low cost, small, safe bias supply - Page 4

I_Forgot · 31st May 2006, 02:00 PM

What are the blue cap values?

I_F

Few · 31st May 2006, 02:52 PM

Printed on them is:

473
6KV

Since I measure 56 pF on each capacitor while it's still soldered to the board (but with its output lead disconnected), I'm assuming they're 47.3 pF caps.

Bazukaz · 31st May 2006, 03:40 PM

hmz ... 473 should mean 47nF caps , shouldn't it ?

Few · 31st May 2006, 05:47 PM

That's what I was going to say, but when I measured it to double check, I had a hard time rationalizing the results. I thought I could get away with measuring the capacitance as long as one of the two capacitor leads was disconnected from the circuit, but I guess I was wrong.

I_Forgot · 1st June 2006, 02:49 AM

The minimal info I can find on that particular inverter say it produces 1000Vrms. ccfl inverters can usually supply 3-5 mA out, though at a voltage a little lower than the open circuit (start-up) voltage value. Your rectifier circuit should charge the 300 pF cap to about -750VDC relative to ground. 750V bias won't make a sensitive ESL- maybe the output is low and you are not hearing it due to the low bias voltage.

When you connect a scope probe to the circuit if you connect directly across the 300 pF cap the probe's voltage divider is going to load the cap and discharge it. What is the scope probe's divider resistance? It doesn't take much to discharge a 300 pF cap. You may be charging and discharging the cap on each cycle, hence the AC that you see on the scope. What frequency is the waveform on the scope?

Another possibility is that your scope ground may be loading the 24V supply and killing the 24V going into the inverter. Check the input voltage while you are making the output measurement.

I_F

rpapps · 1st June 2006, 03:36 AM

Hi Few
I think you need to bypass that output cap and connect directly to pin 6 of the transformer. As it stands, there is no DC return path for the rectifier. Also, connect a 10 Meg resistor accross the 300Pf filter cap.
Cheers
Rob

Few · 1st June 2006, 02:57 PM

Thanks very much for the help guys, I really appreciate it. I'll give things a try and let you know what I discover. I need to track down some high voltage diodes first, though. The ones I was using left with a student I was helping.

Few

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31st May 2006, 02:00 PM	#31
I_Forgot diyAudio Member Join Date: Jan 2005 Location: Phoenix, Az.	What are the blue cap values? I_F

31st May 2006, 02:52 PM	#32
Few diyAudio Member Join Date: Apr 2004 Location: Maine, USA	Printed on them is: 473 6KV Since I measure 56 pF on each capacitor while it's still soldered to the board (but with its output lead disconnected), I'm assuming they're 47.3 pF caps.

31st May 2006, 03:40 PM	#33
Bazukaz diyAudio Member Join Date: Nov 2005 Location: Vilnius	hmz ... 473 should mean 47nF caps , shouldn't it ?

31st May 2006, 05:47 PM	#34
Few diyAudio Member Join Date: Apr 2004 Location: Maine, USA	That's what I was going to say, but when I measured it to double check, I had a hard time rationalizing the results. I thought I could get away with measuring the capacitance as long as one of the two capacitor leads was disconnected from the circuit, but I guess I was wrong.

1st June 2006, 02:49 AM	#35
I_Forgot diyAudio Member Join Date: Jan 2005 Location: Phoenix, Az.	The minimal info I can find on that particular inverter say it produces 1000Vrms. ccfl inverters can usually supply 3-5 mA out, though at a voltage a little lower than the open circuit (start-up) voltage value. Your rectifier circuit should charge the 300 pF cap to about -750VDC relative to ground. 750V bias won't make a sensitive ESL- maybe the output is low and you are not hearing it due to the low bias voltage. When you connect a scope probe to the circuit if you connect directly across the 300 pF cap the probe's voltage divider is going to load the cap and discharge it. What is the scope probe's divider resistance? It doesn't take much to discharge a 300 pF cap. You may be charging and discharging the cap on each cycle, hence the AC that you see on the scope. What frequency is the waveform on the scope? Another possibility is that your scope ground may be loading the 24V supply and killing the 24V going into the inverter. Check the input voltage while you are making the output measurement. I_F

1st June 2006, 03:36 AM	#36
rpapps diyAudio Member Join Date: Jul 2004 Location: nowhere of interest	Hi Few I think you need to bypass that output cap and connect directly to pin 6 of the transformer. As it stands, there is no DC return path for the rectifier. Also, connect a 10 Meg resistor accross the 300Pf filter cap. Cheers Rob

1st June 2006, 02:57 PM	#37
Few diyAudio Member Join Date: Apr 2004 Location: Maine, USA	Thanks very much for the help guys, I really appreciate it. I'll give things a try and let you know what I discover. I need to track down some high voltage diodes first, though. The ones I was using left with a student I was helping. Few

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