Battery Backup reducing current demand

Thread Starter

Russell

Joined Sep 20, 2006
12
Hi

I have just built a digital readout for my milling machine using a design available on the web, and I have built a battery backup for it using a design from the same source. While the battery backup works well I would like to extend the battery life if I can.

At the moment data from the milling machines three capacitive scales is transmitted at 3.3 V to a MSP430G2553. The battery backup uses two Schottky diodes and a capacitor and keeps the scales powered so they don't lose track of their position while shutting down the MSP430G2553.

When running on the battery the current drawn is about 700uA but if I disconnect the three data lines to the MSP430G2553 then the current drops to about 35uA. The battery backup is from AA cells which would give the batteries a life of about 4 months. I'd like it to be over a year if I can and my initial idea is to use a 74HC08 to disconnect the data lines when running on battery. In that idea one input for each gate would come from the main power supply and the other from the data lines coming from the scales. The chip would be powered from the battery.

After thinking about it a bit I am less certain about this. The power consumption would only be reduced if the input currents were very low. I wondered about a 74 series buffer instead. What I would like is something that works like a triple pole normally open 3.3V relay.

I'd welcome any advice.

Russell
 

crutschow

Joined Mar 14, 2008
27,402
The 74HC08 should work since the input is a very high impedance when powered (taking only a very low leakage current).

Actually all you should need is a non-inverting buffer such as the 74HC34 to isolate the input from the output.
That would need no connection to the mains supply.

You can also significantly increase battery life by going from an AA to a C (≈2.5x) or D (≈4.8x) size alkaline.
 

Thread Starter

Russell

Joined Sep 20, 2006
12
The 74HC08 should work since the input is a very high impedance when powered (taking only a very low leakage current).

Actually all you should need is a non-inverting buffer such as the 74HC34 to isolate the input from the output.
That would need no connection to the mains supply.

You can also significantly increase battery life by going from an AA to a C (≈2.5x) or D (≈4.8x) size alkaline.
Thanks crutschow.

I've been thinking about using a buffer and I don't think it would work. The MSP430G2553 is sinking current when it's off, so it would sink current from the buffer too - unless the buffer had a high impedance input when it was off so it wouldn't reduce the backup battery current.

I'm also wondering whether I'm overcomplicating this and could use three transistors to switch off the data leads.

Russell
 

MisterBill2

Joined Jan 23, 2018
9,004
For much longer battery life another option would be higher capacity batteries. And to get the very best battery life use some rechargable cells and a low power float charge system.That should give you a battery life of several years.
 

crutschow

Joined Mar 14, 2008
27,402
The MSP430G2553 is sinking current when it's off, so it would sink current from the buffer too - unless the buffer had a high impedance input when it was off so it wouldn't reduce the backup battery current.
Yes, I think that would be be true if the signal is high.
Better to use the CD74HC08, which will keep all the outputs low when the main power supply inputs are low.
 

Thread Starter

Russell

Joined Sep 20, 2006
12
Thanks Crutschow.

MisterBill2, that's a perfectly reasonable suggestion, but I used a PCB mounted battery holder as I had one that literally fell out of the sky (the remains of a radiosonde weather balloon) and I don't think the larger batteries would fit in the box. I could add some leads and mount the batteries externally but it wouldn't be as neat. That doesn't mean I don't appreciate the suggestion. I'm grateful to all who help.

Russell
 

crutschow

Joined Mar 14, 2008
27,402
An alternate to the 74HC08 is a CD4081 AND gate.
It's slower but tougher, and your circuit likely doesn't need the speed of a 74HC08.
 
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