[ARC5] [Milsurplus] Hello Smart People Again:
J. Forster
jfor at quik.com
Fri Aug 6 19:52:41 EDT 2010
They will make some hash, but because the operating frequency is fairly
low, the switching times can be longer and that reduces higher harmonucs.
To reduce spikes further, I'd suggest twisting wires wherever possible.
This minimizes the net area seen from afar and reduces the antenna effects
of current loops carrying switching waveforms. The Collector leads are
most important to twist together.
FWIW,
-John
============
> Yes indeed. I don't like to blow transistors and intuition said don't
> go too low on resistance, so I never reached that point. All my
> applications have been receiver-size power supplies, and it would be
> interesting to try a large one. This circuit also has the advantage of
> NO RFI hash to trouble the receiver, in contrast to vibrator supplies.
>
> Richard
>
> On Fri, 2010-08-06 at 13:28 -0700, J. Forster wrote:
>> Richard,
>>
>> If you make the base drive resistors too small, the transistors are
>> driven
>> waaay to far into saturation. When the circuit switches, it takes a
>> while
>> to get the excess space charge out of the base region, so both
>> transistors
>> are turned on during the switching transient. This can cause one
>> transistor to be fully turned on, but with double the DC supply voltage
>> across CE, resulting in very high dissipation and operation outside the
>> SOAR. This blows transistors and puts big current spikes on the power
>> lines (making EMI).
>>
>> There are better base circuits, but they are a bit more complex.
>>
>> I spent months studying this on Space Qualified power supplies and blew
>> several $500+ transistors.
>>
>> -John
>>
>> ================
>>
>>
>>
>> > Transistor vibrator replacement:
>> > I have used a simple version of this circuit many times which works
>> > without fail, usually when I have tried to restore a vibrator and have
>> > damaged it beyond help. Two 2N3055 type transistors with collectors
>> > connected to the ends of the LV transformer winding, two 5000 Ohm
>> > resistors connected base to opposite collector, emitters grounded, +
>> LV
>> > to the transformer center tap. The other parts in the referred
>> diagram
>> > are superfluous. Buffer capacitors installed in the power supplies
>> may
>> > contribute to stability, for example in the DY-105 there is a .047
>> > across the primary, and .008's across each half of the HV winding.
>> The
>> > base drive resistors are not critical, and a little experimenting is
>> in
>> > order - if too large it won't start, and if too low the transistors
>> are
>> > driven hard. This circuit is slightly more efficient than a vibrator.
>> >
>> > Richard, AA1P
>> >
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>>
>>
>
>
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