I like when my ideas turn out to be vastly inferior once I find out what the real goal is. Lol

Actually, post #34 does not state explicitly that there is only one DC input path or connector.

As for your circuit:

1. If you replace the two switches with one SPDT switch, you can eliminate both diodes.

2. If the 12 V input has its own, isolated path to the relay, the entire LM317 circuit can be replaced by one 2-watt resistor. That is such a simple solution that the fact that the TS is not going for it is another indication (to me) that there is only one DC input and the circuit has to handle both input voltage ranges automatically.

ak

 

Those tend to change quickly to a high impedance, as a sorta solid state fuse. They are not good for steady-state current limiting.


The operating point of the circuit would have to sit on the curvy part of the FETs transconductance curve, something that is not consistent from part to part and varies significantly with temperature.

ak

Certainly the single resistor in the source line will take some effort to determine the best value. It might even require using mosfets with more tightly controlled transconductance curves. It might work better with bipolar transistors, but the drive source may not be able to supply adequate drive.

 

Actually, post #34 does not state explicitly that there is only one DC input path or connector.

As for your circuit:

1. If you replace the two switches with one SPDT switch, you can eliminate both diodes.

2. If the 12 V input has its own, isolated path to the relay, the entire LM317 circuit can be replaced by one 2-watt resistor. That is such a simple solution that the fact that the TS is not going for it is another indication (to me) that there is only one DC input and the circuit has to handle both input voltage ranges automatically.

ak

I tried to make my circuit simple for a beginner. Looks like from his last post he is in production. I get a kick out this kind of situation because TS should have provided a schematic or poorly drawn sketch. I'd even take a scribble of this apparent automotive device

 

Relays and "limited-board-space" are mutually self exclusive.

6-Volt Cars are really really rare now,
and when You do find one, it's 6-Volt Electrical-System has been meticulously preserved
for either nostalgia-purposes, or Car-Collector-resale-value,
which means they won't be installing any "modern" aftermarket "Electronic-Gimmick" on the Car.

Just install a 12-Volt-Relay, or, better plan, eliminate the Relay altogether.
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.
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I think the idea of using an LDO regulator (<1V max. dropout) set to 6V output would be the easiest, and likely best solution.

 

Relays and "limited-board-space" are mutually self exclusive.

6-Volt Cars are really really rare now,
and when You do find one, it's 6-Volt Electrical-System has been meticulously preserved
for either nostalgia-purposes, or Car-Collector-resale-value,
which means they won't be installing any "modern" aftermarket "Electronic-Gimmick" on the Car.

Just install a 12-Volt-Relay, or, better plan, eliminate the Relay altogether.
.
.
.

Six volt system cars are mostly found (in the US, at least) in collectors collections and in museums, such as STAHLS, about 30 miles north of where I live. But in other parts of the world, and California, they are a bit more common. Watch traffic in Mexico City to see a vast array of what I consider classic cars that are driven daily.

 

1962 VW Beetle. 6 V system, J-box heat exchanger for the cabin, a saucer of engine oil as the air filter. Girlfriend's car. Got very good at driving stick.

It had a *solid state* turn signal blinker, a classic 2-transistor multivibrator driving a relay. Fixed it with a 2N404 I salvaged from somewhere.

ak

 

There are no reference designators, so any real discussion is very difficult. The top-most transistor has no impedance across the base-emitter junction, so it will not turn off quickly or reliably. There is nothing in the circuit that senses the current through the relay coil, so it is not clear what the circuit does to solve the problem. All I see is a saturated switch putting 12 V on a relay coil no matter what the rest of the circuit does, but the (?) opamp output is saturated high and there is no base current limiting for the other transistor, so it burns up and the relay turns off.

Which nodes would connect to a negative feedback circuit?

And - what is the purpose of the diode?

ak

This is a 'Model' not a final circuit. Parts need to be selected. The diode it taking advantage of the fly-back current and using it to measure the current via the op amp. This current is then amplified to produce negative feedback that will reduce the voltage to the coil as the TS requested.

 

The diode it taking advantage of the fly-back current and using it to measure the current via the op amp.

The kick from the relay coil is going to drive the opamp non-inverting input way below GND. As this is the opamp's negative rail, the output will be saturated low no matter how long or deep the flyback spike is.

AND - that flyback current happens only when the coil is turned off. It cannot be used to regulate the steady-state current through the coil.

ak

 

The LDO six volt regulator is also a good idea, but so is the emitter resistor to take the mosfet (or transistor) out of saturation.