recommend a relay
Printed From: the12volt.com
Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=115092
Printed Date: May 10, 2025 at 10:14 AM
Topic: recommend a relay
Posted By: v8gt
Subject: recommend a relay
Date Posted: July 15, 2009 at 8:56 AM
Hello all first post here.... I need to use 6 negative triggers to switch 6 positive controls. Setting up a auto HVAC system in a show car controlled by the onboard pc. I have the control board, it has negative outputs and I need to have positive. I was planning on using a pcb set of relays to do this, does this sound like the way to go as it could be engaged for hours at a time.. Thanks
Replies:
Posted By: howie ll
Date Posted: July 15, 2009 at 11:23 AM
What's the current draw on each, the standard small case PCB relays (look inside an alarm) will handle a constant 10amps at 12v+. If individual draw is less, then the PCB mount is the way to go.
Posted By: KPierson
Date Posted: July 15, 2009 at 12:40 PM
What's the current draw on each? If the current is low (less then 200mA) you can use PNP transistors. They will have extremely low current draw when switched on. If the load is over 200mA nothing you do will work in an engine off situation because any thing over 200mA will drain the battery. ------------- Kevin Pierson
Posted By: v8gt
Date Posted: July 15, 2009 at 12:46 PM
Sorry I should have noted the draw. 5 - 9 amps with key on only.
Posted By: KPierson
Date Posted: July 15, 2009 at 2:48 PM
With a 5-10A current draw with the key on I would just use stackable Bosch relays. The added coil current (~750mA) isn't going to have a drastic effect on battery life when you have 20+A of current draw on the battery.
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Kevin Pierson
Posted By: howie ll
Date Posted: July 15, 2009 at 4:03 PM
The mini pcb relays I mentioned before, Maplin Electronics UK, equivalent to $2 each, SPCO mini format, 10amps, 400ohm coil OR prewired from DEI part no. (I think) 1186, about the same. I would also diode the coils if motors are involved, also if our poster is driving cooling fans, I would only recommend Bosch type with 30amps handling because of the onrush current when the fans start up.
Posted By: v8gt
Date Posted: July 15, 2009 at 4:59 PM
How would this do: https://cgi./ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWAX:IT&item=120438556836 If I read it correctly it will take a negative input and give a positive output. This is to run the vent actuators for the mode doors and the recirc door. Thanks for all your help..
Posted By: KPierson
Date Posted: July 15, 2009 at 5:37 PM
That I/O board is set to trigger on + voltage. You may be able to install a pull up resistor on the input and then use (-) triggers to drop the voltage out. The downside to this is that the relays will always be energized and the fact they already have an extrememly high base resistor, adding a pull up resistor may not allow enough current to switch the relay. The alternative would be to replace the NPN transistors with PNP transistors (or see if they offer a PNP board). ------------- Kevin Pierson
Posted By: v8gt
Date Posted: July 15, 2009 at 5:46 PM
I figured that woud be to easy.. Thanks
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