This has sent me down a different track which is good.

Just thinking aloud here, what about a bog simple "regulator" like say 3 forward-biased diodes (or a zener) in series with a resistor. This will cap the voltage at about 1v8 regardless of the input voltage.

The boost reg then takes it back up.

The resistor has to be sized to allow whatever current the circuit needs.

EDIT: large parasitic load (say 50mA) that you can never turn off if you wan to shut down to save power.

Its a low voltage dropout chip down to 1.3V. So if you programmed it to output 1.5V (resistor network) and supplied it with more than approx 1.6V it will regulate. I didnt look at the power dissipation figure.

mmmm! 50Ma at 0.2V = 4 ohms
4ohms at (12-1.8) = 25Watts, big resistor
Zeners would be carrying about 2.5 amps

I think there's a gotcha, I just saw this in note 5

Thanks for doing the numbers, I thought it might be a problem but that's a bit over the top. Back to the drawing board.

2V is probably at the low end of the battery voltage, normal lead-acid cells should be around 2.1-2.2. Probably a bit close for comfort.

Normally I would be OK. I've been doing some numbers as well, the lowest I ever see on our system is maybe 23v or even high 22s. That's pulling about 70A when the microwave is on and it's also at the meter which is about 3m from the actual batteries, what the losses are in the cable and connections I don't know.

Let's say 23 / 6 = 1v9 if I had 2v batteries, and that's under a lot of stress which is very infrequent.

Maybe I should raise my lower limit to 1v9. I can't go to 2 because it's quite common to be under 24v and I don't want a system to shut down when that happens.

I may have to go back to my dual regulator idea but that had a few holes as well. Another option is to provide external power, something I'm resisting.

Ok, transistor guys, will this work? Does the emitter follow the collector until the zener starts conducting and then the emitter is .6 below the zener voltage?



As I said the other day, transistors aren't my strong point.

Also, I've read in quite a few places that just using a normal linear regulator will do the same job as they tend to follow Vin until they have enough headroom to regulate.

What about using a "Joule Thief " circuit to boost the lower voltage, I have seen much chatter about them and with their use on flat AA batteries to run leds etc.

Pete.

Jeez you're up late, it's after 11 here in WA.

I'll do some research on that, I still need to regulate high voltages though.

We're just east of Norseman, heading out onto the Nullarbor tomorrow, don't know when I'll get the interweb again