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22nd August 2010, 01:54 PM  #1 
diyAudio Member
Join Date: Feb 2009

confused on power transformer ratings
Ive been looking at hammond/edcor/antek transformers but im having alittle trouble understanding their current ratings. Hammond and antek rate theirs in va. Edcor seems to be alittle more cryptic, their dimension drawings seem to hint at the va (last number seems to change from 200 to 300 if the weight goes up).
Whats confusing me is the amp ratings. antek rates there units as 300vac 250ma. so its really 2 300vac windings, and if you parallel them you get 250ma. Hammond/edcor rate theirs as 600vac centertap 250ma. So what happens to the hammond/edcor when you do full wave centertap and it ends up 300vac? is it 250ma, or is it 500ma since you dropped the voltage in half? My logic was 600vac 250ma or 300vac 500ma. On a final note about the antek. Is it better to parallel the outputs and use a full bridge, or run them in series and make a full wave center tap? 
22nd August 2010, 03:38 PM  #2 
diyAudio Member
Join Date: Jun 2002
Location: Macedon NY

Bridge makes best use of a transformer  about 30% more current that a FWCT for the same transformer. For example, if there are two 125 ma windings, it can deliver about 125 mA DC with FWCT, about 170 mA with bridge. This Hammond catalog page will help: http://www.hammondmfg.com/pdf/5c007.pdf

22nd August 2010, 06:32 PM  #3 
diyAudio Member
Join Date: Feb 2009

Uh the hammond page seems to say the opposite of what you just said, and it also confused me more.
FULLWAVE Choke Input Load: V (Peak) D.C. = 0.45 X Sec. V A.C. V (Avg) D.C. = 0.45 X Sec. V A.C. I D.C. = 1.54 X Sec. I A.C. FULLWAVE BRIDGE Choke Input Load: V (Peak) D.C. = 0.90 X Sec. V A.C. V (Avg) D.C. = 0.90 X Sec. V A.C. I D.C. = 0.94 X Sec. I A.C. Whats more confusing, is the dc numbers. Hammond is saying filtered dc will be about .9 x vac. So does that mean that if im shooting for a B+ of 400v, i should get a 400vac transformer, and use caps that are rated for 1.41 x vac? Using hammond logic, a 600vac ct 250ma transformer will put out 270vdc choke filtered at 385ma. Using hammond logic, a 600vac ct 250ma transformer will put out 270vdc cap filtered at 250ma. Using hammond logic, an antek 300vac 250ma full bridge unit will put out 270vdc choke filtered at 235ma. Using hammond logic, an antek 300vac 250ma full bridge unit will put out 270vd cap filtered at 155ma. My brain is now broke. 
22nd August 2010, 08:01 PM  #4 
diyAudio Member
Join Date: Nov 2003
Location: Brighton UK

Hi.
Your losing me, pure choke filtering ~ averages voltages whilst pure capacitance causes near peak voltages. For CLC its peak voltage, the L averages our ripple. For pure choke filtering the load current must be high enough for averaging. Choke versus capacitor filtering does not put out the same voltage. rgds, sreten. 
22nd August 2010, 08:14 PM  #5 
diyAudio Member
Join Date: Feb 2009

the hammond chart has cap or choke/cap.
Im just trying to figure out how to get 400vdc @250ma with either crc or clc filtering. I dont want to buy a 400vac 250ma transformer and end up with 560vdc I dont want to buy a 800vac CT 250ma and end up with something 2x bigger then i need. I need some learnin to figure out how to translate the manufactures numbers into dc voltage and current. 
22nd August 2010, 08:26 PM  #6  
diyAudio Member
Join Date: Nov 2003
Location: Brighton UK

Quote:
As your first component is a C then you'll allways get near peak voltage. Full wave over half wave will give more current, no point in using half wave. rgds, sreten. 

22nd August 2010, 08:28 PM  #7 
diyAudio Member
Join Date: Jan 2008
Location: So.Cal.

General rules of thumb: Choke (LC) input filter will yield about 90% of the transformer secondary voltage, and cap input filter will yield about 141% (sqrt 2) of transformer secondary voltage. By using a CLC filter and adjusting the value of the first cap roughly between <1uf to about 10 uf will allow dialing in the voltage you want/need anywhere between 90% and 141% of transformer secondary voltage. A very tiny uf first cap mimics the choke input condition, and larger values approach the cap input filter.
To really get an idea of what you'll end up with for voltage and current, download & play with Duncan amps free PSUDII software. PSUD2 This will allow you to model the transformer and filter before you solder anything and is fairly accurate once you get up to speed with it. Change the resistive load in PSUD II to a current load when modeling, assuming that you have an idea of your req'd current draw. 
22nd August 2010, 08:43 PM  #8 
diyAudio Member
Join Date: Feb 2009

Ah boywonder i do understand your logic, as im using it to tune the B+ of my current amp.
Can you help me with my orignal question on translating the current numbers of the transformers? As i said before i dont know if a 3000300 250ma is the same as a 300 250ma. Im not sure if the 3000300 could be interpeted as 600 @ 250ma so 300 @ 500ma. 
22nd August 2010, 10:50 PM  #9  
diyAudio Member
Join Date: Jan 2008
Location: So.Cal.

Quote:
One other thing to keep in mind, for FWCT the diodes need to be rated at least 2.82x transformer secondary voltage (think 1200V Fairchild Stealth); for bridge the diodes only have to be 1.41 x transformer secondary voltage. What is your target B+ and current? Last edited by boywonder; 22nd August 2010 at 11:05 PM. 

22nd August 2010, 10:59 PM  #10  
diyAudio Member
Join Date: Jan 2008
Location: So.Cal.

Quote:
Your cap voltage rating needs to be at least 1.41 x transformer secondary VAC. On start up, unless you have some sort of delay scheme, the cold, nonconducting tubes will not draw any current, and the offload B+ voltage will approach 1.41 x transformer secondary. Are you using a choke input or cap input filter? 

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