2012-09-14 9V Battery Substitute

DMMs All Over The Place – I have more than a half dozen of those cheapo Cen-Tech DMMs from Harbor Freight laying around my house, and I use them a lot for checking the AA and AAA cells that I’m constantly trying to drain with my germanium Joule Thiefs with their multicolored LEDs, of which I have several going.  The cells still have enough juice in them to keep the JTs powered for a week, even though they only have a volt when I put them on the germanium JTs.  I’ve been trying to use up those old AA cells; some have date codes of the late 1980s.  Seriously.  But I digress.

All those DMMs have been used for more than a year; I haven’t bought any in quite awhile.  The funny part about it is the test leads or lead wires go bad before the battery does.  I’ve got a couple of the meters where I cut the banana plug end off the wire and threaded the wire through the socket in the meter, and soldered it to the PC board inside.  I can’t remember the last time I needed to unplug the leads, so this works great for fixing the problem with the wire going bad at the plug end.  Sometimes the probe end goes bad and I cut it off and solder an alligator clip on it instead.  That works good, too.

I have a few other good meters, too.  One is a Fluke 77, and another is a B&K DMM with a frequency range which really comes in handy when working on JTs.  It has an auto shutoff so the battery in it has been going for years.  But the Cen-Tech DMMs don’t and they have been left on on occasion until the  9V “Extra Heavy Battery” (that’s what it says!) goes dead.  So I thought about changing the meter so that it will run off a single AA or AAA cell.

I measured the current from the 9V to the meter, and it measures the most when it’s on the resistance range, something close to 4 milliamps.  That’s quite low and the reason why the batteries last so long.  The total power is 9V times .004A or 36 milliwatts.  I could make a Joule Thief DC to DC converter and it would easily be able to supply that much current.

I’ve made my own 1.5V or 3V to 9V DC to DC converters; they’re good designs.  I was looking at this article (some English; mostly in Czech) about a 9V substitute. looking at some other circuits, also.  One of them is Roman Black’s V boost circuit.  His goes from 5V to 13V, I would change it from 1.5V or 3V to 9V.  The circuit uses a zener on the output to do the regulation so it’s inefficient since all the excess power is wasted in the zener.

I’m Looking at other circuits too.  I found one that was posted to a forum, but it’s only accessible to those registered and logged in.  To regulate, it uses a zener and another transistor to shunt the base bias of the driver transistor to ground.  This is less wasteful than the Black brute force regulation method but still wastes a few milliwatts, which is something that could be saved when using a battery.

I have thought about using rechargeable batteries.  I have also thought seriously about  using a supercapacitor in place of the battery.   The advantage of a supercapacitor is that the rechargeable battery may take hours to charge, the supercapacitor just a few minutes.  However, the rechargeable battery can be swapped out for an already charged battery.  The one thing I really don’t like about rechargeable batteries is that they lose 1 percent of their charge per day, so every month 1/3 of the charge just disappears without any use.  This means that one is constantly recharging them no matter how much they get used.

I have some 10 Farad and 50 Farad capacitors, both at about 2.5 volts.  They should  hold enough charge to run for tens of minutes, long enough to make enough measurements.  I have some 2600 Farad ultracaps but they’re the size of a soft drink can and too large and unwieldy to be used, I think.   I have thought about just plugging a 9V AC adapter ‘wall wart’ into the back of the DMM, but this is a safety hazard.  If the meter is used to measure something such as the AC line or high voltage, it could go through the meter, and through the adapter, and cause the meter to short out the measured device, or worse yet shock someone or cause a fire.  When a 9V battery is used, it is completely contained inside of the meter and there is no shock risk.  That is also something I have to consider when I build a battery substitute.  The substitute has to be insulated if it is not totally enclosed inside of the meter.  The insulation will have to handle hundreds of volts, maybe as high as a thousand volts.  The supercapacitor is only 2.5 volts, so it will need a DC to DC converter to bring it up to 9V, and a jack to charge it up periodically.

I still haven’t made up my mind as to which method I want to pursue.  I may just do one of each and see how they work out, and compare them.   Like, learn from experience, huh?

Here is the link to one I made.