Author Topic: Why don't my diodes work?  (Read 1438 times)

Offline nsled

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Why don't my diodes work?
« on: July 30, 2023, 08:30:36 PM »
I'm attempting to rectify a cold cathode driver marked 900v 25khz. By placing a single diode in line with the positive lead I receive a substantial voltage drop and zero rectification, waveform looks the same. The diodes I've tried are a microwave diode, a 2CL82 and a 2CL2FP all do the same thing. They will rectify 240v mains when tested.

I also tried to rectify a zvs driver but to the same non-effect. voltage drop but still an AC sine wave.

Why can't I rectify high frequencies? I understand some diodes will act as capacitors and not work at high frequencies, but these should should they not? The zvs as shown below is only 10khz.

2CL82
https://www.aliexpress.com/item/4000055718772.html?spm=a2g0o.order_list.order_list_main.21.7acc1802dJVM1d

CL01-12 (microwave)
https://www.aliexpress.com/item/1005002510732797.html?spm=a2g0o.order_list.order_list_main.11.7acc1802dJVM1d

2CL2FP
https://www.aliexpress.com/item/1005001500427448.html?spm=a2g0o.order_list.order_list_main.5.7acc1802dJVM1d
« Last Edit: July 30, 2023, 08:52:20 PM by nsled »

Offline AstRii

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Re: Why don't my diodes work?
« Reply #1 on: July 30, 2023, 10:06:27 PM »
High voltage diodes aren't usually much capable at high frequencies due to too high reverse recovery time, they are basically many rectifier diodes in series. If you want to rectify 900V signal, you don't need these anyways. A single BA159 diode should do just fine.
Bc. Marek Novotny
Czech Republic, Czech Technical University in Prague
www.uhvlab.org

Offline davekni

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Re: Why don't my diodes work?
« Reply #2 on: July 30, 2023, 11:25:00 PM »
I'd guess two separate issues.  The CCFL driver likely has a capacitor in series with output.  Check with power off by measuring DC resistance of output.  The ZVS driver is too low voltage.  HV diodes have a high forward drop, perhaps 30V, higher than your ZVS is generating.
David Knierim

Offline nsled

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Re: Why don't my diodes work?
« Reply #3 on: July 31, 2023, 12:31:47 PM »
High voltage diodes aren't usually much capable at high frequencies due to too high reverse recovery time, they are basically many rectifier diodes in series. If you want to rectify 900V signal, you don't need these anyways. A single BA159 diode should do just fine.

2CL2FP diode is much faster than BA159, 100ns compared to 500ns.

https://pdf1.alldatasheet.com/datasheet-pdf/view/476407/GETEDZ/2CL2FP.html
https://pdf1.alldatasheet.com/datasheet-pdf/view/1203688/CHENDA/BA159.html

I'd guess two separate issues.  The CCFL driver likely has a capacitor in series with output.  Check with power off by measuring DC resistance of output.

I believe you but do not understand why a capacitor would have that effect. There is no continuity that can be measured. Capacitance measures 22pf.

Offline davekni

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Re: Why don't my diodes work?
« Reply #4 on: August 01, 2023, 03:41:01 AM »
Quote
I believe you but do not understand why a capacitor would have that effect. There is no continuity that can be measured. Capacitance measures 22pf.
Have fun learning.  Analog simulation would be a good way to understand what you are seeing.
David Knierim

Offline nsled

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Re: Why don't my diodes work?
« Reply #5 on: August 01, 2023, 08:51:50 PM »
This is a small AC flyback driven by a 12v 20kHz PWM and rectified by the 2CL2FP diode. Curious to know if there is anything that can be interpreted by that waveform? Voltage is 1:1000 as measured by Pintech HVP-40 probe.
« Last Edit: August 01, 2023, 09:20:35 PM by nsled »

Offline MRMILSTAR

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Re: Why don't my diodes work?
« Reply #6 on: August 01, 2023, 10:02:44 PM »
It looks basically like DC with ripple. Adding a smoothing capacitor and a load should clean it up a lot.
Steve White
Cedar Rapids, Iowa
Retired electrical engineer

Offline nsled

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Re: Why don't my diodes work?
« Reply #7 on: August 01, 2023, 10:51:55 PM »
I guess I'm asking compared to an ideal half wave rectification why isn't it as rippled as I'd expect, and why every second peak is stunted.

Offline davekni

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Re: Why don't my diodes work?
« Reply #8 on: August 02, 2023, 05:04:37 AM »
Quote
I guess I'm asking compared to an ideal half wave rectification why isn't it as rippled as I'd expect, and why every second peak is stunted.
Pintek HVP-40 is intended for use with a DMM from DC to 60Hz (and for 10meg meter impedance, not 1meg scope impedance).  No specifications beyond 60Hz.  20kHz result may not be close to reality.
Probe and wiring capacitance is likely enough to filter diode output.

Probe capacitance (which may include significant series resistance) affects HV transformer output.  With just the diode for load and additional capacitance on diode output, transformer/driver appears to be experiencing some sub-harmonic oscillation (10kHz component to signal).  I'd guess the diode is hitting it's reverse breakdown voltage (at low enough current to avoid diode failure).  Breakdown may be cause for sub-harmonic oscillation.
David Knierim

High Voltage Forum

Re: Why don't my diodes work?
« Reply #8 on: August 02, 2023, 05:04:37 AM »

 


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