# How to take a nixie power supply and make a low power frequency modulated signal

The project I am working on requires a low input voltage stepped up to a high voltage at a certain frequency and PWM with low power.

I found a nixie power supply circuit that can supply 170V at 20mA with a 5V input found here. I then need to take this output and convert it into a signal of 170V with a frequency of 400Hz at a duty cycle of 25% and power of 800uW. I need to know if this is possible and if it is what I need to do to accomplish this?

I was thinking of using a 555 timer in astable mode to drive a FET but I can only get down to 50% PWM using this method. Plus I am not sure how to get the power to my desired level on the output after that.

Yes it is possible. By adding one or two diodes to the standard 555 circuit, and changing one of the resistors to a pot, you can adjust the duty cycle from 1 to 99 percent -ish. Here is a schematic grab from the innergoogle:

Probably none of the component values are correct for your application, but this shows the concept. Also, this shows the load being switched to GND. If you want to switch the +170 V to the load, that requires one additional transistor. With Q1 rated for at least 300 V (safety margin), it can turn on a p-channel FET that connects the load to the +170 V.

Note: Use the CMOS 555 - LMC555

Note: Delete R3.

UPDATE: Here is the first pass at a ((concept)) schematic.The parts values are close, but the diodes change the basic 555 equations and I have not run the new numbers.

Note: Do not use the FET part numbers shown. They are representative types already in my design library. Select FETs rated for at least 200 V; 300-400 V is better.

In the upper schematic, R2 could be made variable to adjust the overall frequency, with R1 to adjust the duty cycle. In the lower schematic, R3 adjusts the output high time, and R3 adjusts the output low time. Two sides of the same coin.

With the resistors shown, the gate will have approx. -10 V on it. D3 is there to protect it in case R6 opens. R4 and R5 spread out the heat so each resistor is below 0.1 W; these should be 1/4 W through-hole parts.

• For clarification on my part. I think what you are saying to do is to is to take the drain of Q1 to the gate of another p-channel mosfet which I will call Q2. Then have the source of Q2 connected to one side of my load and the 170v to the other side of the load. Is this correct? Commented Dec 11, 2020 at 18:54
• p-channel source to +170 V, drain to load. The gate needs a zener to protect it from overvoltage. I'll try to whip out a schematic, but not right away. Commented Dec 11, 2020 at 19:21
• See the answer UPDATE. Commented Dec 11, 2020 at 21:42
• Thanks for the detailed answer. I appreciate it. Commented Dec 12, 2020 at 1:05