Re: need cap. charge indicator


Jim wrote:
I built a battery tab spot welder using multiple 5 volt 150,000 uf computer
grade capacitors in parallel, as the heart. The capacitors are charged
using a small 5 volt unregulated charger. This works well, except I am not
sure when the capacitor bank is fully charged up, as it takes a long time.

What can I use as a charge indicator? All I really need is for an LED to
light or something like that. Can someone please suggest a circuit or an
idea that would work for this application?

Thanks
Jim

Hi, Jim. One easy newbie way to do this is to rely on the pull-in
voltage of a small DIP package reed relay.

If you were to get, say, a Magnecraft W107-DIP1 (Mouser P/N 528-107-1),
you'd find it has a nominal coil voltage of 5V, with a guaranteed
pull-in voltage of 3.8V and a coil resistance of about 500 ohms. If
you were to place the coil in series with a 1K ohm pot, and adjust the
pot so the relay just clicks in at your desired voltage, you'd have a
reliable and very simple method of indicating charge voltage (view in
fixed font or M$ Notepad):
| ___
| o--------o-|___|--o--------o----------o----o-- . .--o
| | | R | | | | |
| | \ o | | | | |
|6V +| \ | .-. | | |
| --- CRY1\. Vz /-/ .->| |1K | | |
| - o 5.1V ^ | | | | | |
| | | | | '-' +| +| +|
| | .-. | | | --- --- . . .---
| | 220| | | '---o C --- C--- C---
| | | | | | | | |
| | '-' | RY1 C| | | |
| | | | C| | | |
| | LED V ~ | C| | | |
| | - ~ | | | | |
| | | | | | | |
| '--------o--------o--------o----------o----o--. . --o
|
(created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

While the relay is guaranteed to pull in at 3.8V, it will probably pull
in at around 3V. You should tweak the pot so the relay just turns on
when the caps reach the specified voltage (you might want to use a
meter for this if you have one). This pull-in voltage will remain
fairly stable over time and temp, so you can set the pot, and forget
it. And your very simple, reliable indicator circuit will draw less
than 10mA from the caps.

The relay will let go by itself when you discharge the caps. Vibration
may affect the pull-in of the relay, so be sure to mount the piece of
perfboard on something stable.

If you find another 5V relay that has a different coil resistance, just
measure it, and use a pot with a higher resistance than the coil.
Tweak it to the right pull-in point as above, and be sure to take the
extra current draw from the caps into account.

Questions of this type usually find a good response on s.e.b., too.

Good luck
Chris

.

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