dug out this W1209 thermostat that I bought on ebay some time ago:
The idea was going to be to use the module with the MFJ-4225MV power supply to control the noisy fan - see the original blog post:
https://wb9kzy.blogspot.com/2023/07/mfj-4225mv-fan-switch.html
When originally purchased the W1209 was overvolted by accident because unlike the ebay listing picture, the W1209 I received has a zero ohm resistor in place of the regulator (right side). So I complained and got my money back.
But today I also noticed that R1 (left side) is also missing. The W1209 schematic shows that R1 is a 10k ohm resistor from the base of Q1 to ground:
I asked the question: what does that 10k ohm resistor do ? on the PICLIST. The answers (which seem reasonable to me) were that if the bipolar transistor was replaced with a MOSFET, the 10k ohm resistor would serve to pull the gate low in the case of the microcontroller pin being either temporarily or constantly open (tri-stated). This would prevent the relay from being turned on accidentally.
Anyway, I thought I'd add back a regulator but then I realized that another problem with this particular unit is that the relay has a 5 volt coil. It should be a 12 volt coil per the schematic. This means that if the regulator was added, the 5 volt coil would be driven by 12 volts, which would not be good.
I'll have to think about it, maybe an external 5 volt regulator should be used and the series input diode on the W1209 module should be shorted out so that it doesn't ruin the voltage regulation.
On the other hand, since there are 4 problems with this unit (no regulator, no R1, wrong relay, overvolted by accident), maybe it's 4 strikes and it's out ? I should probably just buy a new one.
Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm
The idea was going to be to use the module with the MFJ-4225MV power supply to control the noisy fan - see the original blog post:
https://wb9kzy.blogspot.com/2023/07/mfj-4225mv-fan-switch.html
When originally purchased the W1209 was overvolted by accident because unlike the ebay listing picture, the W1209 I received has a zero ohm resistor in place of the regulator (right side). So I complained and got my money back.
But today I also noticed that R1 (left side) is also missing. The W1209 schematic shows that R1 is a 10k ohm resistor from the base of Q1 to ground:
I asked the question: what does that 10k ohm resistor do ? on the PICLIST. The answers (which seem reasonable to me) were that if the bipolar transistor was replaced with a MOSFET, the 10k ohm resistor would serve to pull the gate low in the case of the microcontroller pin being either temporarily or constantly open (tri-stated). This would prevent the relay from being turned on accidentally.
Anyway, I thought I'd add back a regulator but then I realized that another problem with this particular unit is that the relay has a 5 volt coil. It should be a 12 volt coil per the schematic. This means that if the regulator was added, the 5 volt coil would be driven by 12 volts, which would not be good.
I'll have to think about it, maybe an external 5 volt regulator should be used and the series input diode on the W1209 module should be shorted out so that it doesn't ruin the voltage regulation.
On the other hand, since there are 4 problems with this unit (no regulator, no R1, wrong relay, overvolted by accident), maybe it's 4 strikes and it's out ? I should probably just buy a new one.
Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm
