[Amps] Using HV rectifiers from microwave ovens

Angel Vilaseca avilaseca at bluewin.ch
Mon Nov 19 05:41:41 EST 2007


Thanks for the answers. The 1N 5408 seem to be the best way to go. I 
will buy some.

73

 Angel Vilaseca HB9SLV


k7rdx a écrit :
> The modern 1N5408`s do not require bridging with resistors&capacitors and
> can be found on the internet very reasonable in price. I use series parallel
> strings by matching diodes using my hipot tester to determine reverse
> v.breakdown of each diode and then using  matched diodes in my
> FWB/FWCT,etc...Airflow is always important and strings should be arranged
> for most effective air cooling.The last time I lost a diode string was in
> the late 1970`s and that was using the old 2.5 amp/1kv Radio Shack diodes
> with "Protection" and that during a glitch with the old 4-1000`s...I have
> also tested and used many of Franks (K2AW) Diode blocks and find these test
> very closely,however being enclosed in epoxy are somewhat harder to cool...I
> found a local operator  to test my amps....He likes to get every last
> watt&then some so if things survive him I guess I did it right!
> 73 & happy turkey day to everyone,Jim K7RDX...----- Original Message ----- 
> From: "Manfred Mornhinweg" <mmornhin at gmx.net>
> To: <amps at contesting.com>
> Sent: Friday, November 16, 2007 1:13 PM
> Subject: Re: [Amps] Using HV rectifiers from microwave ovens
>
>
>   
>> Hi Angel,
>>
>>     
>>> I was thinking of using about 1 ohm. This would produce a voltage drop
>>> of about 0.5 volts at 500 mA. Ten times more than the mismatch between
>>> two individual rectifiers. Does this seem right?
>>>       
>> No, it's not enough. The problem is thermal runaway. If one diode gets
>> warmer, it will reduce its voltage drop, take a larger current share,
>> get even hotter, etc. You would end up with most current in one diode.
>>
>> To be safe against this problem, you need about 0.5V drop in the
>> resistor for EACH silicon junction. Since your rectifiers seem to have
>> six diodes in series, you would need about 3V drop in the resistors.
>> With 0.5A total, that's 0.25A in each resistsor, thus about 12 Ohm. A 2
>> Watt rating should provide enough safety margin.
>>
>> But all this is rather academic. I agree with Carl. 1N5408 diodes are
>> cheap, rugged and plentiful. Use strings of them in series, and you
>> probaly don't even need any resistors nor capacitors with them. In my
>> amp I have strings of 5 of these diodes in series, in each leg, for
>> 2800V, which is still really cheap and gives ample safety margin. I'm
>> using just the bare diodes, with no resistors nor capacitors. So far it
>> works, and has worked for years.
>>
>> Manfred.
>>
>> ========================
>> Visit my hobby homepage!
>> http://ludens.cl
>> ========================
>>
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>
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