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Old 5th October 2012, 07:50 AM   #11
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by KatieandDad View Post
I'll try a DC blocker as suggested, it's easy enough to iron out that possibility.

Can anyone tell me why circuit 4 above works. D1 and R2 surely conduct into destruction on the positive half cycle.
Yes it works... the trouble is its been copied out of context and the diode and resistor are part of a worked example explaining the theory.
http://sound.westhost.com/articles/xfmr-dc.htm
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Old 5th October 2012, 05:44 PM   #12
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Question:- My 2 x 500VA transformers blow 5A mains fuse on power up. I've had to use a 10A Time Lag. What would the current rating opf the diodes need to be ?
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Old 5th October 2012, 07:28 PM   #13
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Good question and I haven't got the answer. I suppose the question really is what voltage is allowed to develop across the caps. If the reactance is low enough (and hence no or little voltage developed across the cap) then the diodes never come into conduction anyway. Its what happens at the moment of switch on under worst case conditions that counts.

Putting my "practical empirical" head on for a moment I would say that as we usually specify a hefty bridge rectifier for this application (because if you look at the circuit its just a matter of shorting the appropriate connections on the bridge) it becomes a non issue as the surge current of these things is huge. The voltage rating is unimportant as it never sees more than a volt or two.
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Old 5th October 2012, 07:43 PM   #14
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Quote:
Originally Posted by KatieandDad View Post
Question:- My 2 x 500VA transformers blow 5A mains fuse on power up. I've had to use a 10A Time Lag. What would the current rating opf the diodes need to be ?
1000 VA need a soft start and in particular if those are toroidal transformers.
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Old 5th October 2012, 08:32 PM   #15
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Originally Posted by KatieandDad View Post
Question:- My 2 x 500VA transformers blow 5A mains fuse on power up. I've had to use a 10A Time Lag. What would the current rating opf the diodes need to be ?
extract below says if you do not use a soft start device (like an NTC inrush limiter) diode current requirements go up. I was surprised to see the 25A and 35A rectifier recomendation. IMO, It's much cheaper to use an NTC inrush limiter and the smaller diodes.

--------------------------------extract---------------------------------

The circuit shown above satisfies all criteria. There is more than enough capacitance, and the diodes specified have a continuous current rating of 3A and a peak current rating of 200A (non-repetitive). After this circuit is included, the no-load current falls back to 16mA, and the visible signs of asymmetrical saturation are gone. As transformer load increases, the diodes will take over from the caps, preventing the peak voltage across either capacitor from exceeding 1.3V - even during the inrush current period.

Note that if no soft-start circuit is used, larger diodes are highly recommended. Regardless of whether soft-start is used, a 25A or 35A bridge rectifier assembly is a simple and relatively inexpensive way to obtain very high current diodes, already neatly packaged and insulated. When using a bridge, remember that + and - must be shorted together to obtain 4 diodes in anti-parallel (as shown in Figure 3).
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