<<<<   Index   

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.

<<<<   Index