What Transformer Voltage?

Discussion in 'The Projects Forum' started by DC4me, Oct 10, 2010.

1. DC4me Thread Starter New Member

Oct 5, 2010
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0
Hello, I am building a 300 VDC fixed power supply. I want to use a full wave bridge with a pi (CLC) filter arangement after a 120VAC step up transformer. My question is what secondary voltage do I want on the transformer? I have found different information. Some say the DC voltage will be 1.4 or 1.2 times the secondary AC voltage and some say the DC voltage will be .9 times the secondary AC voltage. I appreciate the help.

2. marshallf3 Well-Known Member

Jul 26, 2010
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If it'sa 120V:300V transformer it's going to put out 300V RMS at the rated current.

When you rectify and filter it that 300V becomes 300 x 1.414 and goes down with the amount of current you draw from it.

How much current do you need for the 300V? Does it have to be really steady or can it vary by a certain amount? This may be simpler than you think.

3. DC4me Thread Starter New Member

Oct 5, 2010
3
0
Well I think I need something less than a 300 volt secondary if I want 300 VDC if the 1.4 x secondary is correct, or a little more than a 300 volt secondary if the secondary x .9 is correct. What I will have after the secondary winding is a full wave bridge, a cap, a choke then another cap. So I don't know if .9 or 1.4 is the correct calc. Thanks for the reply.

4. wayneh Expert

Sep 9, 2010
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4,276
The rectifier will trim off only ~1.4 volts from the peak DC. The output under load will drop much more than that, though, depending somewhat on the size of the cap (the voltage drop during the non-conduction phase), and the ability of the supply to charge the cap back up to the peak, during the brief phase during which the diodes are conducting.

If it helps, the voltage change in the transformer will be proportional to the number of windings on each side. More loops = more volts.

5. Kermit2 AAC Fanatic!

Feb 5, 2010
4,165
1,120
http://en.wikipedia.org/wiki/Rectifier

They even have the diode circuit you require all on the one page.(scroll down just a little)

PLEASE. Use a small 1 to 1 isolation transformer to obtain mains level voltages to work with. AND fuse the primary and secondary BOTH. Shocks from this voltage level of AC or DC can stop your heart. Never work alone around mains level voltage.

Fuses can be replaced, but you can't...so don't blow it!

6. Wendy Moderator

Mar 24, 2008
20,993
2,731
I interpreted the OPs request as to used a step up transformer, which meets the guidelines (thought the voltage is definably on the scary side. 300VDC ÷ 1.414 = 212VAC, so a step up of 1:1.8 or better is needed. A transformer rated 1:2 would work well, and would output approximately 339VDC after rectification, probably a bit less.

7. marshallf3 Well-Known Member

Jul 26, 2010
2,358
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I was thinking aout using something in the industrial line. You can get transformers that deal with 277VAC pretty easily.

What we really need to know is what type of load you're trying to drive - what amount of current do you need at 300V and how accurate does it have to be?

8. DC4me Thread Starter New Member

Oct 5, 2010
3
0
Thank you all for the help. I need about 100 mA. The voltage does not have to be exactly 300 VDC. It can be as high as 350 VDC.

9. DigitalReaper Member

Aug 7, 2010
70
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One of those small transformers that converts US mains to EU/UK mains would probably work well here. 220v output = 311v peak, 240v = 339v peak. Don't forget to allow for some ripple on your smoothed output though, 240v output is your best bet if you can find one.

Jul 17, 2007
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11. DigitalReaper Member

Aug 7, 2010
70
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That transformer won't meet his current requirements, he needs ~100mA after rectification and smoothing.

12. SgtWookie Expert

Jul 17, 2007
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It should work just fine.

If you have a better suggestion, then make it.

13. Wendy Moderator

Mar 24, 2008
20,993
2,731
The 230VAC side is rated for 110ma, why don't you think it wouldn't it work? The currents do not change after rectification and smoothing.

If you look up the part number is it meant for 115V in/230V out.

14. marshallf3 Well-Known Member

Jul 26, 2010
2,358
202
Thought I just mentioned that in post #7.

There are a lot of 277V:120V transformers to be found that could be hooked up in reverse.

15. DigitalReaper Member

Aug 7, 2010
70
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110mA @ 230v AC = 78mA @ 325v DC. I had the same problem with a power supply i'm building =/