2013-03-05 1.5V to 9V DC-DC Converter For DMM

I built this 1.5V to 9V DC-DC converter. It’s a modified   Tim Williams design.  It’s a Joule Thief with the output rectified and filtered, which is compared to the “Vref” zener diode and if the output voltage is too high, it turns off the base bias current to the Q2 driver transistor.  The input is 1.5V (I’ll use an AA cell), and the output is 9.1V at less than 10 milliamps for a cheap DMM from Harbor Freight.

My (very few) Mods

The circuit is a solid, well designed circuit so I didn’t have to do much.  I used a 2N3906 for Q1, a PN2222A for Q2, and a 2N3906 for Q3.  I used a transistor with its collector lead cut off for the “Vref” zener diode.  Its emitter to base junction breaks down at about 8.5V, which when added to the 0.6V base to emitter voltage of Q3 give about 9.1 volts output.  He gave no information about the coil, so I wound a small toroid with 14 turns of 24 AWG telephone wire for the primary and 7 turns of the same for the feedback winding.  The core was a YR41003TC from Surplus Sales, which cost about 40 cents apiece.

I changed the 22 ohm resistor to 47 ohms; I decided that it would reduce the possibility of the ringing caused by the inductance and the capacitance of the transistor’s base.  I want to have as little RF interference as possible.  I changed the 10 uF output filter capacitor to 47 uF, mainly for the same reason.  The Schottky diode was a 1N5817 1 amp rectifier.

I built it on the small piece of birch plywood, by drilling the holes with a #60 drill in a small pin vise.  I had one of these at home, but at work I had a #60 drill, but no pin vise.  So I cut off four inches of a chopstick, put it in a vise, and drilled the tapered end deep enough to hold all of the drill bit except for a quarter inch.  Now someone is going to have to try eating with a single chopstick! (just kidding, they’re free at the Asian food restaurants).

The parts went in kind of haphazardly because it was my first prototype and things just had to be guesstimated.  I connected up a connector from a depleted 9V battery to the green output leads.  Later I’ll add a battery holder and switch to the red 1.5V leads.

I powered it up and measured the performance.  The output was 10V, too high.  I cut out the zener transistor and grabbed another one and soldered it in.  The new output voltage was now 9.1VDC, which is just what I wanted.  I measured the supply current with no load on the output and it was about 25 mA.  I connected a 1k resistor to the output and the output voltage dropped to 8.95 volts, not too bad for a cheap regulated converter.  That gave me about 8.95 milliamps through the 1k resistor.  The supply current had jumped up to 130 milliamps.  Any load current more than this and the output voltage drops quickly.  This converter was not made (and not meant) to have as much power as that needed for running a transistor radio or similar.  They require three or more times the output current.  Also, my experimental results with DC-DC converters used to power AM radios were disappointing: the AM band was filled with the RF interference from the converter.  FM band worked okay, though.

It’s working okay, but I’ll have to see how it works when the DMM is connected to it.  Sometimes the device is sensitive to RF interference and EMI (electromagnetic interference) coming from the battery.  After all, the normal 9V battery is pure DC, nothing else, so the device doesn’t have to filter it at all.  I was looking through these DC-DC converter circuits earlier, and I saw one that operated at 1000 Hz.  Well, if it was used to power an audio device such as an earphone amplifier, the power supply’s switching frequency is right in the middle of the audio band, where the ear is most sensitive.  Even the smallest amount leaking through to the amplifier is going to be heard in the headphones.  It is much better to have the converter working at some high frequency above the audio range to make this situation less objectionable.  Although I didn’t measure it – the frequency changes with the load, this Joule Thief converter is operating at some frequency in the several tens of kHz, well above the audio band, so there’s no problem with hearing it.

Back to thinking about how many of these I’m going to need as the 9V batteries go dead in these cheap DMMs.  Problem is I sometimes go to Harbor Freight and buy another DMM when they’re on sale, so I really should just use the spare ones and cannibalize the old ones with the dead batteries.  But I don’t.  It’s odd, but I have had to repair more test leads than replace the 9V batteries.  In an earlier blog, I talked about making one of these, but instead I wired three CR-123 3V lithium cells in series and taped them to the back of the DMM, and they work great.

 Some Other Ideas

I’m now glad that I’ve done one, and all I have to do now is connect it to the DMM.  I’ve thought about two other enhancements that would be a benefit to this converter.  The best one is to add a timer that shuts off the converter after a minute or two.  The usual DMM measurement takes only a few seconds or less than a minute, and the auto shutoff would prevent depleting an AA cell.  Or alternatively I could power this converter off a rechargeable AA cell.  There are two downsides to this, the biggest is that the typical rechargeable Ni-MH cell loses 1 percent of its charge per day, so a month or two later it has to be recharged, even if it hasn’t been used.  And I can’t remember ever having a charger that will charge a single cell – that’s one reason why I considered using 3V in that earlier blog.  But the single AA rechargeable is only 1.2V, and that means the circuit will not be able to put out as much current with the lower supply voltage.  It would be better if the supply was two AA cells, for a total of 2.5 volts or so, which would also make the current drain less and the cells would last longer.  With rechargeables, the problem of running down the battery is no longer a concern, you just keep a spare set of charged batteries in the same area and swap them if they go dead.  With two cells, the converter would operate on a pair of AAA cells with no problems.  That would make the package lighter and smaller.

Also with a rechargeable pack, the pack could have a jack on it for charging the two cells.  The pack would not need to be opened to recharge the cells.  I don’t want anyone getting zapped by high voltage.  I would put a double pole, double throw switch on the jack, so while it’s charging, it would be disconnected from the converter, thus protecting against operating the DMM while it’s plugged in and charging.

Back to experimenting.