Grounding Problem on AC and DC circuit

I think that I will change the 12Vdc step down part on replacing a small power transformer. Then the 12Vdc will be isolated. However, how can I connect the ground? Thanks~

I read in sci.electronics.design that Electronic Swear wrote (in ) about 'Grounding Problem on AC and DC circuit', on Mon,

28 Feb 2005:

You can't. The two are at very different potentials. You need to make a major change to the configuration, and there are several ways to do that. What can you change, if anything?

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Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

This may be a possibility. ________ O-----+--| |-----------------------+ 220vac | | diode | 311Vdc | | | bridge | MOTOR O-+--(---|________|-------AGND | | | D | | | ________ _______ / | |_| |_______|___, | G || | | BRIDGE |Const-I| | |-----||

I read in sci.electronics.design that Tony Williams wrote (in ) about 'Grounding Problem on AC and DC circuit', on Tue, 1 Mar 2005:

Doesn't the current generator need an improbable 311 V compliance voltage?

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Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

You could be right. Perhaps he should evade the problems wih a small isolating transformer.

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Tony Williams.

I read in sci.electronics.design that Electronic Swear wrote (in ) about 'Grounding Problem on AC and DC circuit', on Tue,

1 Mar 2005:

The negative of the DC output from the 220 V rectifier is not at 'ground' potential at all. You must connect the negative of your 12 V

**isolated** supply to it, so that the thyristor will work, but on no account connect the safety ground to it!!!

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Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

"Tony Williams" a écrit dans le message de news: snipped-for-privacy@ledelec.demon.co.uk...

It might be possible to connect GND and AGND together and the step-down/command bridge on the 311vdc high side.

I can't think of a strange internal wiring that'd prevent to do this but it's better to carefully check anyway.

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Thanks,
Fred.

Maybe you'll consider this a silly question, but why can't you just connect GND and AGND directly together in the original circuit?

Larty.

In article , Fred Bartoli

The low voltage bridge might be being fed via a capacitive dropper?

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Tony Williams.

I read in sci.electronics.design that Larty wrote (in ) about 'Grounding Problem on AC and DC circuit', on Wed, 2 Mar 2005:

Maybe if you draw the circuit out using circuit symbols you will see why. One ground is connected to the neutral supply and one to the negative output of the bridge rectifier. Look what happens if you connect them together; you short out one of the diodes in the bridge. BANG!!!!

--
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

I read in sci.electronics.design that Fred Bartoli wrote (in ) about 'Grounding Problem on AC and DC circuit', on Wed, 2 Mar 2005:

Doesn't that get you 320 V DC out of the 'low power' supply (assuming the bridge diodes and/or filter cap don't explode)?

--
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

"Tony Williams" a écrit dans le message de news: snipped-for-privacy@ledelec.demon.co.uk...

So connect it before the power bridge :-)

In fact, if the 2 supplies rectifiers are really full bridges, the only issue in connecting them, (AC together and GNDs together too) is that the power bridge may force too much current into the GND side low power bridge diodes. Since the low power bridge is just for a command module, i.e. will supply low currents, it might be possible to power it through 2 power resistors. They will of course create a small dissipation penalty, but will also allow to control the currents injected into the low power bridge by the power one.

I probably won't recommand this to somone not knowing very well what he's doing, though.

Oh, another idea: just replace the power resistors by 2 diodes, and the pb is solved.

+-+----------------- A A --------+-----+ | power -----+--|-----(-+ | | A A | | +-+-----------+----- | | | | | | V V | - - | | | +-+-----------(----- | | A A | | '-----+ | | low power '--------(-+ | A A | +-+-----------+----- (created by AACircuit v1.28 beta 10/06/04

Be sure to have the AGND and GND well connected: if they get disconnected, you've lost the low power supply.

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Thanks,
Fred.

"John Woodgate" a écrit dans le message de news: snipped-for-privacy@jmwa.demon.co.uk...

Sure. According to the OP, the low power supply is to be powered directly from mains and have a non isolated stepdown converter thereafter. I don't know why such an arragement, but that were the inputs.

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Thanks,
Fred.

I don't see either of the grounds (GND or AGND) in the originally posted circuit connecting directly to the neutral supply line.

I can see that there may be a problem if a dropper resistor is used in the live line prior to the bridge rectifier for the lower voltage supply. GND and AGND will then be at different voltages. If a small isolating transformer is used prior to the bridge rectifier for the low voltage supply then surely it should be ok to connect GND and AGND together?

Cheers, Larty.