Stupid Bridge/Diode Question

[richie00boy]

If I build a discrete bridge rectifier using 4x 100V diodes, is the bridge rectifier as a whole rated to 200V?

 

[anatech]

Hi richie00boy,
I would think 100V. It is reverse biased on alternate cycles with one end being the rectified voltage close to peak, the opposite goes the other way.

-Chris

 

[richie00boy]

Yes, but the current has to pass through two diodes on the journey. I an interested in the theoretical answer but also from a practical viewpoint as I currently have a bridge made from 100V diodes running off a 45VAC winding and has been given some abuse without blowing.

 

[peranders]

45 VAC and 100 V diodes is a bit weak. 200 V at least. You must also have incoming voltage transients in mind.

 

[richie00boy]

Yes but from a purely theoretical view, is a bridge made from 100V diodes rated to 200V?

As 45V * 1.414 = 63V which when doubled to take into account of reverse bias bridge gives 126V yet my bridge has not blown up.

 

[moamps]

As 45V * 1.414 = 63V which when doubled to take into account of reverse bias bridge gives 126V yet my bridge has not blown up. [/B]

No.
Reverse voltage on each diode is Vpeak-Vdiode(on)=ca Vpeak
like Chris already said.

Regards,
Milan

 

[richie00boy]

Thank you, that is the kind of answer I was looking for.

 

[richie00boy]

So what about half wave voltage doublers? I used 60V diodes on a scheme similar to this

An externally hosted image should be here but it was not working when we last tested it.

Schematic copyright Rod Elliot, sound.au.com

but they blew up with an 18V transformer. Yet 18*2*1.141=51V which should be within their capabilities.

 

[AndrewT]

Hi,
a bit off topic but why on earth are you feeding 51Vdc into a 7815/7915? They are almost certain to blow. With or without a load on the end. Or is your circuit only delivering 25Vdc to each?

So back to topic

Is a 4 by 100V diode bridge equivalent to 100V or 200V PIV?

 

[Nigel Goodwin]

Is a 4 by 100V diode bridge equivalent to 100V or 200V PIV?

Interesting question!

In theory it would be 200V, as the two diodes are effectively in series in each side of the bridge. BUT!, there's no voltage sharing resistors across them, so they won't be accurately balanced - so it will be greater than 100V but less than 200V (the exact value depending on the exact spec of the individual diodes).

Personally, I'd still rate it as a 100V bridge, if you want 200V, then use better diodes! - the price difference is pretty insignificant.

 

[richie00boy]

Hi,
a bit off topic but why on earth are you feeding 51Vdc into a 7815/7915? They are almost certain to blow. With or without a load on the end. Or is your circuit only delivering 25Vdc to each?

Please ignore the regulators, I'm not using those ones and I put the schematic up purely to show the half wave voltage doubler arrangement. As you mention with a 18V transformer each reg only sees approx 25V.

So back to topic

Is a 4 by 100V diode bridge equivalent to 100V or 200V PIV?

That is my question and the whole point of my thread From earlier responses it would seem the bridge is rated to 100V PIV.

 

[moamps]

.... as the two diodes are effectively in series in each side of the bridge. ...

Two diodes are in series when conducting.
But diodes aren't in series when are reversed biased!

Regards,
Milan

 

[richie00boy]

Milan, thanks for your consise input hitting the spot! Can you care to speculate why my diodes blew in the half wave voltage doubler arrangement I posted earlier?

 

[moamps]

Can you care to speculate why my diodes blew in the half wave voltage doubler arrangement I posted earlier?

The max. reverse voltage in the halfwave rectifier is Vpeak+Vc1=ca 2*Vpeak.
Which diode do you use (type number)?

 

[richie00boy]

MBR160 which is 60V rated Schottky. I actually only used a 12V transformer but it was lightly loaded so the voltage was a bit higher. Your formula followed what I had calculated and should be within the diode rating

So what about half wave voltage doublers? I used 60V diodes on a scheme similar to this

An externally hosted image should be here but it was not working when we last tested it.

Schematic copyright Rod Elliot, sound.au.com

but they blew up with an 18V transformer. Yet 18*2*1.141=51V which should be within their capabilities.

 

[moamps]

I actually only used a 12V transformer but it was lightly loaded so the voltage was a bit higher. Your formula followed what I had calculated and should be within the diode rating

Hi,
you are correct. Diodes should work fine.
Did you check PCB and other elements?
Put a small resistor (10 to 22ohms/1 or 2W) in series with the secondary and check voltages on the caps and outputs.
Regards,
Milan

 

[richie00boy]

The PCB I believe is fine, but will triple check. The diodes failed short after a few minutes operation, just driving a simple op-amp board. I measured the capacitance of all caps and they are consistent between types but give strange capacitance values compared to their designation, which I presumed to be because they were in circuit with other parts.

 

[peranders]

Yes but from a purely theoretical view, is a bridge made from 100V diodes rated to 200V?

As 45V * 1.414 = 63V which when doubled to take into account of reverse bias bridge gives 126V yet my bridge has not blown up.

As a professional I would never have chosen 100 V diodes. The price difference is nothing compared what you can loose.

I have no doubts that it would work though.

 

[richie00boy]

It is more difficult to get >100V Schottky diodes for me.

 

[peranders]

... but does the application need ultrafast diodes of course, this is DIY