[Amps] Alpha 78 voltage sag

[TexasRF@aol.com]

Martin, your hv power supply has a capacitor style filter circuit. The peak  
current drawn from your AC line can be as high as 10 times the average 
current.  When you observe the 2.5vac sag this is an average reading. The actual peak 
 voltage sag can be as much as ten times this amount due to the ten times 
peak  current.
 
If you monitor the AC line voltage with your scope you should be able to  see 
this effect easily. You may need to add an isolation transformer to do this  
test safely. Any small power transformer will work for this. If the voltage 
sag  is due to the wiring in your house, you might be able to improve it, just 
as  using 6mm wire already did. If the voltage sag is seen at the service 
entrance  then the only hope for improvement is through your power company.
 
Just knowing the cause of the voltage drop may be enough to satisfy the  
problem. Most amplifier users here in the USA are fortunate to have very stiff  ac 
line voltage with very little sag under load. These users will not see the  
big voltage drop you are seeing. At my remote qth I decided to install my own  
underground AC service because the power company wanted too much money to  
install theirs and they wanted to use power poles. The cable is 4/0 aluminum  
about 900ft long. I knew there would be a voltage drop issue just as you are  
seeing but it is as expected and therefore not an issue. I see key up to key  
down ac and dc voltage sag similar to yours and it has not caused any  problems.
 
Hope this helps.
 
73,
Gerald K5GW
 
 
 
In a message dated 3/2/2006 8:45:53 A.M. Central Standard Time,  
msole at loxinfo.co.th writes:

I had a  good number of replies to my questions concerning likely output for
the  Alpha 78 and in the process uncovered what is clearly a problem here
with  the AC supply line. The distribution is done from the breaker panel to
the  wall outlet with 2.5mm. Breaker panel to main breaker is 35mm and the
feed  from the street is in what looks like the next size up from  that.

Measured about 10 volts sag in the AC line at the wall outlet  with the amp
key down 1.5K out. Sag drops to 2.5 volts when I use 6mm cable  Also measured
about 90 volts AC sag on the secondary of the transformer in  the SSB
position. Seeing approximately 350 volts sag at the output of the  power
supply filter. Why so much?

Measured the ripple by looking at  the voltage across the 180K HV metering
resistor. Saw approx 8 volts pk-pk  off load rising to approx 25 volts pk-pk
at full output. Added a bank of  6x470uF capacitors and the ripple reduced to
approx 6 volts off load and  neglible on load, can see the 20 metre RF
getting in the scope but that's  all. DC still sags down 350 volts on load.
Changed the diodes out replacing  the RCC1733 with a string of 3x1N5408's in
each case, no difference, DC  still sags 350 volts.

If this is about right can anyone explain why?  Others have indicated to me
that they see approximately 200 volts of drop  between no and full load.
Think I could maybe accept that but not yet  cofortable with 350 volts drop,
Any ideas?

Just as a thought , not  that it would make that much difference but I really
couldn't say.  Electrical supply here is 50Hz not 60Hz, would that be enough
to render the  transformer a Peter Dahl 2.4KVA Hypersil meant for 60 Hz, a
little on the  wimpy  side?


73
Martin

_______________________________________________
Amps  mailing  list
Amps at contesting.com
http://lists.contesting.com/mailman/listinfo/amps