Kick-starting a circuit?

Been pondering about using up every last watt in a cell using various circuits. Some of the circuits I'm comtemplating use components that require 3-4.5V input (but very little current 5mA down to 10uA). This isn't a problem to deliver once the circuit is running and the inductor is boosting the output voltage. But is it possible to do something to jumpstart the circuit without using an external powersource?

I was thinking about a capacitor of suitable specs (220uF 6.3V perhaps?) hooked up to the cell. The capacitor should charge up a little. Then, when the circuit is hooked up, the capacitor would give the output voltage a little boost, just enough to start up the components and start the oscillations to get the inductor running.

Possible?

Pardon me if this has been covered before. Thanks.

Such circuits are often called 'bootstrap' (as in to 'pull one's self up by your own bootstraps') and while possible, in my experience, are often tricky and problem plagued.. Other solutions might use a small supply for just this purpose (often shut down later in the cycle), although this much increases the complexity and cost.

In a strange way it's kinda like a W.W.II B29 bomber, the 'all electric wonder'. The plane had seven 400 Amp starter/generators on board to run the plane. Two each on inboard engines, one on the outboards. These provided the huge electrical load of the plane ('everything' ran on electricity, including gun turrets). The seventh was in the tail with a small gas engine and small battery. You'd start this one first, when up to speed it would give you enough current to crank up an inboard engine which when once running could start up the rest of the ship. The seventh was then shut down. The combined weight was in favor of this against a large heavy battery.

While I understand (and somewhat share) the enjoyment of 'getting the last bit', let's agree that there is a diminishing returns issue in play. Percentage wise, there is little real energy left in such 'spent' cells. OTOH, I went to school with a guy that kept his (small) Honda motorcycle fed by draining as much gas as he could from the hoses at closed gas stations into a coffee can.....

Doug Owen

Hey, Thanks Doug. You are full of interesting information. I didn't know that about the B29. In fact, I think I know very little about the B29 except, IIRC, the Enola Gay was one. Don't even know how it got its name.

Perhaps the issue about "getting the last bit" is not quite what it is. I have several lights that run 4 cells. But some of the circuits I have been looking at use components that need at least 4.5V input. With circuits that use the 555 timer IC, I can at least use transistors to do the astable vibrator bit of the circuit and so run the battery down lower, but the rest of the circuits would be dead while there was still the equivalent of 3 cells left.

That's why I was so interested in the regulated SatCure. A 4 cell light would last quite a while with such a circuit. An astable vibrator would work too, but would involve at least 2 more transistors, 2 more capacitors, 3 more resistors and that's before we get in to regulated output.

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Steelwolf said:
Hey, Thanks Doug. You are full of interesting information. I didn't know that about the B29. In fact, I think I know very little about the B29 except, IIRC, the Enola Gay was one. Don't even know how it got its name.

Perhaps the issue about "getting the last bit" is not quite what it is. I have several lights that run 4 cells. But some of the circuits I have been looking at use components that need at least 4.5V input. With circuits that use the 555 timer IC, I can at least use transistors to do the astable vibrator bit of the circuit and so run the battery down lower, but the rest of the circuits would be dead while there was still the equivalent of 3 cells left.

That's why I was so interested in the regulated SatCure. A 4 cell light would last quite a while with such a circuit. An astable vibrator would work too, but would involve at least 2 more transistors, 2 more capacitors, 3 more resistors and that's before we get in to regulated output.

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Hey, strange information is only one of the many things I'm full of......

FWIW, Enola Gay was Tibbits' (plane's pilot) mother's madmen name (somewhat of a departure I guess). I come by it honestly this time, one of the two last flying B29s ("Fertile Myrtle", the pretty one of the two) used to hang out at the Oakland Airport, where a friend was a member of the ground crew. You can see her 'staring' in the Disney movie 'The last Flight of Noah's Ark".

I guess I don't really understand what you're after. The 555 bit confuses me. I trust you understand that 'your circuit' won't work at all if the battery is higher than Vf? Trace the path from battery plus through the primary, rectifier diode to the LEDs. And such circuits typically have short working voltage ranges and big changes in performance within that range.

Perhaps you can tell us just what you're trying to do?

Doug Owen

Doug, Nothing concrete just yet. But essentially, a stepup/stepdown circuit. One that I'm looking at uses an astable vibrator to provide pulses to a MOSFET, which chops up the power to provide just enough to run the LEDs when Vin is greater than Vf. When Vin drops below Vf+0.5V, an inductor cuts in. Still working on it. But I had hoped to use the 555 as the vibrator to reduce parts count.

Another circuit that came to light recently was because I was searching for those pesky transistors to use in the regulated SatCure circuit. This new electronics store has the MC34063 monolithic DC-DC IC. (and quite cheap too.) The IC apparantly can do both step up and step down depending on external configuration (and I think there is one slightly less efficient configuration that can do both step up and down). However, the minimum input voltage is 3V. I wondered if it might be possible to somehow use it in a 2-cell flashlight. Of course I already have somethings planned for it in my 4-cell lights.

Still not that great though. With 4 cells, it would run to 50% of battery capacity at about 80% efficiency. The maximum input is 40V, so this could be used in bigger flashlights and extract more energy out.