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[ultradianguy]

Hi - I want to use  a powerflash module to sense an output from a thermostat, which is a 24v output.  Powerflash specs say 0-18v input.  Does anyone know if exceeding that and using the 24v output would fry or damage the powerflash?  It's strange to have the 18v max given that 24v is standard for so many devices one might want to automate.

Thanks,
Michael

[Brian H]

Is the thermostat powered by the 24 volts or controlling a 24 volt device now and you want to sense when the device is On?

Reason I asked is some here have just used a thermostat's output to control the Dry Contact mode of a Powerflash.

I have seen a schematic of an older PowerFlash input but don't know if the added voltage may damage the unit. I will study it further.

If you had to I would think drive a 24 volt AC relay and then use its contacts to control the Powerflash in the dry contact mode.

[dave w]

Brian, what if a diode was put in series with the 24VAC line to the Powerflash?. The Powerflash would be looking a half wave 12V.

[ultradianguy]

The thermostat has configurable outputs that will get 24v when a particular thing happens (for instance, humidity falls below set point).  So 0v when in range, 24v when out of range.  (It's the Venstar T1800)

[Brian H]

dave w, The schematic of the Powerflash I have. Has a diode on the input as it uses DC or lumpy DC from AC already.

Thanks for the updated information on the T1800. Yes it does use AC to power it.

[Brian H]

I had another thought and will try and do some tests tomorrow.
Maybe a resistor can be used in series with one of the input terminals.

[dave w]

I had another thought and will try and do some tests tomorrow.
Maybe a resistor can be used in series with one of the input terminals.

Either that, or a couple as a divider. I think your 24V relay suggestion would probably be the best way to go.

[Brian H]

I did some tests and I believe a 5.6K to 10K resistor in series with one of the input terminals may do it.

Though I have seen an internal photo of the T1700 and I believe the T1800 just stuffs the added components not in the T1700.
It looks like it uses triacs to switch the outputs on and off. I am not 100% sure the PowerFlash Inputs would draw enough current to keep the T1800s outputs happy. 24 Volt AC relay if you can find them maybe a better choice if the resistor and PowerFlash are not working correctly.

[ultradianguy]

Thank you so much!