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Old 2nd February 2006, 07:39 AM   #21
AndrewT is online now AndrewT  Scotland
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Hi Onvinyl,
I cannot find a schematic in any of Eva's posts.
Are you working from a description Eva posted?
Or fig5 in Borbely?

Please post your breadboarded schematic, so we can try to debug.
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Old 2nd February 2006, 09:33 AM   #22
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Hi Andrew,
I'm working from eva's description.
Rüdiger
edit: circuit picture
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Old 2nd February 2006, 10:48 AM   #23
AndrewT is online now AndrewT  Scotland
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Hi On,
are you sure you have posted the correct schematic?

It seems all wrong to my inexperienced eye.
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Old 2nd February 2006, 10:53 AM   #24
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something is wrong for sure, else it would work
It's that what I understood from the description
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Old 2nd February 2006, 11:55 AM   #25
AndrewT is online now AndrewT  Scotland
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Hi Eva,
your description has me beat. The Pchannel in the +ve line has reverse control and similarly Nch in -ve line.

Onvynil,
the + &- halves of your schematic do not match.
+ve half collector to gate, -ve half base to gate.

If you have 24V on output then Vds is only 4V, so I think this may be right on the limit of operation of the FETs.

Consider +ve half control. If emitter is connected to 2*6.8V zeners then emitter voltage is -24+13.6=-10.4V. Base is at +24V, therefore Vbe=34.4V
A suitable zener would be 2 times 24V- 0.6V =47.4V.

But wait for Eva to reply because his description of the common source reg does not appear to work. If the toplogoy was changed to Source follower (swap N & Pch) then the control can be made to work.

I notice you want to use 6v8 Zeners. To get these into a good operating range it is normally recommended that they carry a minimum current of more than 10% of maximum. Using low voltage Zeners forces you to use very high current to meet this requirement.
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Old 2nd February 2006, 12:14 PM   #26
Eva is offline Eva  Spain
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I'm sorry, I think you didn't understood my description. These are two versions of the simple regulator that I proposed. Note that it's not a source follower at all, and that the drain terminal (the tab of TO-220 devices) is conveniently grounded to allow to use a metal case directly as a heatsink.

Click the image to open in full size.

The version on the left is the simplest one with only 5 components, but its minimum dropout voltage is determined by Vgs. The version on the right features a dropout voltage only limited by Rds-on at the expense of two additional components. The version on the left may require a zener for gate protection and some form of startup current limiting for the small transistor, while the version on the right has an inherent Vgs limitation to 17.6V (12V*1k/680) and the small transistor is protected.

Note that these are just simple circuits aimed at a low parts count and a lot of improvement may be obtained by just adding parts.
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Old 2nd February 2006, 12:20 PM   #27
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Quote:
Originally posted by AndrewT

Consider +ve half control. If emitter is connected to 2*6.8V zeners then emitter voltage is -24+13.6=-10.4V. Base is at +24V, therefore Vbe=34.4V
Argh, this strange arrangement makes me dizzy, I looked for Vbc instead of Vbe.


I notice you want to use 6v8 Zeners. To get these into a good operating range it is normally recommended that they carry a minimum current of more than 10% of maximum. Using low voltage Zeners forces you to use very high current to meet this requirement.
[/QUOTE]
Thanks for the hint.
Rüdiger
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Old 2nd February 2006, 12:24 PM   #28
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Have you considered an LM317T ? That will use less parts and be smaller and easier to wire up, and probably regulate better as well.
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Old 2nd February 2006, 12:25 PM   #29
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Quote:
Originally posted by Eva
I'm sorry, I think you didn't understood my description.
Obviously *loool*
Thanks for clearing,
Rüdiger
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Old 2nd February 2006, 12:29 PM   #30
Eva is offline Eva  Spain
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Quote:
Originally posted by richie00boy
Have you considered an LM317T ? That will use less parts and be smaller and easier to wire up, and probably regulate better as well.
It all depends on voltage and current requirements. LM317 is quiet and precise, but it has its limitations in comparison with some standard power MOSFET like a IRF540 when the requirements are just a high output current and a very low dropout voltage.

By the way, I've seen a lot of people with their brand-new transformers producing too high voltages for their amplifiers and asking for some simple high-efficiency anti-overvoltage pre-regulator. This circuit may be very useful for them.
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