Re: 1502 TDR Battery problems

Tom Gardner

On 11/06/20 17:24, Chuck Harris wrote:
How is a zener dissipating the power necessary to get the
voltage down to 12V any different from a shunt resistor dissipating
the power necessary to get the voltage down to 12V?
A fair question.

My reticence is based on not having thought through how the "unique" characteristics of the 1502's charging circuit interact with a resistor or clamp.

Consider the /hypothetical/ example where a charging circuit had a 16V output and a very low output impedance.
With a resistor R, the max dissipation would be 16*16/R W.
With a clamp it would be determined by the clamp's impedance, which ought to be much lower than R. Hence the current might be much higher than 16/R and the power dissipation correspondingly higher.

The magnitude of any such effect would depend on the detailed operation of the charging circuit, which I haven't considered in this context.

Seems to me they are both the same, and both dissipating the same
power the nicad pack is taking in. Granted, a charging nicad pack
is endothermic, and a resistor is exothermic...
And the case is tightly sealed. Measurement would be required!

-Chuck Harris

Tom Gardner wrote:
On 11/06/20 16:02, Mark Pilant wrote:
I tried for quite a while to get the "battery fooler" to work *reliably* in
the 1502 and 1503 I have.  I was never successful.

The zener concept isn't too bad, but there is the power dissipation in the enclosed

I did try the Batteries Plus route and had them make up a replacement pack.
While it looked fine, it wouldn't fit.  The original Tek design used flat
top "C" cells, and pretty much all that is available are button top "C"
cells.  According to the specifications I could find, the "original" flat
top "C" cells were slightly shorter than the button top "C" cells; by the
height of the button.
I have found these acceptable

The diameter is slightly too small, but that is easily rectified with some
closed-cell foam.

BTW, I did some checking, and these batteries will last about a month before
needing to be recharged when "sitting" in TDR that has been turned off.
Yes. There is an ~1mA (higher if the chart recorder is present) drain even with the
front panel switch off. If the battery is left in then eventually one or more of the
cells will be damaged by being reverse charged. That's the reason the manual
specified the battery should be charged monthly.

Personally I just leave the battery pack out of the case. If I wanted to keep it in
the case, e.g. for shipping or long-term storage, then I would remove the fuse in the
battery pack.

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