Re: Switching +/-12V from 6/0V

On Jul 15, 9:18 am, George <gh424NO824S...@xxxxxxx> wrote:
In article <1184515263.294863.297...@xxxxxxxxxxxxxxxxxxxxxx
ps.com>, dagmargoodb...@xxxxxxxxx says...

> The short version of all this is that the NPN transistor
> driving your relay (Q2) needs a certain amount of base
> current to do its 'thing,' to make sure it turns fully
> 'on' and drives your relay. We're all just trying to
> make sure it gets enough.

Yes, I understand that. And I'm just trying to make sure it
doesn't get more than it needs and waste battery power for
no good reason.

> The amount needed varies with the gain of the NPN
> transistor. To switch 120mA, for example, a nice
> transistor might need a base current that is only 1% of
> that, or 1.2mA. So 1.6mA might even be enough, if
> you're lucky. A random junk-box transistor might need a
> base current as much as 1/10 its collector current to
> really pull all the way down. *We* over here don't know
> what your Q2 transistor is capable of--only you know
> that.

Well, the transistor in question is a 2N2222A in a TO-18
case. Now you know as much about it as I do.

Not quite. What you know (and we don't) is whether Q2's collector
pulls all the way to its emitter supply. Really, I don't think it's
that important here: since you've got supply voltage to spare, it
doesn't hurt if Q2 fails to saturate.

After checking, your 2n2222a is guaranteed to have a gain of not
less than 100 at 150mA and 1.0 volts collector-to-emitter, so that
sounds like a pretty good choice. In theory, therefore, 1.6mA is just
enough--even in the worst case--and most 2n2222as will have more gain
and would operate with even less drive.

And, you know, in theory since the cap starts at +/-12V,
we're looking at a 240ma current at the very beginning,
although that drops very quickly. I guess if I could rig up
a +/-12V regulated supply, and a 100-ohm resistor, then I
could actually measure whether a given base current turns it
on fully. Let's see - if I park two cars next to each
other, then I could....

> However much current your particular Q2 needs, it's a
> tiny fraction compared to the amount of your MAX232
> supply's current that goes to driving your relay.
> Therefore, Q2's 'switching current' isn't very important
> UNLESS you're leaving this thing on for a long time.

> If you leave the circuit on all the time, then Q2 (and
> the relay) will drag on your +/-12v and prevent them
> from building up.

It's on for less than a second each time.

A second here's a pretty long time! Fortunately you're using higher
than the coil's rated voltage. That gives your mechanism a big push
initially, when the magnetic pieces are farthest apart and hardest to
move. Once the solenoid pulls 'in' it's easily held there with a much
lower current.

But, even considering that, what size capacitor do we need?

Suppose that the capacitor starts at 20v, and we need an average
i=80mA, at 10v or more, for a second. This means we can afford to
discharge the capacitor by 10 volts. Well, rearranging the
approximation i*t = C*v, we get C = (i * t)/v, or (0.08amps * 1
second)/(10 volts) = 8,000uF !! FYI.

That pretty much
fully discharges the cap. Then it slowly (about 3 seconds)
builds up charge again during the Off period, which in my
circuit is adjustable from about 5 seconds to over a minute.

> You're relying on the voltage built up in your filter
> capacitors to do all this work, so once you start it has
> to be done quickly--push that button, then release it!
> If your MAX232 supply doesn't have enough energy to do
> that for as long as you'd like, then maybe you need some
> bigger capacitors.

Well, at that point the MAX232 isn't contributing much. In
fact, it isn't enough to keep the relay pulled in. So, it's
basically all in the cap.

Any and all contributions are welcome though! I think the MAX232
may be working harder than you suspect, augmenting the (marginal)
energy stores from your big storage capacitor.

>> Maybe I should reverse everything and put the load
>> below the transistor, so that the switching current
>> also flows through the load, but I think that may
>> complicate things more than it's worth. I don't know.
>> I'll play with that.

> I don't see how that would help. If it's already
> working, then heck, you're pretty close, right?

Hell, it was working back when I was using three
transistors. But everybody said it would be better this way
or that way, so I'm just trying to make it as good as I can.
From playing with it, it's clear to me that getting that
initial full current dump through the coil is critical to
getting a reliable pull-in, even when the batteries are
partially used up and all the voltages are lower. So I
understand the need to turn the transistor fully on. I just
don't want to use 4ma to do that if 1.5ma is all that's
needed.

Right, understood. We just don't quite understand why you're
worried about 2-3mA when the relay is burning 100-to-200mA. But it
does seem you're more 'right' than we were--your 2n2222a doesn't need
very much drive, and even 1.6mA may serve fine.

Anyway, I've played with your no-base-resistors version and
it does seem to work quite well. So, I'll use that, with
the only issue being the emitter resistor value.

But it does seem to me that putting the load below the NPN
would make all the current flow through the coil, and thus
not waste any of it no matter how large the switching
current.

Ah, now I understand. You want to use the NPN Q2 as an emitter
follower, and put that base current to work. Sorry, but a) the
circuit isn't set up that way and b) there's no free lunch--the
circuit that *would* do that will pull a lot of current, even when
off. And remember, even if any given new configuration worked, it'd
only save you 1-5% of the current being used. It's not very much.

I just don't know what complications that
introduces when the switching current has to deal with that
inductance. It's also not clear where you would put the
snubber diode(s).

So, I'll play with it some more. It would be nice to get it
right - just in principle. Otherwise, I would have farted
it off three days ago when it first worked. I mean, you're
having fun, aren't you? I certainly am. It's all in the
journey.

Of course I'm having fun!

Hey, if you're really dead-set on cutting that switching current just
for the challenge, you could make Q2 a Darlington--or even an n-
channel MOSFET--and raise the resistor values to suit. That'll save
oodles of switching current.

Thanks very much for your help.

You've been cheerful, communicative, clear, willing to try things &
eager to learn. It's been my pleasure sir.

Sincerely,
James Arthur

.

Quantcast