Project: Installing a late model electronic HVAC control unit in a 1985 El Camino.
Problem: The AC request that comes from the HVAC head is 12V, but very low current ( I believe- I'm not Elecronics savy). When the HVAC requires that the AC compressor turns on, this line goes from 0V to Ign Voltage ( approx 12.8v). The diagram in the Service manual shows this line as IGN ---1000K ---out to controller. I think that going through the 1K resistor is protection, and also lowers the current available.
Initially, not knowing that his was low current, I tried hooking it up to an automotive relay, which would then drive the AC compressor. This relay has a current of approx 150 MA. Did not work.
Then I tried a PCB mounted relay, drawing 30 MA. Did not work
Next I tried a PCB mounted relay drawing 15 MA. Did not work.
Finally, I tried a MOSFET (ZVN4206AV), drawing 1 MA, attempting to drive the 15MA PCB relay using the drain on the MOSFET. Did not work.
http://www.diodes.com/_files/products_appnote_pdfs/zetex/an10.pdf
Last option I have now, which I have not yet tried, is a Darllington Octal Driver, ULN2804 http://www.datasheetcatalog.org/datasheets/105/366825_DS.pdf
Somehow, I need a solution that will take the 12v+, very low current signal coming from the HVAC and convert that signal to a means to drive a 12V 150MA relay. Theoretically, either of the PCB relays I have should be able to drive the Auto relay. Even the MOSFET is supposed to have enough capacity ( 600MA) to be able to drive the coil on the relay directly. I guess I'm just having problems figuring out how to take the signal from the HVAC unit and use that to drive something else.
Note: All wiring is being done inside the vehicle. All through-firewall wiring uses existing wiring from the manual HVAC system.
Thanks for helping a novice through this problem.
Problem: The AC request that comes from the HVAC head is 12V, but very low current ( I believe- I'm not Elecronics savy). When the HVAC requires that the AC compressor turns on, this line goes from 0V to Ign Voltage ( approx 12.8v). The diagram in the Service manual shows this line as IGN ---1000K ---out to controller. I think that going through the 1K resistor is protection, and also lowers the current available.
Initially, not knowing that his was low current, I tried hooking it up to an automotive relay, which would then drive the AC compressor. This relay has a current of approx 150 MA. Did not work.
Then I tried a PCB mounted relay, drawing 30 MA. Did not work
Next I tried a PCB mounted relay drawing 15 MA. Did not work.
Finally, I tried a MOSFET (ZVN4206AV), drawing 1 MA, attempting to drive the 15MA PCB relay using the drain on the MOSFET. Did not work.
http://www.diodes.com/_files/products_appnote_pdfs/zetex/an10.pdf
Last option I have now, which I have not yet tried, is a Darllington Octal Driver, ULN2804 http://www.datasheetcatalog.org/datasheets/105/366825_DS.pdf
Somehow, I need a solution that will take the 12v+, very low current signal coming from the HVAC and convert that signal to a means to drive a 12V 150MA relay. Theoretically, either of the PCB relays I have should be able to drive the Auto relay. Even the MOSFET is supposed to have enough capacity ( 600MA) to be able to drive the coil on the relay directly. I guess I'm just having problems figuring out how to take the signal from the HVAC unit and use that to drive something else.
Note: All wiring is being done inside the vehicle. All through-firewall wiring uses existing wiring from the manual HVAC system.
Thanks for helping a novice through this problem.
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