Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
Power Supply Voltage Drop Under Load




 
The AM Forum
August 20, 2019, 02:11:26 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1]   Go Down
  Print  
Author Topic: Power Supply Voltage Drop Under Load  (Read 503 times)
0 Members and 1 Guest are viewing this topic.
W9ZSL
Contributing
Member
*
Offline Offline

Posts: 768


« on: August 03, 2019, 06:25:54 PM »

Just bread-boarded a PS with a bridge rectifier, a pair of 10 ufd caps and a 4 Hy choke in a "pi" capacitive input filter. Bleeder is 75K. I'm getting 1150 VDC, no load. I'll be using it for the plate supply for a single 4-65A drawing 120 ma. Screen supply of maximum 250 volts at 40 ma will come from a slider on the bleeder. Really basic. Nothing else will be tied to the B+. How do I go about guesstimating what the voltage will be under load? Need to know so I can calculate the output tank impedance. I'm going to try screen modulation, but I also have a plate modulator, so that is covered. Neither plate nor screen mods will draw power from the 4-65A plate supply. Thanks!
Logged
K8DI
Contributing
Member
*
Offline Offline

Posts: 31


« Reply #1 on: August 04, 2019, 06:47:51 AM »

A little mental math says your load is in the region of 10k. So pull out some junk box wirewounds out and make a 10k load. It need not handle the 130 watts it will have going into it, because you’ll only run it for five seconds to measure the voltage. Hook it up to the meter, then plug it in, read it, unplug it, and you’ll know what the sag is. Using junk box resistors means you can toss them after if you’re worried that they’ve been damaged...

Ed
Logged

Ed, K8DI (fmr. KB8TWH)
W7TFO
WTF-OVER in 7 land Dennis
Contributing
Member
*
Offline Offline

Posts: 2219


IN A TRIODE NO ONE CAN HEAR YOU SCREEN


WWW
« Reply #2 on: August 04, 2019, 09:34:49 AM »

Cap input, more Voltage, more droop.
Choke input, less Voltage, less droop.

DG
Logged

Just pacing the Farady cage...
NW2K
Member

Offline Offline

Posts: 7


« Reply #3 on: August 04, 2019, 10:15:57 AM »

No need to guess.  This free software will get you to within a couple percent given some simple measurements.

http://www.duncanamps.com/psud2/

I've used it extensively and the models are very good. 

73, Dean NW2K
Logged
K8DI
Contributing
Member
*
Offline Offline

Posts: 31


« Reply #4 on: August 05, 2019, 08:25:22 AM »

I’ve played with this software a bit too. Note that it requires you to halve the secondary voltage when using a bridge rectifier (4 diodes) — if you don’t the voltages it shows are of course twice reality...

Ed
Logged

Ed, K8DI (fmr. KB8TWH)
W9ZSL
Contributing
Member
*
Offline Offline

Posts: 768


« Reply #5 on: August 07, 2019, 05:34:14 PM »

I tried a few loads from 6.5 to 10 K and got an average of 1045 VDC. I think I'll just go for 1050 with a Q of 10 and let it go at that.
Logged
K1JJ
Contributing
Member
*
Offline Offline

Posts: 7859


"Let's go sailing, Tommy!" - Yaz


« Reply #6 on: August 07, 2019, 07:03:03 PM »

Interesting that 1150VDC no load drops to 1045VDC under load.

In general, a no load voltage X 0.9 (with a choke, cap input) will give an approximate loaded estimate.  In your case, 1150V X 0.9 = 1035V.  Pretty close.  That's within 10 volts of your test considering the many variables.  

T
Logged

Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
W9ZSL
Contributing
Member
*
Offline Offline

Posts: 768


« Reply #7 on: August 07, 2019, 07:38:25 PM »

Yeah, I was expecting a bigger drop than that. When I hooked it up with choke input, I got 750 volts with a 10 K load across a 75 K bleeder.
Logged
Opcom
Patrick J. / KD5OEI
Contributing
Member
*
Offline Offline

Posts: 7240



WWW
« Reply #8 on: August 07, 2019, 09:19:39 PM »

Did you mention the AC voltage of the secondary? Is it somewhere around 820-835V?

Looks great!
Logged

Radio Candelstein - Flagship Station of the NRK Radio Network.
K1JJ
Contributing
Member
*
Offline Offline

Posts: 7859


"Let's go sailing, Tommy!" - Yaz


« Reply #9 on: August 07, 2019, 10:27:15 PM »

Yeah, I was expecting a bigger drop than that. When I hooked it up with choke input, I got 750 volts with a 10 K load across a 75 K bleeder.

Another quick estimate...   For a choke input supply under load:    Take the AC RMS and multiply X 0.9.  In your case IF you are using 830 VAC like Opcom asked, that's 830VAC X 0.9 = 747 VDC... close to the 750VDC you are measuring with the choke input now.

Of course these are just rough numbers and will vary based on actual components used and how heavy the load is, but when quickly calculating supply outputs in your head, it works FB.

T
Logged

Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
W9ZSL
Contributing
Member
*
Offline Offline

Posts: 768


« Reply #10 on: August 08, 2019, 11:21:13 AM »

Using .9 as a multiplier comes out to 1035 VDC cap input. That is really in the ballpark because I measured 1025 with a 6.5K load and 1050 with a 10K. 7.5K gave me 1045. The transformer is a 120 VAC primary with 900 VCT rated at 250 ma. secondary. I'm putting the 900 through a commercial bridge with ratings well above those of the tube (4-65A). This is a funky AM rig I've been thinking of for quite some time. Strictly experimental for 160 and 80 meters. I picked the 4-65A because I have a couple and have plans and parts for building a 2KV supply down the road for another amp covering all bands with either an 813 or a 4-125A running around 350 to 400 watts.  The 4-65A would work at the higher voltage but I'm tied to the 120 ma. plate current. My modulator will put out 250 for either the 813 or 4-125A OR this particular project though I'm going to try screen modulation first.  I have a 500 ohm to grid iron and just happen to have a limiter amp I built years ago that has a 500 ohm output. It uses a crazy output circuit with a pair of 12AU7 tubes The two triodes in each tube are run in parallel with each other and the two tubes are run in push-pull parallel so basically there are four triodes at work. I used a UTC input transformer and Triad hi-fi output iron. Built it in the late 60's for my recording studio and man it sounds sweet!!!
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone © 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.086 seconds with 18 queries.