Extending functionality, power concerns

A project log for State-Based Nixie Digital Voltmeter

A digital voltmeter built without any microcontrollers or integrated ADC chips. Based entirely around discrete logic and comparator ICs.

kittankittan 06/13/2014 at 17:230 Comments

Additionally there exist a pair of nixie tubes more or less specifically designed for meters. One has units (like V, A, F etc) and the other scales (m, u, k etc) so you can pair them to do unit and range outputs. I could use a set of shunt resistors and low-side amplifiers to measure current, feed the output through the same voltage measurement loop and, depending on the range selection, have the tubes display the proper units. A simple latching overcurrent detector cutting out the relay making the range selection would prevent roasting shunts. I gotta say, I'm pretty excited about this project.

One thing to consider though, is isolation. If I want to be able to measure stuff in circuit without the risk of explosions, I'll need an isolated power source. The nixies I've got to play with can pull around 4mA at 150V, so a 6-digit display (4 decimals, 2 units) could eat upward of 3.6watts continuous draw by themselves. Last winter I built a 150VDC power supply for driving nixies, designed for 7.5V input and runs above 80% efficient; that takes input power requirements to 4.5W to the drive supply.
I hope the electronics don't eat too much, but assuming I run all 5V logic and amplifiers and burn half an amp, we're looking at needing at least 7W continuous. Say I stack two Lipo 18650s to get a nominal 7.4V 2Ah, we'd have two hours continuous runtime per charge. Not so great for bench equipment. Gonna need to find an isolated mains supply, either something simple with a transformer or an isolated switching supply.

Thinking about it, if I used a switching supply I'd have to probably build it myself using either comparator PWM drive or something like the TL494 to make sure there's no controllers in it. Probably the easiest thing to do will be a rectified transformer output into a BJT linear regulator. It'd be totally wasteful but also really easy, unless I wanted to build a buck driver using only an LM339 - which actually wouldn't be that hard. I already have a PWM driver designed around it that I was using for 4-wire fan control; it'd be super easy to strap a high-side PNP (TIP42 anyone?) to the open-collector output and cook up a basic RC output stage good for ~8V/2A. I could even push relatively unregulated DC into the nixie driver and buck a flat 5V for my logic boards. Well heck why don't I just do that then?