Re: LM317 Charger Problem
From: Lord Garth (LGarth_at_Tantalus.net)
Date: 03/16/05
- Next message: GTD: "Looking for advise on PC FlowMeter"
- Previous message: Larry Brasfield: "Re: Resistance of ammeter caused voltage drop"
- In reply to: hboothe_at_gte.net: "LM317 Charger Problem"
- Next in thread: hboothe_at_gte.net: "Re: LM317 Charger Problem"
- Reply: hboothe_at_gte.net: "Re: LM317 Charger Problem"
- Messages sorted by: [ date ] [ thread ]
Date: Wed, 16 Mar 2005 03:44:32 GMT
<hboothe@gte.net> wrote in message
news:1110942121.407450.280620@l41g2000cwc.googlegroups.com...
> Hey, folks, I know the topic of LM317 chargers for batteries has been
> beaten to death but I've got a problem that I can't solve and my
> reading of the groups has yet to yield a similar problem; maybe my
> searching capability stinks, but who knows.
>
> Anyway, I have a cordless drill that I want to charge in my vehicle
> (13.8 volts when running (12V system)) and I built a charger using a
> LM317 that has about 7 to 10 Ohms between the Adj and the Out pins,
> runs the output through an LED and finally to my driver (marked as a
> 2.4v device). The output current is about 75 mA, upped from the prior
> value of 50 mA (after I found it wouldn't charge up with the 50 mA
> output). It still wouldn't charge up (slow running, no power) so I
> checked it *again* against the factory wall wart and it draws, even
> after being hooked up for 24 hours, around 110 mA (which explains why
> the thing lasts for about 2 days off the charger before, even without
> use, the battery goes dead). So, it seems to have some internal
> leak-down circuit; I say that because I bought 3 of them (they were
> cheap at Home Depot) and my friend has one and all four of them go dead
> after a couple of days off the charger. Anyway, I decided to try to
> build another charging circuit from scratch and test it on the
> workbench before I tear into my vehicle to work on the circuit in it.
>
> What I got is this, and it's messing with my head (which isn't well
> tuned for electrical stuff anyway); I built the charger according to
> the "Typical Applications" section of the National Semi documentation
> to be a constant current charger (having been told two years ago when I
> first built the charger by someone here in the groups that these types
> of devices are current sensitive not voltage sensitive). It should, I
> thought, be really simple: Vin = 13.8, tie Adj to the output, tie Out,
> via some resistance, to the output, and BAM, battery charger. Not
> quite. I read the notes and the calculation for the Iout should be
> 1.25/R (1.25 nominal voltage diff between Out and Adj, and R being the
> resistance between Out and the load. So, Vin =13.8vdc, Adj goes
> straight to the load, and Out goes through R to the load. R=10 Ohms,
> so Iout should be 1.25 / 10 = 0.125 or 125 mA, and since the driver
> eats (sinks) about 110 mA when fully charged the 125 mA should be
> enough to charge the device (slowly) and then provide enough juice to
> keep it up.
>
> The only difference between this installation and the one in my vehicle
> currently is that I didn't run the final output through a LED before
> the load.
>
> Finally, the problem; I measured the current through the device with
> two different meters and I get about 460 mA...? I double checked my
> pin locations on the TO-220 package, double checked my wiring (it's one
> IC, one 10 ohm resistor, 3 pieces of wire or so...it can't be too
> difficult to get it right...can it?), cleaned the breadboard of other
> projects to make sure I'm not seeing another circuit involved, and yet
> it's still drawing the same amount of current. I'm afraid to leave it
> running for more than a few seconds because I think it might blow that
> battery up. I'm going to try a constant voltage setup to see if that
> might not fix it.
>
> But, in case that doesn't work, and even if it does, just so I'll know,
> is there something blatantly stupid I'm doing here? I even tried
> another LM317 and got the same results (with the way I do electronics I
> never buy just one of anything <GRIN>).
>
> Hope someone can shed some light on this for me. Thank you for your
> time and help.
>
> --HC
You need a constant current at more than 12 volts to charge the battery.
The
LM317 will lose a couple of volts across itself and that alone puts you
below
what is needed.
You need to boost the voltage first and then regulate the current. There
are
many IC's that can boost the applied voltage but their output current will
be limited.
I doubt you'll achieve a rapid charge though you will probably be able to
trickle charge.
That could take about 10 hours which I'm sure is unacceptable.
How about going the easy way and get an inverter...this will create 120 VAC
and you
can use your home charger to do the job. This is not efficient so watch the
drain on
your vehicle battery.
- Next message: GTD: "Looking for advise on PC FlowMeter"
- Previous message: Larry Brasfield: "Re: Resistance of ammeter caused voltage drop"
- In reply to: hboothe_at_gte.net: "LM317 Charger Problem"
- Next in thread: hboothe_at_gte.net: "Re: LM317 Charger Problem"
- Reply: hboothe_at_gte.net: "Re: LM317 Charger Problem"
- Messages sorted by: [ date ] [ thread ]
Relevant Pages
|
