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Old 6th November 2009, 03:49 AM   #1
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Default LM4702 Theory

Me again

Just wanted to test a theory. I know I will get some bashing, but it is just a theory.

AC voltage is 230v. After rectification it is 336v. If you halve that you get 163v.

Now if you accept that a LM4702B is safe up to 200v or 100-0-100 and look at the circuit below....then the theory schematic below with two LM's in series....will it work as the 'apparent' voltage across each of the LM's is a 84-0-84 supply

[IMG]Click the image to open in full size.[/IMG]

[IMG]Click the image to open in full size.[/IMG]

I realise that it will not be isolated from mains, but is it possible.

Wizard
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Old 6th November 2009, 06:55 AM   #2
Mooly is online now Mooly  United Kingdom
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Do you really have to ask...

And to elaborate, provided the opamps as you have drawn them (floating) current share equally (and have theoretically identical characteristics at power up) then yes they would "work" The instant you unbalance that in any way the voltage seen by each would alter and go outside the limits.

I don't think it needs discussing further really
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Old 6th November 2009, 07:21 AM   #3
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The other problem is with the signal ground, for the top chip it is at it's '-Vee' while for the bottom chip it is at its '+Vcc'. No joy, I'm afraid!

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Old 6th November 2009, 07:22 AM   #4
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Thanks Mooly

Makes sense I suppose, but what if two banks of capacitors were placed across the supply lines, but in the region of 50000uF per bank.

[IMG]Click the image to open in full size.[/IMG]
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Old 6th November 2009, 07:35 AM   #5
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The only answer to this is theoretical.
If the caps differed in value the charge times would be unequal at switch on and so the IC's would be destroyed. Tolerance of elecroylitics cap could be -/+ 20 to 50 %

And as Janneman correctly says mentioning inputs etc all this is before you even begin to think of interfacing them to anything.

You will never create anything useful from this.... just getting them to survive a switch on event would be remarkable.

If you want to see what happens build it up from 741's on a 30 volt supply and have a play and a measure.
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