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-   -   Pedja Rogic discrete regulator question (http://www.diyaudio.com/forums/chip-amps/216595-pedja-rogic-discrete-regulator-question.html)

Nuuk 22nd July 2012 10:13 AM

Pedja Rogic discrete regulator question
 
Next to the SMPS, my favourite power supply for chip amps is the Pedja Rogic discrete regulated. I've just built another pair of these regulators, and they appear to work OK but I have a question about the measured voltages.

I'm using BZX55 C27 zeners (500mW) that are quoted as having a voltage between 25.1 and 28.9. However, in my case I am getting voltages of 29.5, 29.8 29, and 29.4, all above the maximum quoted in the datasheet.

The corresponding output voltages are: 27.1, 27.3, 27.3, 27.4 - so would appear to be what is expected from the diode voltages.

My question is - are these diodes just a little bit out of spec, or is there something wrong with my circuit?

http://www.decdun.me.uk/Pedja-Rogic/...fb-sup_sch.gif

Juergen Knoop 22nd July 2012 03:42 PM

hard to say...the zeners voltages depend on their operating current which is set by R1/R2. Those resistors have to match the FET parameter spreads.
The larger the oprerating current the larger the zener voltage. So to which current you've set the current sources to?

Nuuk 22nd July 2012 04:38 PM

Quote:

Originally Posted by Juergen Knoop (Post 3100682)
hard to say...the zeners voltages depend on their operating current which is set by R1/R2. Those resistors have to match the FET parameter spreads.
The larger the oprerating current the larger the zener voltage. So to which current you've set the current sources to?

I've got between 7.4 and 7.6 mA to all for J309's (as Pedja recommends).

Juergen Knoop 22nd July 2012 05:30 PM

you could try 5mA at one diode as specified in the datasheet. Thus you would know!

Nuuk 22nd July 2012 05:32 PM

Quote:

Originally Posted by Juergen Knoop (Post 3100777)
you could try 5mA at one diode as specified in the datasheet. Thus you would know!

Thanks, I'll try it.

Nuuk 22nd July 2012 06:31 PM

5mA brings the voltage down about half a volt so it's still a bit on the high side. I've got to go out now but will lower it further when I get time.

Nuuk 22nd July 2012 06:41 PM

Alternatively, how about using some BZX85/27 diodes?

BZX85 pdf, BZX85 description, BZX85 datasheets, BZX85 view ::: ALLDATASHEET :::

Juergen Knoop 23rd July 2012 09:37 PM

Seems the zeners are indeed inside their specified parameter range. Using other types won't help you very much, since they have an app. ten percent range as well. Anyway those simple regs tend to sag under load, so they would never be 'DC accurate'.
Luckily the chipamp doesn't care about volt more or less!

Nuuk 23rd July 2012 09:47 PM

Well it's been fun (and educational) trying different zeners and changing R1/R2. I must look in the original modules that I built as I have ~25 volts on all the outputs with them.

I'll soon be comparing these Pedja Regs against the SMPS supply (both amps use LM3875's).

Nuuk 26th July 2012 01:35 PM

The BZX85/27 diodes arrived this morning, are fitted, and it all measures as it should so I would suggest that anybody building the Pedja regs uses those (in conjunction with 150R for resistors R1/R2. :)


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