The Appa51 thermocouple thermometer.
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Modifying an Appa51 thermometer for lower battery consumption
The Appa51 is a digital thermocouple thermometer.
Someone asked me to take a look at it, because the battery in the meter was in
short time empty, even when the meter was (almost) not used.
The Appa51 thermocouple thermometer.
Backside of the meter
A first check showed the meter was consuming constantly 0.3 milli-Ampere in
the off-state.
Which means it will draw a new 9 volt battery in 2 or 3 month completely empty,
even if the meter has not been used.
I found the high current consumption in the off state was caused by a design
error in the power on-off circuit of the meter.
The on-off circuit uses a 4013 type flip-flop, which is constantly connected to
the 9 volt battery.
When you press the power button, the output of the flip-flop is turning on the
rest of the meter.
When you press the button once again, the output of the flip-flop is turning off
the meter.
So far, so good.
But the 4013 contains two flip-flops inside.
The second one is not used in the circuit.
The error is, they left all inputs of this second flip-flop open (floating).
When an input of a digital gate (in this case a flip-flip) is open, the gate
will draw excessive current.
So, the solution was simple, I connected the inputs of the second flip-flop to 0
volt, which solves the whole problem.
Circuit board before modification.
Circuit board after modification.
A piece of wire is connecting all 4 inputs of the second flip-flop to 0 volt.
Detail of the wire, connecting pins 7 (0 volt), pin 8 (set input), pin 9 (data input), pin 10 (clear input) and pin 11 (clock input) of the 4013.
The power consumption is now 2 nano-Ampere in the off-state, so 150000 times
less the first.
In the on-state the power consumption is by the way 5.5 milli-Ampere, which
means it will work about 100 hours on a good quality 9 volt battery.
Hoping this article helped you, if you have this same thermometer with the same
problem.
Later I found someone with the same type of meter, but without this problem, so
probably the producer has also solved this issue in newer production series.