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Old 13th December 2012, 06:19 PM   #21
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Thanks kevinkr for the tips. I heard a bit about 5ar4's arcing issues. Pros and Cons?
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Old 13th December 2012, 07:17 PM   #22
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Originally Posted by FoMoCo View Post
The center tap is particularly a bad idea. If one of your rectifiers shorts, it doesn't protect at all against that.
If a rectifier shorts, then it will put full AC voltage over the filter capacitor making to drain sufficient AC current to blown the fuse. If you put a fuse on both rectifier anodes, and one of them blows, the conducting anode will make a DC circulate inside the secondary of the transformer, saturating the core, and overloading this half secondary. Putting the fuse in the common line avoids this situation.

I make obvious that there was a fuse in the primary too.
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Old 13th December 2012, 11:41 PM   #23
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Originally Posted by Osvaldo de Banfield View Post
If a rectifier shorts, then it will put full AC voltage over the filter capacitor making to drain sufficient AC current to blown the fuse. If you put a fuse on both rectifier anodes, and one of them blows, the conducting anode will make a DC circulate inside the secondary of the transformer, saturating the core, and overloading this half secondary. Putting the fuse in the common line avoids this situation.

I make obvious that there was a fuse in the primary too.
It also places one rectifier directly across the secondary. I think the fuse in the center-tap is a bad idea. The filter cap that now has AC on it may very well rupture before opening the fuse.

That's why I think only a fuse in the primary is the way I'd go.
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Old 14th December 2012, 01:55 AM   #24
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If the fuse sits on the center tap (series) and a failure triggers it to blow, theoretically it will stop the current right? But because the primary may still be functioning and the transformer is still producing, each leg is at the high voltage potential all though the circuit appears to have been disconnected right? Is this safer than just the primary side fuse? Having a center tap fuse will have the opertunity of saving the equipment right? Or is it going to allow transformer damage to acour?
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Old 14th December 2012, 11:51 AM   #25
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Originally Posted by woodrough View Post
If the fuse sits on the center tap (series) and a failure triggers it to blow, theoretically it will stop the current right? But because the primary may still be functioning and the transformer is still producing, each leg is at the high voltage potential all though the circuit appears to have been disconnected right? Is this safer than just the primary side fuse? Having a center tap fuse will have the opertunity of saving the equipment right? Or is it going to allow transformer damage to acour?
No. Draw out the circuit. Replace your fuse in the center-tap with an open. Now replace one rectifier with a short. You now have a rectifier across the full secondary. Every half cycle there's a short across the entire secondary.

If it's the only fuse you've included, you have no protection. If you've included the primary fuse, and sized it right, it'll open. But, that center tap fuse doesn't afford any more protection than the correct primary fuse.

Take a look inside most commercial equipment. The norm is to place the fuse as close to power entry as possible.
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Old 14th December 2012, 02:45 PM   #26
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I have been told that if the center tap is disconnected the circuit is considered open. Although... I always viewed ct secondaries as one continuous winding with a lead attached half way (making it not an open circuit when ct open).
I would hands down use a primary side fuse, but putting one after the transformers secondary proves to be not the healthiest method of protection.

I think I will be sticking to the primary only side fuse only.
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Old 14th December 2012, 02:51 PM   #27
FoMoCo is offline FoMoCo  United States
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Originally Posted by woodrough View Post
I have been told that if the center tap is disconnected the circuit is considered open.
Just draw out the circuit as I described. It'll be instantly clear that the circuit is only open if both rectifiers are operating properly.

As long as you're using a primary fuse, that is properly sized, you're golden from the point of safety, fuse wise.
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Old 16th December 2012, 11:51 AM   #28
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Originally Posted by kevinkr View Post
Place the fuse on the hot side and before the switch, that way if the switch develops a short to chassis (I have seen it happen) the local fuse can blow rather than relying on a distant circuit breaker to protect the intervening wiring. Make sure that the chassis is grounded.

I use the series diodes with all modern 5AR4 as it greatly reduces their propensity to fail due to arcing during warm up and during a hot start event. (Supply interruption)
+1 on the diodes as kevinkr said, same with the fuse. To me it's a safety (fire) thing. I use them on all my PSUs after suffering from an arc over... $.13 part vs loosing a $100 transformer and a $30 NOS rectifier (in my case) is a no brainer.
Not much hurts worse than a rectifier arc over... you wont make it to the power switch fast enough to save your transformer. In the OPs first post that yellow pic in the middle is how I do it. I'm not entirely comfortable depending on a slow blow fuse to save the power tranny either before or after the primary..

As mentioned already... HT = you die if you are not careful. Another good tip I learned is using BLEEDER resistor on one of your PSU caps. Specially helpful when building the PSU when you are taking voltage measurements. A 500V cap holds that 500V for quite some time without a bleed resistor. Short the leads on the cap.. BANG...ouch if yer lucky. One hand in the pocket as they say.. it's no BS. Another good safety tip = X/Y on the input ac line (X cap across the 120V and 0v AC input is what i use). It's a safety thing and a side benefit is you get AC line filtering for RF noise on the line. Specially helpful if you dont use any external line filter/conditioner (I.E. you plug your amp directly into your AC outlet).

Be careful.

Cheers,
Bob
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Old 17th December 2012, 03:52 PM   #29
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Thanks bobrown, I appreciate the tips greatly. I just installed the safety diodes. and should hopefully prolong the life of the rectifier when necessary.

Also, thanks everyone for discussing everything rectifier related!
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