voltage divider

[hpibmx]

Hi,

I'm getting crazy here now, I have got this aneng AN8009 which operates at 3v (AAA x2).

I have got 4.2v battery old iphone and even 18650, All I need to bring this done to 3v so I'll use this non stop.

I have tried 10k pot and a diode for 18650 but it didn't work.

Any idea why? or any other suggestion?

Thank you.

[mariush]

I would first open it up and look at the circuit board and try to figure out what chip it uses, and see that chip's datasheet.
A lot of chips can function with a higher voltage range, like 2.5v ... 5.5v ... your 2 AAA batteries could give the multimeter anything between 2 x 1v (discharged rechargeable)  to 2 x 1.65v = 3.3v (fresh alkaline ni-mh batteries)

some multimeters already contain a small voltage regulator which converts the battery input to a fixed 2.5v output or something similar, in which case you'd only have to check if that voltage regulator can handle 4-5v from your battery,

Anyway, if all the above fails, the easiest would be to use a fixed output voltage regulator set to 3.3v or 3v - you can easily find lots of them at stores like digikey, mouser, newark/farnell etc

Linear regulators would be the simplest, just the chip and a small ceramic capacitor on the output would be enough... but they will not be very efficient, because the difference between input voltage and output voltage is lost as heat. You can get 1-2$ step-down (buck) voltage regulators which will be more efficient.
   
Alternatively you could put several diodes in series with the batteries. For example, a simple 1n4001...1n4007 diode will cause a voltage drop of around 0.5v..0.8v so if you add two diodes in series with your  3.6..4.2v battery, you'll get 2v...3v just like you'd get with your 2 aaa batteries. 
that extra battery power will be wasted in diodes as heat so just like linear regulators it wouldn't be efficient as a step-down regulator

 

[palpurul]

I don't really understand. Are you trying to get 4.2V to 3.3V level try to supply your Multimeter?
It's never going to work because you make the resistance of your supply too high to supply anything.
Can you share a simple schematic, so that I can understand better.

[Damianos]

Using an external battery the internal circuitry is exposed to the outside! The first thing that must be done is to put a big and clear sign that this cannot be used in any voltage higher than 30V or a current over a few hundreds mA!
For the voltage limit I think that it is easy to understand. For the current limit, imagine the case that there is a high inductance in the circuit and the fuse breaks; what happens at that moment? The kickback voltage from the inductor will be present to outside, instead of absorbed inside (even destroying part of the circuit).

Something more: If a device is designed to operate with batteries, then any ripple from an inappropriate regulator may create false functioning. Murphy's law says that this will happen when performing a critical measurement.

[Kleinstein]

Most simpler DMMs don't take that much power and last some 1000 hours with a set of normal batteries. Under normal use this may very well be a few years before a battery replacement is needed.  So it it really worth to modify the supplies and this way compromise saftety ?

The current needed may vary over time. So the could be short bursts of high current needed and with the simple series resistor this would fail. 1 or 2 series diodes would be the slightly better choice.
One may not notice of the rechargable battery discharges to low - so one may end up with only 1 discharge cycle.

[Gyro]

A variant of the thread / thread title seems to come up pretty much every week. Voltage dividers don't work (at least not as expected) if you draw current from them. Maybe we need a sticky.

With regard to the DMM, I'd try just putting a couple of NiMH cells into it. You can still recharge them and they won't leak like Alkalines!

[BeBuLamar]

actually if you simply connect to 4.2V I think it would work. The voltage divider would not work. A voltage regulator is better.

[tunk]

And there's no cover; you risk ripping it open through
wear and tear. A worst case would be shorting it.

[Terry Bites]

Make yourself a little buck converter eg TLV62569.