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CRASHED! A topic for SEVERE and immediate Hardware and Operating System FAILURES. We will try to get you up again. NOT for Optimization questions!


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  #1 (permalink)  
Old 20th April, 2002, 11:35 AM
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Unhappy Dead PC..

Well. After using and abusing my main PSU module, one of the two async converters decide it was going stop working.

One moment, the PC is running fine, and the next moment black screen, but with the power light on.

The converter picked up that it's 12V line failed, and stopped the PowerGood signal. PC then instantly went into reset.

I've taken the converter apart, and found one of the pair of fast recovery diodes went short circuit.

Now I just need to find a packaged half rectifier to replace them with.

AidanII
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Old 20th April, 2002, 05:17 PM
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Re: Dead PC..

Quote:
Originally posted by AidanII

Now I just need to find a packaged half rectifier to replace them with.
I come fully contained and I half rectify things :-D

Cuteness aside, what's a packaged half rectifier (other than my lame guess above )??
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Old 20th April, 2002, 06:37 PM
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I have to ask what a "half" rectifier is, too. I do not, never have, and never will say that I'm an EE, btw.
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Old 20th April, 2002, 07:30 PM
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Do you meant the ones that you use to make the two psu to turn on at the same time? If not youve lost me.
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Old 20th April, 2002, 11:04 PM
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A half rectifier is nothing to do with turning two PSUs on at the same time.

Inside the PSU, the mains AC is turned into DC. Then there's a few transistors that convert the DC back into AC, but at a MUCH higher frequency (like 40KHz). In order to simplify wiring, a half rectifier is used to turn the high frequency AC back into DC. A half rectifier is two diodes with a common anode or cathode.

I'd draw a picture, but I can't be bothered to right now. If you really want one, I'll do one.

AidanII
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Old 21st April, 2002, 04:54 AM
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I got it now, but no lead at all for them... don't buy electronic parts anymore. I'm lame and gave up a few years back, sorry
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Old 21st April, 2002, 09:42 AM
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I'm not an electronics bod (incident w/ a soldering iron when I was 10 yo put me off) but, Did you try RS or Maplins ? to look for what ur after?

Otherwise, there's one other big place in the UK who do electronic compos (but I don't know what they're called offhand.
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Old 21st April, 2002, 11:58 AM
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Maplin don't do anything worth looking at now. They're little more than whitegoods box shifters, which is a bit sad. They used to be very good.

Farnell, RS, AVNet to name a few. I need to work out what the equivelent part would be, then overspec it.

I'm half considering building a syncronous rectifier instead, but I need some more information on whether it's possible to do syncronous half wave rectification. I don't see why not though.

AidanII
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Old 21st April, 2002, 12:37 PM
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Quote:
Originally posted by AidanII
I'm half considering building a syncronous rectifier instead, but I need some more information on whether it's possible to do syncronous half wave rectification. I don't see why not though.

AidanII
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Old 22nd April, 2002, 03:25 AM
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Quote:
Originally posted by surlyjoe


I love it when he talks dirty like that
And I thought it was just me!
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Old 22nd April, 2002, 03:30 AM
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Quote:
Originally posted by AidanII
Then there's a few transistors that convert the DC back into AC, but at a MUCH higher frequency (like 40KHz).
Just out of curiosity, does it convert to a higher frequency to allow for a physically smaller transformer, or what? And another just out of curiosity question; is that high frequency square or sign wave? And em, is the first AC to DC conversion just a high voltage(300V?) cap or two and a rectifier circuit(no transformer)?

I'm trying to visualize here, help me out
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Old 22nd April, 2002, 06:28 AM
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I think that you have lost us in exactly what is broken........... Spyder
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Old 22nd April, 2002, 05:03 PM
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Quote:
Originally posted by Spyder462
I think that you have lost us in exactly what is broken........... Spyder
No, it's just two diodes. They are like a gate that swings one way. Electricity flows one way through them, but not the other. Another good analogy would be to compare them to hydraulic check valves . . .
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Old 22nd April, 2002, 08:27 PM
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Quote:
Originally posted by dimmreaper
Just out of curiosity, does it convert to a higher frequency to allow for a physically smaller transformer, or what? And another just out of curiosity question; is that high frequency square or sign wave? And em, is the first AC to DC conversion just a high voltage(300V?) cap or two and a rectifier circuit(no transformer)?

I'm trying to visualize here, help me out
Ok.. in no particular order.. The following is at 240V.

The mains comes in through some standard filtering components (4 capacitors and a coil). That mains is then fed into a bridge rectifier, which throws out ~340V. This is stored in the capacitors to do some basic smoothing. (There's some tricks around here that acts as a voltage doubler for a 110V supply, so you end up with ~311V). This is why it's dangerous to play with a PSU. There's supposed to be a resistor to bleed power away from the two capacitors, but sometimes these fail, and sometimes they never quite make it into the build... If the capacitors are still charged, contact with 340V worth of DC can be very lethal.

Next step, it's fed into the two switching transistors. This allows one transistor to create a positive flux, and the other to create a negative flux. The two transistors are driven with a square wave, as this (massively) cuts down energy loss in the transistors, which means cooler transistors and more power transfer. These transistors are driven somewhere between 20KHz and 40KHz. The high frequency means the transformer can be physically small and still manage the power throughput. I've seen transformers the size of the ones in a PC power supply that are good for 500W worth of power when they're running at 500KHz.

The transformer provides physical isolation from the mains supply, so no nasty shocks happen. On the secondary side, there's a bunch of paired windings, 12V, 5V and 3.3V. The 5V and 3.3V are often the same winding, just taken off earlier for the 3.3V. (Hence the combined power rating of 3.3V and 5V). The windings are paired because the centre tap acts as ground, and a pair of diodes do half wave rectification on each half of the pair of windings.

This lot then runs through some filters (coils) and capacitors to remove the high frequency switching noise. This is why a motherboard monitor can't pick up a noisy PSU line. The hash on the PSU line that's crashing your computer is happening at 20-40KHz and upwards due to harmonics. The motherboard monitors just can't manage to pick up things that fast!

Somewhere in that lot, there's a supervisor IC, which monitors the average error across the 3.3V and 5V lines (and if you're lucky 12V), and adjusts the duty cycle of the two switching transistors to bring the power up or down. Yes, that means your PSU checking the power several thousand times a second and making adjustments. The average error bit is why often, when you adjust the 3.3V line, the 5V line changes too. After all, there's only one pair of transistors for all the voltages!

Hope that answers your questions. Feel free to ask more if you want!

AidanII
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Old 22nd April, 2002, 09:07 PM
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OK, now I see how it's all working. I've been staring at an old AT PSU I have here for about a year trying to figure it out exactly, and it seems about the same as what you just said.

The voltage doubling "trick" for 110V, does this work in a similar way that photo-flash power supplies work?
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