Re: 1502 TDR Battery problems

Chuck Harris

OK, fair enough.

The circuit description tells us the average current out
of the charger is 150ma... regardless of the charge state
of the battery pack.

It also tells us it is a pulse charger, so, it could be creating
larger current pulses, as long as the average is 150ma.

A quick look at the schematic shows the transformer secondary
is connected directly (+/- a current shunt resistor) to the
Nicad pack. The SCR triggers every cycle where the average
current is below 150ma, and doesn't trigger when the average
current is above 150ma.

One thing that any engineer who has worked with nicad packs
knows is Nicads are wily beasts. They short circuit, and they
open circuit, almost at will.

Another thing they know is Nicad's are very, very low impedance
devices, and will not easily be bullied into going above about
1.7V/cell... especially at 1/10th C (trickle) charging rate.

I can't see any risk in trying. The 12V 2-5W zener will have
much higher resistance than the 1AH Nicad pack.

Quick calculation: P = I x E = 0.15A x 12V = 1.8W max zener

-Chuck Harris

The Nicad pack is
Tom Gardner wrote:

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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