2013-06-08 DC-DC Converter For DMMs, 2nd I’ve Built

This is the second one I’ve built.  The first one I built is in my earlier blog – links to the schematic can be found there.

I used the following parts: a 2N3906 for Q1, and PN2222A for both Q2 and Q3.  R1 was 470 ohms, R2 was 10k and R3 was 8.2k.  Both the battery bypass capacitor and the output capacitor were 470 uF.  For the rectifier, I used a 1N5817.  The Zener diode was the emitter to base junction of a selected transistor.  I used a jumper across VR1 and left CDS1 empty.

I have been trying to get the required 6 milliamps output with at least 8 volts output before the circuit starts to limit itself.  The two parts I’ve been changing are the capacitor and the inductor.

I have been using a milliammeter between the power supply and the circuit during these measurements.  The supply is set at 1.5V, but about .05 volt is dropped across the milliammeter, so the actual voltage at the circuit is about 1.45 to 1.48 volts.  I feel that it is better to have the circuit working at less than 1.5V just to make sure it can do the job with a battery that’s getting low.

I first used a 100 pF for the capacitor, and tried a few different inductors.  The circuit would  only put out 7 to 7.5 volts when I put a 1400 ohm load on the output.  I should be able to get 6 milliamps with 8.4 volts across the 1400 ohm load.  Changing inductors helped, but the supply current went up a lot higher than with the first one I built , over 100 mA,  So I added  a variable capacitor, and the 1400 ohm load had about 8.5 volts when I had the capcitor adjusted.  I removed the variable capacitor and measured it, and it was 59 pF.  So I removed the 100 pF capacitor and put a 150 pF capacitor in, and the circuit easily gave me 8.5 volts with the 1400 ohm load was on.  All it took was that added 50 pF to make the circuit put out a lot better.

The first inductor I used was a 100 uH I got from Mouser.  I made most of these inductor changes using the 100 pF capacitor.  With the 100 uH, I could not get enough current with the 1400 ohm load, so I put a 180 uH in parallel.  This increased the output enough to where I was getting 8.6 volts with the 1400 ohm load on.  But the supply current was higher than the first circuit.

I tried a 56 uH surface mount inductor and it did well enough to give more than 8 volts out with the 1400 ohm load.  But the frequency was more than 250 kHz, so I decided to try using an inductance that was greater than 56 uH.

I removed the 56 uH and put two 180 uH inductors in parallel into the circuit.  The circuit would not put out 8 volts with the 1400 ohm load on.  I added a third 180 uH choke in parallel.  This was equivalent to 60 uH.  This gave more than 8 volts with the 1400 ohm load on.  But obviously I now had three chokes hanging on the circuit board.  That wasn’t going to work – there was no room for all three.

So I removed the three chokes and put a single 100 uH choke into the circuit.  This was a larger choke, with lower DC resistance, it did better than the first 100 uH choke, but it still couldn’t put out 8 volts with the 1400 ohm load on.  But it would fit into the board, with very little room to spare.  This is when I decided to try changing the capacitor to 150 pF.  With this choke and the 150 pF capacitor, the output stayed up around 8.5 volts with the 1400 ohm load on.

With the above parts, I measured the no load voltage at 8.85V, and 8.55V with the load.  The frequency was 142 kHz no load and 119 kHz with load.  The supply current was 5 mA no load and 50 mA with the 1400 ohm load.

More about this converter in my blog about a 1.5V to 9V DC-DC converter for a Superregenerative Radio Receiver.

Back to experimenting…