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Old 30th March 2004, 04:32 AM   #21
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Thanks for the tips. At this point the transformer(yup, a toriod) hasnt been bolted down. its just sitting on my ironing board(sic!) for testing before i actually assemble. The sound appears to be coming from the rectifier(maybe the transformer as well i'm not sure) but both the rectifier and trans get very hot in a few split seconds. I havent used the bolt yet at this point.

the transformer works fine with proper voltage measurements meeting spec when the recitfier is not connected.

I will difinitely be trying to get some new recitfiers to try again(sigh)

What i fear is if the problem persists with the new bridges, then, i will really be stuck wont i....

Here's the rectifier i plan on getting: does it look ok?

http://www1.jaycar.com.au/productVie...Max=&SUBCATID=

I will be running 36V approximately for the rails and it will be a 4 channel gainclone(planning on building linkwitz orions one day and a 4 channel would be great for running the mid/tweet active) and the whole amp will be powered off my 300VA toroid. The rectifier seems to exceed spec by quite a large margin but since recitifers arent that expensive i though "what the hey..."
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Old 30th March 2004, 05:37 AM   #22
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About the resistance readings:

A diode needs around 0.6 volts to conduct any significant current.

It's quite common that a regular DVM doesn't put out enough voltage when measuring resistance, and thus the diodes won't conduct regardless of polarity. Some DVM:s have a dedicated diode test mode, which will measure voltage across the diode.
Without that, you can take a 1.5volt battery and a resistor (100-1000 ohms, doesn't really matter much). Connect those in series and in series with the diode. Measure the voltage across the diode. Should read 0.6 volts with the diode in conducting mode, and 1.5 volts in reverse mode. See attached drawing.

However, it seems your briddges aren't short circuited, so that doesn't explain the currents. It sounds like you've connected the transformer windings to the output pins of the bridge, rather than the input pins. That would result in large assymetric currents, which can easily get the transformer humming wildly.

To protect your gear, connect a 60W light bulb in series with the primary winding. Makes for excellent current limiting when testing.

Rune
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Old 30th March 2004, 03:33 PM   #23
lgreen is offline lgreen  United States
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Default Circ

OK, sorry I couldn't help. Check your power supply circuit carefully and then let it rip with the new bridge (that one looks fine). You know when I made my first diy project about 15 years ago, I killed off about 10 diodes doing the same thing as you, for no apparent reason (exept there was no heat or noise). I solved the problem using a massive diode bridge like you have indicated on your post. Keep at it. If you are really stuck, post a picture of everything with measurements.
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Old 30th March 2004, 06:06 PM   #24
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Default nervous reply!

I consider this not really rocket science but it is difficult enough for me to again reply, I am not a rocket scientist anyway.

Matjans was right but maybe i should translate it in Flemish advice.

your red ac needs to go to a rectifier , let's call the rect "lul".
your black wire 0v also needs to go t the same "lul", ofcourse the other ac entry!!!


your yellow ac goes to the second (yes you need 2 recs) rectifier, let's call this on "lol"
your orange 0v goes to the same rectifier "lol" but again ofcourse the other ac entry.

now after haveing done this
one new wire goes from LUL's dc neg to LOL's dc pos and from this connecting wire you will have a 0 volt out that you NEED to have AND use!!!!!!!!

now from LUL's DC pos there is one wire going to the board or wiring p2p as the positive (plus)
and from LOL's DC neg there is the other wire going to the board or p2p as the negative (minus)

so you end up with 3 wires, it is to say : a pos, a neg and a 0 volts

before you start connecting these wires to a p2p or board, please measure them, and post the result here.

J-P
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Old 30th March 2004, 11:51 PM   #25
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Hello Blackreplica, it looks like to me you may have wired your bridge recifier incorrectly. I downloaded your schematic and revised it (I will post it here). You may want to wire your bridge rectifier as shown in the modifications to the schematic. Hopefully you didn't blow one of the diodes in the bridge.

Represented in the schematic, I drew in a 60 watt lamp (as posted by runebivrin), you want that in series while testing to limit damaging current in case things go wrong. If there is a short in the wiring or a mistake, the lamp will glow brightly, while protecting your transformer and bridge rectifier from damage.
(Please bear with my bad drawing of the lamp).

Good luck Blackreplica, just wire the bridge rectifier as shown in the schematic (if it is alright, I highy doubt you damaged the transformer, as it would start smoking), things should be OK.

P.S.>Be careful around the mains voltage, I see yours is 230 volts. You don't want to get a hold of that!
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Old 31st March 2004, 01:23 AM   #26
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Hello everyone,

This is great, and everything here is really helpful. I think the way to go is with the new bridge and start again(am planning on getting em tomorrow, unfortunately, decent electronics parts are really troublesome and expensive to get in australia)

Just to clarify, i have been suggested with 2 alternative methods of wiring up the PS with one bridge(as suggested by rimband(thanks so much for editing the diagram, it makes so much sense now, i think that was how i was wiring it from the start though) and another with 2 bridges(as kindly pointed out by mat and uvodee). Aside from sound quality implications, is there really a difference between using one bridge or 2? Specifically, Will it result in a different rail votage between the 2 methods? I ask this because i need to get 25VAC to the rectifiers. My trans is a twin secondary 25V transformer(i believe both secondaries are 25 volts). Between the single and double rectifier method, are they both situations where the secondaries are wired in parallel?(thats what i should be aiming for right, since if i wire in series i would be double the voltage to 50V instead?)

Just a curious piece of info, i know what a 230V shock feels like. No no, no accidents with the amp, its just that when i was 8 years old i think i had an unfortunate accident when i pulled out a plug from a wall socket with the switch still in and one of my fingers touched the pins on the plug while it was still coming out of the socket(lol, dont ask). I think i was electrocuted for about 10secs before i even realised what was happening to me. thats an experience i wont forget anytime soon!

Edit: Just in case i may have mis-specced the transformer i thought i'd post a link directly to the transformer i have right now

http://www.rsaustralia.com/cgi-bin/b...toid=-83963296

its the 20-25V, 60A one.

I am aiming to get 25V AC output going to the rectifiers and not 50V. If there is a difference in the 2 abovementioned methods of wiring the secondaries(1 bridge vs 2 bridge method) do let me know! Thanks so much!

Also, I may be getting a little ahead of myself by asking this too but i was also wondering that when actually wiring up the amplifier, would i eventually connect power/signal ground from the amplifier circuit to the 0V point of the PS or would i just leave them floating(not sure if thats the right term)?
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Old 31st March 2004, 04:12 AM   #27
Fossil is offline Fossil  Singapore
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Quote:
Originally posted by blackreplica

I am aiming to get 25V AC output going to the rectifiers and not 50V. If there is a difference in the 2 abovementioned methods of wiring the secondaries(1 bridge vs 2 bridge method) do let me know! Thanks so much!
Hi

u can construct a +/- VDC supply using a 2-sec windings and 1 rectifier.

Check out the AKSA website:
http://www.aksaonline.com/products/p...ksaasmbly.html

if u do so, u gotta short the black and yellow leads and tied them to ground.
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Old 31st March 2004, 04:30 AM   #28
Variac is offline Variac  United States
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Quote:
most guys here use a 18, 22,24 or 25 volts CT (centertapped) tranformer and so after rectification this # goes up to 38 volts or so, pretty high for it's purpose in this case.
Hello Uvodee,
Since we have been discussing this before , I feel you won't be offended if I question this statement, and ask for another opinion.

PLEASE SOMEONE:

I thought that most people use transformers that have dual outputs, each pair is 18-25 volts.
1. Isn't this the equivalent to a 36-50 volt center tapped transformer?

2. So... if you use a center tapped trannie, don't you want a 36 to 50 volt center tapped, which has two 18 to 25 volt legs? and one common?
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Old 31st March 2004, 04:39 AM   #29
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I think we're getting close to the reason you're having problems. You really don't want to parallel the secondaries, neither before nor after recitfication.

What voltage do you want after rectification and filtering?

With 2x25V AC, you'll wind up with 35V, which is on the high side for a classic GainClone. There's no really good way to turn that into 17.5V, which would be a bit on the low side anyway.

I'll leave the voltage issue for now, and focus on the wiring. Just wire one of the secondaries to the AC connections of a single bridge. Leave the other scondary unconnected. You should now measure somewhere between 25 and 35 volts across the DC terminals of the bridge, depending on how your multimeter reacts to unfiltered DC.

Rune
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Old 31st March 2004, 05:48 AM   #30
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Default thank you

Thanks Runebivrin.

I could not have expressed myself better than with your message.

25 volts before and 38 volts after rectification and a 10 pct tolerance on capacitors comes close to the maximum 84 volts/2 = 42 volts per rail you can have using a LM 3875.........

it is as simple as that


J-P
if you feed the 3875 anything higher, you get a load roarring noise ... and other consequences
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