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Carbon_Rod

Wed Nov 20 2013, 12:56PM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
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Posts: 1155

I was looking at LTC3108, bq25570 (98% efficient), and the classic joule thief...
IIRC the 2n2222 based joule thief limit is around 100:1...

However, if one is interested in converting low-voltage high-current thermopile energy sources.
Is there a low-tech solution that can efficiently take 200mV @10 watt to something over 5 V ?

Cheers,
Rod

Patrick

Wed Nov 20 2013, 03:22PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

0.2v X 50A = 10W. There may not be some simple way to manipulate that difference at that power.

However, I see no reason why this can't be done. You'll need ultra low resistance mosfets, and I think you'll need a special inductor.

EDIT: I presume you'll want to use a shotky diode to catch the increase V out, or use a transformer.

Sulaiman

Wed Nov 20 2013, 07:01PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

The first thing I'd do is see if there is a way to re-wire the thermopiles
for higher voltage at lower current.
0.2V,10W=50A, 0.2V/50A=0.004 Ohms TOTAL circuit resistance
a FET of Rds(on) <1 mOhm @ 50A would be required....

then there is the requirement for an external power source
of voltage sufficient to switch the FET gate,
if the inverter is supplying a battery then that could be the source.
.provided that the battery is never allowed to fully discharge.

BigBad

Wed Nov 20 2013, 09:50PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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You could use electromechanical switching; that has no lower voltage limit.

Basically, low voltage Tesla coil, rectifier.

Patrick

Thu Nov 21 2013, 02:30AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I agree with sulaiman, raising the voltage generated should be considered if it's possible.

Carbon_Rod

Fri Nov 22 2013, 03:50AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
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Posts: 1155

Your opinions are appreciated,
the TEG piles have undergone few design changes over the years.

Sulaiman

Fri Nov 22 2013, 01:26PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

The idea is to use several small TEG in series rather than of one large TEG (or several small TEG in parallel)

mikeselectricstuff

Sat Nov 23 2013, 10:15AM

Registered Member #311 Joined: Sun Mar 12 2006, 08:28PM
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Posts: 253

The main problem with low voltage supplies is starting them up in the first place. Once started, they can power their control circuitry from their own output. A crude but cheap and effective solution may be to use a small battery to provide an initial startup supply. Or perhaps a small motor used as a generator that can be spun by hand to get it started.
Actually thinking about it, an old-fashioned rotary converter (motor+generator) could be an option.
Or perhaps a secondary small TEG stack to provide a startup supply.
As regards conventional EH products,

make some harvesting
products based on their zero-threshold MOSFETs

Patrick

Sat Nov 23 2013, 07:57PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

An ultra capacitor or watch battery perhaps, for the bootstrapping. Mosfet gates just need voltage after all.

Carbon_Rod

Sun Nov 24 2013, 09:58AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

Not a bad idea Patric,
Perhaps a classic joule thief charging a small mcu 25uA supply would suffice for the driver logic.

Driving a low voltage Fet may be interesting, but the gate drive calculation gets a little odd.
If we assume 40nC for a gate to fully switch on than its draw is not zero as the switching frequency increases. In this case, the driver would have to hibernate awhile to accumulate enough power to fully start the main drive circuit.

@mikeselectricstuff
IIRC, n-channel fets traditionally have lower Rds on ratings than its matched p-channel version.
I am considering a low Vgs n-channel driver, but haven't really thought that design through at this stage.

@BigBAd
An electromechanical buzzer circuit may provide enough back emf to charge a starter supply. The low drop-out-voltage fast reed relay is worth a try, but fairly rare to find these days. The only one I found that could do 0.2 was a surplus â€œRES-55A 04.02â€ on ebay, and the article by MEBER on thermopile relays seems to suggest few modern setups go under 0.45 :

I plan to test driver designs with a much smaller 200 to 750 mV Honeywell thermopile, as they are under $25 locally and can handle 700 'C

Cheers,
Rod

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