micropower boost circuit

can't use a depletion mode mosfet or a Normally closed relay?

On a sunny day (23 Sep 2013 08:17:34 GMT) it happened Jasen Betts wrote in :

Or a simple JFET, driving transformer, driving normal MOSFET:

Q6 is readily avail and only rated for 30V max Vce, so your design should not exceed this.

Just remember that Zetex is now Diodes Incorporated but they still use the ZT part numbers..

If you need more margin, choose a higher Vce rated part.

In an automotive app this may well need a lot more than 18 volts. I'm not certain where this is connected in the wiring, but load dumps in autos often produce voltages well over the battery voltage, by 10's of volts.

We were designing a PSU for a military vehicle and had a transient spec of 100 volts! I think the spec in today's cars is some 40 or 50 volts.

The OP didn't say it's for automotive. If it is, then it either needs to be 80V tolerant or have a surge disconnect.

I guess technically he didn't say "automotive". I assumed when I read "six volt generator" it was for a car.

If so, then that must be a really, really old one. Like my 16-horse Citroen when I was young :-)

Its 6V DC generator was not capable of a serious load dump. In fact, I rode it sans battery most of the time because 6V batteries were expensive. Yet I still used the high beams, the radio and so on. The crank gave me a good workout, especially in winter.

If this is all so that you can turn on the high-side of an N-channel MOSFET, why not rethink? Why not use a PNP BJT to turn it on?

You may end up with fewer parts if you use a light-duty PNP to get things going until a chip-based solution can kick in, and then just run with that.

Or, you may end up with the fewest parts if you just use a heavy-duty enough PNP as your switch, and forget about using a MOSFET entirely.

I tossed that out as an example, and mostly chose that chip because I was using LTSpice.

My main point was to suggest the use of a chip, rather than a bunch of transistors.

Why else would he be building his own regulator? I figured it was for an antique auto. The OP said, "(the armature voltage of an antique six volt generator)".

I thought it was the starter motors that produced the load dump?

But then as I think about it a bit more, this is not really a regulator circuit as much as it is a boot circuit. In a car you have a battery to supply the field coil current on startup. So maybe this is for a six volt battery, but not a car. I just can't think of another reason to use "an antique six volt generator"... maybe to power an antique six volt radio?

That was after my post and my crystal ball is in the shop right now :-)

Usually it's the alternator, the kind with rectifiers. Happens if the battery suddenly comes off or, if operating without battery, a major load such as the rear window defroster goes offline.

I don't know if there even are 6V alternators, except maybe for smaller two-wheelers. They were usually brushed DC generators plus an external regulator. With the one in my Citroen 2CV I was quite impressed. It had the old style non-electronic point contact "bzzzt" regulator. Load dumps left it fairly unfazed, I checked that with a scope to avoid wrecking my precious old Becker car radio with shortwave range. It contained Ge transistors that had become nearly unobtanium by then.

Best of all, the whole car had no belts. The generator sat right on the crankshaft in a coaxial fashion.

These generators' fields don't switch on the high side. There are not lots of solid-state regulators available on the market for people that want to upgrade old six-volt bikes to solid-state from the original technology, mechanical devices. I only know of one company that makes six volt solid-state regulators for these old bikes, which have fields that switch on the low side (negative ground). And I think those regulators require a battery on the bike to work. However, some people use magnetos, and run without a battery. Nobody makes regulators for them, because it's a pain to design. These marvelous suggestions you've made aren't going to solve the problem, unfrotunately... I've come up with a design that takes extremely low voltage and boosts it, and it uses only jelly-bean parts: eight transistors and about thirty passives. In the original post I asked for some feedback from the people on the forum about the transistor that switches the inductor.

BINGO! Takes eight transistors.

And cheap ones to boot :-)

And got such good advise that there was no point in me chiming in!

We so often get folks on this group that are stuck in a narrow spot of the entirely wrong path, that most of the regulars will, at some point, try to make sure that you are, indeed, stuck at a narrow spot on the _right_ path.

That behavior has certainly benefited me in the past, when I was losing sight of the forest for the trees.

Are you actually supplying field to the generator with this boost circuit, or are you using it to power some switch?

If you need to boost just to get field -- yes, you need to boost starting from nothing, and go up from there.

Can you post a neater version of the circuit so we can actually see what's going on?

Why?

Not sure if that is necessary here but if one ever needs to get a field going where there is no source for a gate or base voltage anywhere there is an often forgotten part: The depletion mode FET. Those don't only come in small JFET varieties but also as big fat MOSFETs with rather low Rdson values.

A joule thief based on a decent JFET will do the trick.

Best regards, Piotr

Below is something to begin with. One can add an additional winding and get a flyback-like topology with an autotransformer or to use this circuit to power/start a real switcher etc. If "below one volt" means ~0.8V, then a BJT will be fine, otherwise a JFET would be necessary. The high input voltage sector is an exercise left to the reader. ;-)

Best regards, Piotr

Version 4 SHEET 1 880 680 WIRE 128 48 -64 48 WIRE 192 48 128 48 WIRE 240 48 192 48 WIRE 192 80 192 48 WIRE 240 80 240 48 WIRE -64 112 -64 48 WIRE 192 176 192 160 WIRE 240 176 240 144 WIRE 240 176 192 176 WIRE 128 192 80 192 WIRE 80 208 80 192 WIRE 128 240 128 192 WIRE 144 240 128 240 WIRE 80 304 80 288 FLAG -64 192 0 FLAG 192 272 0 FLAG 128 112 0 FLAG 80 304 0 SYMBOL njf 144 176 R0 SYMATTR InstName J1 SYMATTR Value 2N4393 SYMBOL ind2 176 64 R0 SYMATTR InstName L1 SYMATTR Value 100µ SYMATTR Type ind SYMBOL Misc\\battery -64 96 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 0.6 SYMBOL cap 112 48 R0 SYMATTR InstName C1 SYMATTR Value 10µ SYMBOL ind2 64 192 R0 SYMATTR InstName L2 SYMATTR Value 900µ SYMATTR Type ind SYMBOL cap 224 80 R0 SYMATTR InstName C2 SYMATTR Value 1n TEXT -98 328 Left 2 !.tran 0.1 TEXT 240 328 Left 2 !K L1 L2 1.0