Fun with discretes: low-V inverter for isolated power supply - Page 1
Forum section: electronics / projects

author: Analog Kid
author_rank: Super Contributor
timestamp: February 14, 2025, 03:06:40 am
content: In today's episode we're playing with discretes again--transistors mainly--in an attempt to provide an isolated low-voltage supply from another source (a 9-volt battery).This is actually a sub-project of another project, a transistor beta tester. Since the tester uses a DMM panel meter, and since the meter requires a power supply that's totally separate from the measurement input, this inverter came to be.The tester is going to be powered by a single 9-volt battery, which can directly power the panel meter. Now the simple solution would be to power the DMM from another 9-volt battery. But I just can't bring myself to do that; every fiber in my body rebels at the idea of having not one but two batteries for this simple device. So I came up with this solution, a step-down low-power inverter.I have to say that this is a lot more complex than I'd hoped for; 4 transistors, a transformer, and a voltage regulator. But it became a challenge in and of itself, and I was happy when I finally got it to work. It should all fit on a small piece of perfboard (my preferred construction method).Here's the circuit:The heart of it is my homemade transformer. I tried some transformers I had laying around, but the small ones didn't work well or at all, and the larger ones were just too big. So I decided to try winding my own. The first couple of attempts kinda-sorta worked, but not well or reliably. At first, I was just using one emitter follower after the oscillator, and that barely worked.It occurred to me that I really should be driving the transformer in push-pull fashion to actually generate a true AC wave. I wound another transformer with a split primary, and lo and behold, it worked pretty well. This version gives me a good solid 20mA output. It's not very efficient: looks like about 7%. However, since it seems to work reliably and is stable, that is forgivable.Here's my coil-winding rig. I was thinking of cobbling something together in my workshop with a chuck and a crank, which would be fairly complicated. Then I spotted my trusty hand drill on my bench: hmmm, wouldn't that work if it were clamped down? Yep, it sure would:I thought I could clamp the ferrite cores that I had (2.5 x 15mm) directly in the drill chuck, but they wouldn't stay in, so I ended up chucking then in a small pin vise and chucking the vise in the drill. Worked like a charm.Here's a closeup:That white mark is to let me count turns. It's kind of a pain holding the wire with one hand, cranking with the other and trying to watch both the coil and the white mark, but it can be done.Here's the finished transformer:The coil windings are nice and neat in the center, but got kind of messy at the ends. It would be better to use a longer rod and put a couple of fiber washers on the ends as a stop to the windings. However, this probably works just as well as a totally neatly-wound transformer.The turns ratio is about 1:8.5 (60/60t primary, ~500t secondary, using 34 ga. magnet wire), which gives me about 7-8 volts AC on the secondary, just enough to power the 7805 regulator.Here's the breadboarded project:

author: RoGeorge
author_rank: Super Contributor
timestamp: February 14, 2025, 03:36:16 am
content: Nice workbench!To confess, I'm often lazy, so I will probably just connect the 9V battery to the 7805 directly.Yes, some energy will be lost in the 9 to 5V linear regulator, but I can use 2 series rechargeable LI-ion (2 x 3.7V) instead of a 9V, and recharge the Li-ion any time the battery goes flat, such that the low efficiency of analog regulation won't matter much.

author: Analog Kid
author_rank: Super Contributor
timestamp: February 14, 2025, 03:44:48 am
content: Thanks for the compliment. It's a former watchmaker's bench that I rescued from an antique dealer.First mod: the regulator is totally unnecessary. Without it I get ~5V @ a good solid 20mA (the regulator was probably doing nothing anyhow as the secondary voltage was a lot lower than I thought).Now to integrate it into the beta tester ...

author: prosper
author_rank: Regular Contributor
timestamp: February 15, 2025, 12:08:12 am
content: I think you put your FETS backwards in tge diagram. Did you mean to use source followers?these work. Commonly referred to as 'zvs' or sometimes 'royer' oscillators. Just a self oscillating resonant converter.I used ee10 cores without an air gap. the primary is wrapped to an inductance of 30-100uH and the secondary at whatever ratio you want. you can current-feed it with inductors (either at the center tap, or either side of the primary) approximately 3 to 10x the primary inductance, or with plain resistors for a lower power level

author: prosper
author_rank: Regular Contributor
timestamp: February 15, 2025, 12:11:00 am
content: you can also make a push pull multivibrator like so, and drive a charge pump with it

author: Analog Kid
author_rank: Super Contributor
timestamp: February 15, 2025, 01:08:59 am
content: Interesting. Where do you take the output from that?

author: prosper
author_rank: Regular Contributor
timestamp: February 15, 2025, 01:14:05 am
content: off of the collectors. you can swap 0r shunts for the 15r resistors. or pick the collector of the transistor closest to the rail you need the most current from

author: Analog Kid
author_rank: Super Contributor
timestamp: February 15, 2025, 01:20:13 am
content: I don't understand exactly how that's supposed to work; I mean I know how a multivibrator works but I don't see what the point of the two pairs of transistors is. (I know there is a point, just don't know what it is.)Does this give you more drive (voltage or current) than just the standard 2-transistor multivibrator?That's why I added the pair of MOSFETs to mine, to be able to drive my transformer.Still very interesting.

author: schmitt trigger
author_rank: Super Contributor
timestamp: February 15, 2025, 02:19:22 am
content: Many years ago, the Popular Electronics magazine had a column labeled “DANDY”, discretes are not dead yet. Their all-discrete semiconductor circuits were quite ingenious.But if we want to go further, let’s step back to vacuum tubes. The fact that engineers back then could design circuits for analog color TVs using only tubes, is beyond comprehension to me.

author: prosper
author_rank: Regular Contributor
timestamp: February 15, 2025, 02:22:19 am
content: yes, a standard multivibrator is open collector - it can only ever sink current, the only way to source it is through its pullup resistor. The circuit I posted can both source AND sink current, and so it can drive different sorts of loads, like a capacitive charge pump, for example

author: Analog Kid
author_rank: Super Contributor
timestamp: February 15, 2025, 02:52:59 am
content: Sorry to keep bugging you, but I kinda have a mental block visualizing sources & sinks, etc.Could you draw that circuit driving a charge pump?Much obliged.

author: prosper
author_rank: Regular Contributor
timestamp: February 15, 2025, 03:16:03 pm
content: https://www.radiolocman.com/shem/schematics.html?di=657935

author: Benta
author_rank: Super Contributor
timestamp: February 15, 2025, 08:05:53 pm
content: You should use a bridge rectifier on the output (or cantre-tap the secondary). One of those transistors is not doing any work at all.

author: Analog Kid
author_rank: Super Contributor
timestamp: February 15, 2025, 08:16:39 pm
content: Quote from: Benta on February 15, 2025, 08:05:53 pmI thought about that.Maybe a bridge of Schottkys for less voltage drop?Also curious about combining prosper's H-bridge circuit with a transformer (I have to have a transformer for isolation). Combining a charge pump with a transformer stage.