Low loss/dissipation xistor or ...

[Bob Engelhardt]

The project:
I am making electrically heated socks & gloves. For power I'm using the
9.6v NiCad batteries from my cordless drill. The working assumption is
that each sock & glove will need 1w, but that's very tentative & I'll
have a regulator to adjust as needed. The critical parameter is the
9w-hr available from a battery before needing to be recharged. So the
regulator must have minimum losses. The 2 socks & the 2 gloves will be
in series & independently regulated.

The current plan is to use a 555 timer for each pair & vary the duty
cycle to achieve regulation. But the 555 can't drive the heaters
directly - they'll require about an amp. I'm thinking of using SMT
BJT's from ON Semi:
http://www.onsemi.com/PowerSolutions/parametrics.do?id=808
because the very low VCEsat (as low as 50mv) will minimize the regulator
losses.

The questions:
- is there another device that I could use instead of BJT's? Switching
10v & carrying an amp at low loss (1/10w +-). (OK, now it's obvious - I
don't know anything about semiconductors <G>)

- are there through-hole BJT's with very low VCEsat? (I've not used
SMT's & I suspect that they'll be a fabrication challenge.) Googling on
"low VCEsat BJT" was not helpful.

Thanks,
Bob

BTW - I hang around rec.crafts.metalworking mostly & when I subscribed
here I was REALLY impressed the the minimal amount of OT posts. No
political OT that I recall. Keep up the good work.

 

[Eeyore]

The project:
I am making electrically heated socks & gloves. For power I'm using the
9.6v NiCad batteries from my cordless drill. The working assumption is
that each sock & glove will need 1w, but that's very tentative & I'll
have a regulator to adjust as needed. The critical parameter is the
9w-hr available from a battery before needing to be recharged. So the
regulator must have minimum losses. The 2 socks & the 2 gloves will be
in series & independently regulated.

The current plan is to use a 555 timer for each pair & vary the duty
cycle to achieve regulation. But the 555 can't drive the heaters
directly - they'll require about an amp. I'm thinking of using SMT
BJT's from ON Semi:
http://www.onsemi.com/PowerSolutions/parametrics.do?id=808
because the very low VCEsat (as low as 50mv) will minimize the regulator
losses.

The questions:
- is there another device that I could use instead of BJT's? Switching
10v & carrying an amp at low loss (1/10w +-). (OK, now it's obvious - I
don't know anything about semiconductors <G>)

- are there through-hole BJT's with very low VCEsat? (I've not used
SMT's & I suspect that they'll be a fabrication challenge.) Googling on
"low VCEsat BJT" was not helpful.

Well there are Mosfets but if your Vce is only going to be 50mV, I don't see
much advantage.

Graham

 

[neon]

now 1w is significant drain on any small battery it will not last very long at that rate. anyhow why using a timer to control current? I know you don't know. use a lm317 T0220 REGUALTOR good for an amp and all you need is a pot of 100 ohm to regulate the current essentialy the heat. the idea of design is to make it simple less parts more reliable.
get an LM317 add a 100 ohms [pot] from out to adjust apins and there you go. the adjust pin is now your power source for. you need current so adjust that screw the voltage is immaterial.

 

[Bob Engelhardt]

If you put the transistor in the sock, then any losses from the
transistor will heat the sock just as well as a resistor would.

Ah, good idea. I'll make it a consideration.

In fact, if you controlled it right you could just use the transistor in
linear mode, and forget the resistor.

But why not just use a resistor with the right value to dissipate a watt
with a 9V input? You'll see a bit of extra dissipation with fully
charged batteries and less at low, but it won't be noticeable.

The heater is not a discrete resistor - it's heating wire from an
electric blanket. About 2 ohms per foot. Better heat distribution.

And the 1 watt I'm figuring with is very tentative - I'm sure that it
will change once it's working & in "trials".

(actually, IMHO, you shouldn't sweat the "constant power" issue at all.
What you should concern yourself with is a circuit to shut the thing down
when the batteries get to the end of their charge, to avoid damaging the
pack).

Actually, I'm not concerned with constant power at all. I'm assuming
that occasional adjustment will be desirable.

I did some measurements on my battery with a 1 w load: fully charged
it's about 11v, declining over a hour (linearly) to about 9v & then
going to hell very quickly. Knowing that, I was planning on having a
low battery detector.

Thanks,
Bob

 

[Bob Engelhardt]

Something doesn't jibe here. You're using a cordless drill pack, which
should deliver about 1A-hour, and you say a 1W load lasts about an hour.
But that 1W load should consume about 110mA (only slightly more at 11V),
which means that you should have a good 8-9 hours from a 1A-hour battery.

Even the wimpiest imaginable electric screwdriver pack should be at least
500mA-hour, so we're still not down to one hour.

Are you sure you weren't running 10W (1A)?

You're right - good "catch". I wrote about the battery from memory &
had it wrong. I was just using a 10 ohm load - 1A. And fully charged
was 10v, not 11 - just to get all the facts righted.

A low battery _shutdown_ is, IMHO, a Very Good Thing. The references I
have on the care and feeding of NiCd batteries claim that you can run a
single NiCd cell down to nothing with little damage, but charging it in
reverse is Very Bad Juju. With a pack, there will always be some
mismatch in the cells, so when you get down to 0.9V/cell or so you can
pretty much count on some cells being well above 0.9V and at least one
poor bastard that's at 0V and getting charge rammed through the wrong way.

....

I've read that too, but had forgotten it. My plans for a low battery
detector was motivated by a desire to keep the heater going & my toes
warm <G>. Thanks for the reminder.

Bob