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I added a DC-DC converter in my truck so that I can charge the Lithium batteries in my RV. To turn on the converter, I am using a relay that ties to upfitter switches in my Ford F350 truck. This all works fine and as intended, however, if I leave the truck in ACC key position, then there is a possibility that I can be drawing up to 40 amps from my truck battery to charge the Lithium batteries (assuming I forget to switch off the upfitter switch). I want to "forget-proof" the system such that the only way the relay can be energized is if the upfitter switch is on AND the alternator is running. There is probably a wire on my alternator somewhere that is hot only when it's running, but I don't know for sure (it's a smart alternator system, so there is likely some logic in the system I don't know, so I would rather not tie directly to an alternator). Instead, I'd rather use the logic that if the system voltage is over 13 volts, then the alternator must be running.

I tried the idea of simply putting a 13V zener diode in series with the upfitter switch (see attached crude powerpoint diagram), thinking it would only conduct if the voltage is greater than 13 volts (i.e., alternator is running), but that didn't work - the zener does turn on, but if there is a 13 volt drop across the zener, that leaves only about a volt drop across the relay coil, and the coil is not energizing. Looking for other ideas that are not too complicated or expensive...

enter image description here

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  • \$\begingroup\$ Bad solution. The relay supposed to be activated with only ~0.5V. \$\endgroup\$
    – user263983
    Mar 20 at 19:35
  • \$\begingroup\$ Look for a "hot in run" only wire. \$\endgroup\$
    – Passerby
    Mar 20 at 20:58
  • \$\begingroup\$ No matter which solution you end up with, you most definitely will want to have a flyback diode across the relay coil. 1N400x or such. \$\endgroup\$
    – Lundin
    Mar 21 at 14:53

3 Answers 3

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I did this by driving a low powered relay connected between the ignition battery warning light and the alternator.

The relay coil is connected to the warning light wire and earth.

When the ignition is on and the engine not running that bulb is earthed through the alternator diodes and the voltage is not enough to energise the relay coil.

Once the engine is running and the alternator is charging that batt warning light goes out due to a voltage balance. The alternator side now has sufficient power on that circuit to drive the relay coil and turns the relay on.

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  • \$\begingroup\$ this makes sense, but finding the alternator warning light wire would be like looking for a needle in a haystack - actually, that's true for about any wire under the hood of this truck. I do have a wiring diagram, but the wire bundles are tight and very hard to follow (this is a 2019 - so has all sorts of gizmos and gadgets and computers and such - things that would affect the warrantee if I messed something up... You're right in your answer to Peter Bennet - there are LOTS of things on in IGN even if the engine is not running. \$\endgroup\$
    – SteveO
    Mar 20 at 22:35
  • \$\begingroup\$ @SteveO Did my apprenticeship as a vehicle electrician and had to completely rewire trucks due to fire damage - not the sort of experience many electricians get. \$\endgroup\$
    – Solar Mike
    Mar 20 at 22:42
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Simply power the relay from the Ignition terminal of the switch (or from some circuit connected to that point), not from the accessory terminal. That way, the relay will be on only when the engine is running.

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  • \$\begingroup\$ The ignition can be on and the engine NOT running. Often done for radios, gps etc when stationary. \$\endgroup\$
    – Solar Mike
    Mar 20 at 20:41
  • \$\begingroup\$ @SolarMike but there are other circuits where that's not the case like the ac/heater. The circuitry is there you just need to tap into it. \$\endgroup\$
    – Passerby
    Mar 20 at 20:48
  • \$\begingroup\$ @Passerby heater fan works without engine running or even with engine running and alternator not charging but that depends on idle speed. \$\endgroup\$
    – Solar Mike
    Mar 20 at 20:53
  • \$\begingroup\$ @SolarMike: I would expect radios, GPS and similar devices to be powered from the Accessory position of the ignition switch, not the Ignition switch. Having the ignition switch in the "Run" position with the engine stopped would be rare. \$\endgroup\$ Mar 20 at 21:22
  • \$\begingroup\$ @PeterBennett not all ign switches had an "acc"position... Some had Off, Ign, Heat, Starter . \$\endgroup\$
    – Solar Mike
    Mar 20 at 21:36
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You could try the following circuit modification to your setup. The Zener diode voltage may need to be lowered to 12V to accommodate the 0.7V turn on threshold of the NPN transistor. NPN selected to handle the 12V/80ohm=150mA coil current.

enter image description here

You could choose to tie the top of the relay to the switched side of the uplifter switch if it is inconvenient to attach directly to the PWR connection.

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  • \$\begingroup\$ thanks! I think I understand the concept here - please check my understanding... The relay only turns on when the transistor is on. When the switch is on, the zener would remain off unless there is 13 volts, and when that occurs, the transistor turns on. I think I will try this - but want to ask about current first. With the alternator running at about 13.6 volts, and an 80 ohm coil, that would be about 170 mA. I just read that the 2N3904 is good for no more than 200 mA - any recommendation on what would give me a little more design margin? \$\endgroup\$
    – SteveO
    Mar 20 at 22:20
  • \$\begingroup\$ You can use a TIP120 which comes in a convenient TO220 package. They are rated at 5A so should offer much more design margin than the baby brother 2N3904. The TIP120 is internally two NPN transistors connected as a darlington to provide very much higher base current to collector current gain. Since darlington devices have two Base-Emitter junctions to be biased to turn on the device you would need to adjust the Vz of the zener to something in the order of 11.5 volts or so. With the TIP120 the 220 resistor can become 2.2K and the 10K should be adjusted up to 33K or 47K. TIP120's should be $1. \$\endgroup\$ Mar 20 at 22:45

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