Low voltage step up design?

Old germanium might be the only thing that'll let you run that low.. I mean if you keep a higher voltage supply (1-5V) at all times, you could even use FETs and get better efficiency still, but silicon BJTs need 0.7V or more to turn on and more than 2V for MOSFETs. If it shuts down and you don't have anything to start it below the operating voltage, you'll have to charge it again before it can be used.

Ge gets Vf ~= 0.3V, as compared to Si ~= 0.7V. It's usually slower, so you'll need an iron cored transformer for the inverter. Such circuits have been used for single solar cells.

How much current do you need, anyway?

Tim

-- Deep Fryer: a very philosophical monk. Website:

Search the converter sites for "single cell." There are a bunch. When you're cut off at 0.8V, keep in mind that it's only 10% of the energy you're leaving in the 2.5V cap, not 30%. You may be challenged to find the current you want if it's excessive. If you're fine with low current, you may need to boost with the single cell converter first and boost to 14V second. You could also use the single cell converter to bootstrap a better boost converter. Options abound.

With care a two-step converter isn't necessary. Bob is starting with a charged cap, so he can power the converter IC from the cap to start, and from the converter's output as it starts running.

I think a flyback _transformer_ is the best configuration for Bob. E.g, a 1:6 step-up ratio would allow for a sensible switching duty cycle, especially at the low end of the input-voltage range. If he wishes, Bob can add a second winding to run the switcher IC at its sweet-spot voltage after it's operating.

Hi, Win,

There are lots of small, cheap, stocked 2-winding surface-mount toroidal transformers around. This can sometimes be handy...

Vin----=========--*--=========-----ak----+-----Vout | | | | d c ---g | s | | gnd gnd

It's a sort of autotransformer bootstrapped onto the input voltage. You don't get a lot of choice on turns ratio, I guess.

John

Hello Tim,

Ge is expensive and not well suited for any serious production. You are basically dealing with a limited stock, almost collector's items.

Another option might be to add a nifty start-up circuit around a JFET or other depletion mode device.

I would've suggested tubes, but they typically don't go that low.

That's true, he didn't mention if this was production or what...

Tim