Do you also need to control the relay, or is that just how it happens to be originally wired? If you are just looking for a light that comes on when the alternator is not charging the battery and you have access to the bulb wiring, that can be done with a simple comparator circuit. No fiddling with relay pull-in/holding voltages or worrying about changes in system voltage.
That's pretty much what the TL431 does. It's something of a comparator and zener diode. In the SF product the TL431 "zener" switched in at a set-able voltage and shorted out the LED. In this case the comparator should switch in at a set-able "high" voltage, energize the relay and open the NC contacts of the relay.lyndon:
… that can be done with a simple comparator circuit.
At least that’s the concept.
Lydon
I think the bulb is internally ‘fed’ B+ with one wire going to the original alternator. I think I have to have the relay to ground the bulb.
I am a little lost with the .185 that was derived with an example and then used to set the pot.
Thanks
I read your earlier post that called it a comparator, but I couldn’t see anything in the datasheet that confirmed that mode of operation. Oh well, if it works, it works.
[edit]
OK, I found a reference to using it as a comparator in an NXP datasheet. Interesting application, I wonder how sharply it switches.
Mee_n_Mac:
That's pretty much what the TL431 does. It's something of a comparator and zener diode. In the SF product the TL431 "zener" switched in at a set-able voltage and shorted out the LED. In this case the comparator should switch in at a set-able "high" voltage, energize the relay and open the NC contacts of the relay.lyndon:
… that can be done with a simple comparator circuit.At least that’s the concept.
The datasheet is a bit obtuse. It shows a comparator and a pass transistor, but that transistor shorts anode to cathode. The way I inferred it to work was that when the ref voltage was > the internal 2.5v reference, the pass transistor conducts ... and in a way similar to a 2.5v zener between the anode and cathode. Prior to that only a trickle current ran between the anode and cathode (just enough to run the circuitry) and the voltage could be anything up to 32v.lyndon:
I read your earlier post that called it a comparator, but I couldn’t see anything in the datasheet that confirmed that mode of operation. Oh well, if it works, it works.
A simple LTSpice model semi-confirmed this but is (admitted by everyone) to be a barely functional, functional model. I’ve yet to have success at importing a better, true ‘internal components’ model.
The TL431 compares the ref input voltage to an internally generated 2.5v. When the ref input voltage is > 2.5v, it conducts electricity. The pot acts acts as a voltage divider, making a smaller voltage (from the battery voltage) to be connected to the ref input. So the pot needs to be set such that when the battery voltage = whatever your trip voltage is, the divided down ref voltage = 2.5v. k is that ratio, = 2.5/Vtp.lutronjim:
I am a little lost with the .185 that was derived with an example and then used to set the pot.
If you choose Vtp = 13.5v, then k = 0.185.
If you choose Vtp = 13.0v, then k = 0.192.
As did I. I got a low freq model (finally) working in LTSpice. The OP's circuit is partially modeled (just the coil resistance) while I ramped the "battery" voltage slowly from 12v to 15v. I set the "pot" so that 13.51v = 2.5v @ the ref input (open ckt). As you can see the current through the relay coil is zero until Vref = 2.47v (Vbatt = 13.45v) and goes linear w/battery voltage @ Vref = 2.56v (Vbatt = 13.9v). The relay will energize (and de-energize if the ramp is reversed) somewhere between those points.lyndon:
Interesting application, I wonder how sharply it switches.
Sorry I haven’t been back on but the war department has had me doing other things.
I built circuit and set reference with key on -not running. After cranking, the ref volts went to 25.6 and the relay turn on. I then turned off the engine and the ref volts stayed there. The cap evidently kept the volts up. I took the cap out of the circuit and got it to work. However the battery voltage is slow to come down after just being charged but it is workable.
Setting the pot is very sensitive because the voltage swing is not much. Got to wondering what it would look like with a ‘pulled down’ battery. It might not see enough voltage to trigger relay.
Anyway I appreciate your time. I will try to install next month. I need to build some kind of container with connectors for mounting.