Hi, i have with me a mains-fed nixie tube power supply which i was trying to use for my tube amplifier. I don't have a schematic but i can tell it's using a 555 timer ic as MOSFET PWM driver to control a flyback transformer and it outputs a fixed 250vdc. More details that i can tell you so far:
1. Input is mains voltage 220Vac, fullwave rectifier to 310Vdc then goes to some filtering caps and finally to the flyback transformer
2. PWM chopping MOSFET is FQP5N60C with small heatsink and something i believe a diode wrapped inside heatshrink tube attached to the heatsink (heat sensor?)
3. Output is halfwave rectified using two UF400x diodes in series (maybe UF4007)
4. There are two 4N25 optocouplers. One monitors the voltage through a string of three 82v zeners + RED LED for a regulated output of 250v. The other one i believe is for overcurrent protection as the internal LED+470 ohm resistor is shunted by a current sensing resistor (8.2 ohm). The hot side collector of the 4N25 both goes to pin 5 of the 555.
5. Other minor details: common mode choke at the output, output is isolated from the input. Sellers says it's good for 25W of output (which against 250v yields 100mA of output current).
Problem:
I have two of these power supplies and i was connecting the output in series so that i have a 500v output with 250v tap. It was used to feed an OTL poweramp input stage. During my first trial runs, i accidentally burned 10k/2W resistor across the 0-500v output (so there is at least 500v/10k = 50mA of current which means 25W of dissipation for that poor resistor). That might have caused the problem i currently have: the upper power supply (which provides the 500v tap) can no longer provide stable 250v. If i try to measure its output using a voltmeter, the voltage then quickly drops from 500v to 250v-ish. If i measure it individually (not in series with the lower power supply), it drops from 250v to around 130v. Before this trial, it successfully provides the intended 250v and 500v output.
Do you have any idea what would be the problem? So far,i tried checking the half wave rectifying UF400x diode, the 82v zeners + red LED and they seems to be ok. I have a pocket oscilloscope which might help if needed.
1. Input is mains voltage 220Vac, fullwave rectifier to 310Vdc then goes to some filtering caps and finally to the flyback transformer
2. PWM chopping MOSFET is FQP5N60C with small heatsink and something i believe a diode wrapped inside heatshrink tube attached to the heatsink (heat sensor?)
3. Output is halfwave rectified using two UF400x diodes in series (maybe UF4007)
4. There are two 4N25 optocouplers. One monitors the voltage through a string of three 82v zeners + RED LED for a regulated output of 250v. The other one i believe is for overcurrent protection as the internal LED+470 ohm resistor is shunted by a current sensing resistor (8.2 ohm). The hot side collector of the 4N25 both goes to pin 5 of the 555.
5. Other minor details: common mode choke at the output, output is isolated from the input. Sellers says it's good for 25W of output (which against 250v yields 100mA of output current).
Problem:
I have two of these power supplies and i was connecting the output in series so that i have a 500v output with 250v tap. It was used to feed an OTL poweramp input stage. During my first trial runs, i accidentally burned 10k/2W resistor across the 0-500v output (so there is at least 500v/10k = 50mA of current which means 25W of dissipation for that poor resistor). That might have caused the problem i currently have: the upper power supply (which provides the 500v tap) can no longer provide stable 250v. If i try to measure its output using a voltmeter, the voltage then quickly drops from 500v to 250v-ish. If i measure it individually (not in series with the lower power supply), it drops from 250v to around 130v. Before this trial, it successfully provides the intended 250v and 500v output.
Do you have any idea what would be the problem? So far,i tried checking the half wave rectifying UF400x diode, the 82v zeners + red LED and they seems to be ok. I have a pocket oscilloscope which might help if needed.
Nevermind. Found the problem: the current sense resistor (8.2 ohm) was open. I guess it once flowed much more current than i calculated.
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