[MRCA] [ARC5] RCA AVT/AVR: Strange Vibrations.

[J. Forster]

If you use Zeners to clamp the spikes, the duration of the spikes will
increase. V = L dI/dT.

Furthermore, any Zeners have to be MORE than double the supply voltage.

-John

==============




> Hello Dave,
>
> How about putting 17 V or 20 V Zener diodes across the primary windings?
> Now that you scoped the waveforms, you know which way to put the Zeners. 1
> W size should be more than adequate. I have used this with electronic
> discharge ignition systems to control the hash from them.
>
> A vibrator supply works not by transforming sine waves from primary to
> secondary, but by change of current in the primary. The main power
> transfer occurs with the current inrush as the vibrator contacts 'make'.
> High Voltage spikes on the primary as the primary current collapses (when
> the contacts 'break') will get partly swallowed in the hysteresis of the
> transformer core - though the collapsing current will generate spikes on
> the secondary - hence the need for a snubber; it's really a low-pass
> filter.
>
> 73 de Brian, VK2GCE
>
>
>
>> David Stinson <arc5 at ix.netcom.com> wrote:
>>
>> Whomever said vibrator power supply design was voodoo
>> had it exactly right.....
>>
>> I got a chance to put a scope on the power supply.
>>
>> The squarewave waveforms on the primary -
>> Nominal 12 V PTP, are indeed "spiky,"
>> with a very nasty and fast rise-time, very short duration
>> spike at the pin-7 contact up to 50 volts P-T-P during that
>> contact's "OFF" time, which means it's originating in the
>> other side of the primary (pin 1), I suppose.
>> Also a couple of lesser "nasties:"    One at half the peak
>> value of the primary square wave during the rise-time of
>> the pulse and one near the end of the square-wave peak,
>> 10 volts above the nominal value.
>>
>> However- the waveform in the secondary looks good.
>> The snubber is doing just what it should.
>> The secondary trace is 700 V PTP, just as one
>> would expect for transmit B+ of 350 Volts with very little "noise."
>> Removing the snubber for a quick "snapshot" shows the
>> nasty primary spikes in the secondary
>> and replacing the snubber clears it up.
>>
>> If I put a 22 uFd 400 VDC capacitors
>> from the vibrator primary connections to ground,
>> that cleans-up the fast primary spikes, but dumping them
>> that way causes so much current to flow through the caps
>> that even these high-Voltage rated ones get warm
>> and the transformer overheats.
>>
>> So much energy getting eaten in that snubber,
>> yet it seems to be working.
>> Input current to the supply under full load transmit
>> is about 5 Amps- well below the specified 7.4 Amps.
>>
>> So.... I installed a 10-watt wirewound resistor
>> (which also gets hot but can handle it).
>> I put the rig on the air for 30 minutes and have
>> been making local contacts on 3880 KC with
>> the mighty 8 watts out.  The transformer runs cool
>> and everything but that resistor seems.... normal.
>>
>> Here is the rig before I build a mount for it:
>>
>> http://home.netcom.com/~arc5/AVRAVT/RCAonair.JPG
>>
>> I dunno.... think I should count my blessings and move on??
>>
>> TNX OM ES 73 DE Dave AB5S
>>
>>
>>
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