Re: Multimeter damaged when powered from regulated power suppy.

Geogle wrote:

Thanks much Chris!

That is really a cool idea!, though the power requirement of the
measuring device goes up with the inclusion of a relay. A solid state
relay of reasonable cost would be a nicer choice!.


I think we'd all love "a solid state relay of reasonable cost" :-)


It is just for home use, nothing serious. When the meter got damaged, I
thought
of asking for experts help - so the posting.



I think simplest would just be one NiCd pack, connected to a
trickle* charger by a SPDT switch. In the off position, the NiCd
is connected to the charger (C/20 or lower) and in the on position
it's connected to the DMM. Add an auto-off circuit if you are
afraid of forgetting and leaving the thing on.
*= for the trickle charger you want to keep the charge rate
very low once the battery pack is charged. You can use a LM317
set for constant c and a 431 to pull current once the terminal
voltage is reached.

If you have to have 24x7 meter operation, here's what you can do
using a DPDT relay, charger and NiCds:

---------
Wallwart===|charger +|---------+ +------+----+
| | | | | |
| -|---+ o o | D
--------- | | | [C] M
| |>| <| |>| <| | M
+--------+ | | | | | |
| | | | | +----+
+---NiCd1----+ +---+ | |
| | | | |
+---NiCd2--- | ---+ | |
| | | |
| +-------------+ |
| |
+-----------------------------------+


The idea is that while one NiCd pack is charging, the other
is powering the meter. This will give you 24x7 use of the DMM.
A large C will power the DMM during the ~10 mS transition as
the relay switches. You can switch the relay based on the
voltage of the NiCd in use with a comparator & driver, powering
that circuit from the wallwart. Or you could do it based on
time.

Ed




.

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