Rectifier diodes

akis

Member
2008-07-18 1:09 pm
I have read through some old threads on the subject of choosing rectifier diodes for a PSU.

I learned a bit about possible EMI interference and "snubber" circuits.

In this thread I would like to know if I can use a 5A Shottcky to replace the 1N540x rectifiers simplybecause the forward voltage drop is less on the Shottcky and I get more voltage on the output (and they should get less hot too I'd imagine).
 
Schottky diodes have a fast and soft recovery. They generally work fine for rectification, but you need to take care in selecting appropriate Schottky diodes. If you are not comfortable reading and interpreting datasheets, then maybe you should stay away from Schottkys. With Si recitifiers, it's easy, just stay inside the reverse voltage specs and current specs, and chuck it in there. Schottkys are more difficult. Except for some newer exotic types, Schottkys have a relatively low reverse breakdown voltage. In general, there is a compromise between maximum reverse voltage, and forward conducting voltage. So if you want to minimise voltage drop and heat, pay attention to both specs; don't simply buy the highest reverse voltage you can find. They also must not be allowed to get hot, because they become leaky (in terms of reverse current) when hot. That's why the larger current ones are often heatsink mounted. You can get into a thermal runaway mode where the reverse leakage current contributes to heating, which increases reverse leakage, which makes more heat, etc...
 
They actually make silicon schottkies up to 300V nowadays, which is amazing. The way they do it is pretty neat.

If you're not doing high voltages, then the main problem is really leakage. High conductance, high leakage types can go into thermal runaway quite easily, as mentioned. This, and probably expense, is the main reason junction diodes are used in supplies over 12V.

Anyway, EMI / snubbing is no problem at 60Hz, anyway, even for the crummiest of 1N540x's.

Tim
 

akis

Member
2008-07-18 1:09 pm
The only reason I am considering Schottky diodes, eg the SB560 (5A 60V) is for their very low forward voltage drop which means I can squeeze an extra volt or more out of my transformer.

However I did not know about the thermal runaway problems. Is there a particular characteristic I am looking for in order to decide if the diodes are suitable? Eg are the SB560s suitable in a bridge configuration for 5A at 33 Volts DC?
 
The only reason I am considering Schottky diodes, eg the SB560 (5A 60V) is for their very low forward voltage drop which means I can squeeze an extra volt or more out of my transformer.

However I did not know about the thermal runaway problems. Is there a particular characteristic I am looking for in order to decide if the diodes are suitable? Eg are the SB560s suitable in a bridge configuration for 5A at 33 Volts DC?

Be carefull with the voltage rating. For a 33V DC supply, a 60V diode is not enough. If the diode is cutoff with the secondary of the xformer at neg wave, it will probably be at -35V. The other side of the diode will be at the capacitor DC of 33V (in your case). Total diode voltage is 68V so the 60V diode may say poofff...

For rectifiers, the diode reverse voltage should be at least twice the rectified DC voltage plus some safety margin. In your case I would use minimum 100V diodes. Check the rating on the existing diodes and use that.

jan didden
 
The gains in using shottkey diodes are relatively small at 35V, Keep in mind that supply regulation is +/- 5% for most power companies, if your transformer is toroidal another turn around the core will compensate for diode drop.

Be carefull with the voltage rating. For a 33V DC supply, a 60V diode is not enough. If the diode is cutoff with the secondary of the xformer at neg wave, it will probably be at -35V. The other side of the diode will be at the capacitor DC of 33V (in your case). Total diode voltage is 68V so the 60V diode may say poofff...
That is true for CT full wave rectifiers but with the bridge rectifier the negative half of the cycle is shunted by the other rectifier to the negative rail so it can only swing to 0V - diode drop. If the transformer is 24V then 60V should be fine if it is 33V then I would be looking at a 75V diode just to give a safety margin for mains transients.
 

akis

Member
2008-07-18 1:09 pm
I bought a 230v/2x25V/6A toroidal at a very good price, and then realised it might not be good enough for providing 30VDC/5A.

mains: 230V
trans reg: 6%
filter: 20mF
diode drop: 0.8V each / 6A

DC output base: 30.76VDC
plus ripple 3V = avg 31.82V DC

So I need just like maybe an extra volt to go over the losses on the PSU (pass transistors etc) so as to get a net 30V, hense asking about the diodes.

But if I can simply remove a few turns of primary winding from the toroidal would that be a better idea?
 
I bought a 230v/2x25V/6A toroidal at a very good price, and then realised it might not be good enough for providing 30VDC/5A.

mains: 230V
trans reg: 6%
filter: 20mF
diode drop: 0.8V each / 6A

DC output base: 30.76VDC
plus ripple 3V = avg 31.82V DC

So I need just like maybe an extra volt to go over the losses on the PSU (pass transistors etc) so as to get a net 30V, hense asking about the diodes.

But if I can simply remove a few turns of primary winding from the toroidal would that be a better idea?

Hi,

Why is this exact 30V so important? As pointed out, mains fluctuations are quite large. If you are worried about power amp output, that varies even more with speaker impedance which itself varies with frequency. And even with 1V delta in supply, the difference in output power is probably too small to notice anyway. So what's the real issue here?

jan didden
 

akis

Member
2008-07-18 1:09 pm
ok another question: can I use the *same* secondary winding to power two independent bridges with their own rectifiers? so when one is delivering many amps, the other one is mostly unaffacted and can be used to provide a more stable supply for driving the various components of the PSU which require very little power but steady voltage.
 
ok another question: can I use the *same* secondary winding to power two independent bridges with their own rectifiers? so when one is delivering many amps, the other one is mostly unaffacted and can be used to provide a more stable supply for driving the various components of the PSU which require very little power but steady voltage.

No.

Draw it out: one winding to 2 bridges doesn't work!

Wind another secondary? Only needs thin wire and most toroids have some "space" inside the donut (US spelling!) to allow this.

Cliff
 
I want to use the transformer to make a PSU "30V 5A" - obviously if it can only reach 29V then it is no good :)

Well, unless it is a regulated supply, you will find that your magical 30V varies all over the place over the time of day, the load, the weather, etc.
And if you go for a regulated supply, the xformer voltage gets much less critical.
I think you are chasing the wrong ghost, but it's your time and money ;) .

jan didden
 
Well, unless it is a regulated supply, you will find that your magical 30V varies all over the place over the time of day, the load, the weather, etc.
And if you go for a regulated supply, the xformer voltage gets much less critical.
I think you are chasing the wrong ghost, but it's your time and money ;) .

jan didden

I 100% agree with Jan.
 
No.

Draw it out: one winding to 2 bridges doesn't work!

Wind another secondary? Only needs thin wire and most toroids have some "space" inside the donut (US spelling!) to allow this.

Cliff

It can work - http://www.passdiy.com/pdf/a75p2.pdf Used here to provide some isolation between the two channels.

That said, if you are looking to provide regulated output for the amp's front end, either a separate higher voltage transformer, a light gauge second winding or a doubler feeding the regulator (as shown in the A75 supply) would do better than just having a separate bridge.
 
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akis

Member
2008-07-18 1:09 pm
I am building a general purpose bench PSU. Money is not a consideration because if I ever build it, it will have cost more money and trouble than buying a new one from a shop. But it is a hobby.

The transformer is 25VAC, and that does not leave enough room for 30VDC at the output. A 27VAC is much better.

If I wanted to wind a few more turns on the secondary as suggested, how would I do that? There is not much free wire left, it might not even do one turn. The only solution I can think of is to solder some more enameled copper wire on to it and as smooth as possible, then put electrician's tape around the solder and wind a few more turns. Is that correct?
 
.....

The only solution I can think of is to solder some more enameled copper wire on to it and as smooth as possible, then put electrician's tape around the solder and wind a few more turns. Is that correct?

Why?

Just wind some more turns (do 10 and measure the voltage which will give you the volts/turn figure) and connect the windings in series with the existing ones OFF the transformer.
 

akis

Member
2008-07-18 1:09 pm
I have opened up the transformer and noticed an abundance of ordinary sellotape/scotch tape :)

The secondary is 1mm or 1.25mm wire and it is wound so as to form just one layer - leaving a small gap just for solder connections to be attached. Those connections are also insulated with a few turns of sellotape...

All I have to do is to get same diameter wire, solder it on where the external wires are now attached, wind it a few turns to raise the voltage from 25VAC to say 27VAC, and then attach the external wires back on it.

In addition I can also wind another layer of wire to provide a 3rd secondary, still 27VAC but much lower current. I can wind it on top of the existing secondary winding but using much thinner wire as I only need 400mA at most.
 
I have opened up the transformer and noticed an abundance of ordinary sellotape/scotch tape :)
Polyester tape, more like packing tape with no glue, old sellotape was cellophane based (modified cellulose)

All I have to do is to get same diameter wire, solder it on where the external wires are now attached, wind it a few turns to raise the voltage from 25VAC to say 27VAC, and then attach the external wires back on it.
I never bother with details like that, I just solder a bit of hookup wire of adequate section to the end of the original lead and wind it around the core with fairly widely spaced winding to spread the winding out over the core. Test the turns per volt first by winding a couple of turns of hookup wire around the core and measuring the voltage produced in them. Test the phasing before taping up or varnishing the extra winding. You can use enamelled wire if you like, it can take more heat and is much smaller also it can be any size as long as it is the same or larger diameter than the original wire.

If you have stripped off the tape layer then you might as well use the space left at the end of the winding for the extra turns
In addition I can also wind another layer of wire to provide a 3rd secondary, still 27VAC but much lower current. I can wind it on top of the existing secondary winding but using much thinner wire as I only need 400mA at most.
You sure can but put varnish the new winding first, wrap it in tape (polyester not cellophane) then wind the next winding. I like toroidal transformers because they are easy to fine tune the voltage to suit the job, EI are OK if there is still space in the window and the voltage needs to go up, if down I hook the required number of turns up antiphase which increases copper losses a bit, but it is much easier than stripping windings