Grounding Problem on AC and DC circuit

  • Thread starter Electronic Swear
  • Start date
E

Electronic Swear

Guest
I have a circuit on controlling the MOSFET to on and off in order to
control the motor operation. The motor input is 220Vac rectified DC
and there is a control circuit also using 220Vac rectified stepping
down to 12VDC. But there are no transformer and two grounding is
happened (one is motor grounding and another is 12VDC grounding).

The circuit is similiar as followed:

________
O-----+--| |----------------------+
220vac | | diode | 311Vdc |
| | bridge | MOTOR
O-+--(---|________|-------AGND |
| | D |
| | ________ _______ /
| |_| |______| | G ||
| | BRIDGE | 12Vdc|CONTROL|-----||<-
|_____| AND |__ | | || |
| STEP | | | | \|
| DOWN | | |_______| S |
|________| GND | |
| AGND
GND

There are two grounding in the circuit, but the step-down voltage is
not using transformer and hence no isolated. What can I do on
connecting the two grounding?
 
I read in sci.electronics.design that Tony Williams
<tonyw@ledelec.demon.co.uk> wrote (in <4d44cf028ftonyw@ledelec.demon.co.
uk>) about 'Grounding Problem on AC and DC circuit', on Tue, 1 Mar 2005:

If the Bridge+Stepdown generates a constant current
it can be locally regulated in the Control block
with a 12V zener diode, and the whole of the Control
block referenced to AGND.
Click to expand...
Doesn't the current generator need an improbable 311 V compliance
voltage?
--
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
 
In article <n0OcbAFKxHJCFwtA@jmwa.demon.co.uk>,
John Woodgate <jmw@jmwa.demon.contraspam.yuk> wrote:

Doesn't the current generator need an improbable 311 V compliance
voltage?
Click to expand...
You could be right. Perhaps he should evade the
problems wih a small isolating transformer.

--
Tony Williams.
 
"Tony Williams" <tonyw@ledelec.demon.co.uk> a écrit dans le message de
news:4d44e002f4tonyw@ledelec.demon.co.uk...
In article <n0OcbAFKxHJCFwtA@jmwa.demon.co.uk>,
John Woodgate <jmw@jmwa.demon.contraspam.yuk> wrote:

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

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

--
Tony Williams.
Click to expand...
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.


--
Thanks,
Fred.
 
I read in sci.electronics.design that Electronic Swear
<swear_terence@hotmail.com> wrote (in <910532f6.0503010921.6943c318@post
ing.google.com>) about 'Grounding Problem on AC and DC circuit', on Tue,
1 Mar 2005:
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?
Click to expand...
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!!!
--
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
 
In article <42249d1d$0$31353$636a15ce@news.free.fr>,
Fred Bartoli

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.
Click to expand...
The low voltage bridge might be being fed via a
capacitive dropper?

--
Tony Williams.
 
"Fred Bartoli" <fred._canxxxel_this_bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote in message news:<42249d1d$0$31353$636a15ce@news.free.fr>...
"Tony Williams" <tonyw@ledelec.demon.co.uk> a écrit dans le message de
news:4d44e002f4tonyw@ledelec.demon.co.uk...
In article <n0OcbAFKxHJCFwtA@jmwa.demon.co.uk>,
John Woodgate <jmw@jmwa.demon.contraspam.yuk> wrote:

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

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

--
Tony Williams.

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.
Click to expand...

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.
 
I read in sci.electronics.design that Fred Bartoli <fred._canxxxel_this_
bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote (in <422622fd$0$31344$6
36a15ce@news.free.fr>) about 'Grounding Problem on AC and DC circuit',
on Wed, 2 Mar 2005:
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 www.tech-chat.de)

Be sure to have the AGND and GND well connected: if they get
disconnected, you've lost the low power supply.
Click to expand...
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
 
"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> a écrit dans le message de
news:4R8LywFCiiJCFwdw@jmwa.demon.co.uk...
I read in sci.electronics.design that Fred Bartoli <fred._canxxxel_this_
bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote (in <422622fd$0$31344$6
36a15ce@news.free.fr>) about 'Grounding Problem on AC and DC circuit',
on Wed, 2 Mar 2005:
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 www.tech-chat.de)

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

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)?
Click to expand...
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.


--
Thanks,
Fred.
 
I read in sci.electronics.design that Larty <lartimus@hotmail.com> wrote
(in <65bc2010.0503020252.6228b39f@posting.google.com>) about 'Grounding
Problem on AC and DC circuit', on Wed, 2 Mar 2005:

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

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
 
In article <910532f6.0502282232.35e0a545@posting.google.com>,
Electronic Swear <swear_terence@hotmail.com> wrote:
________
O-----+--| |----------------------+
220vac | | diode | 311Vdc |
| | bridge | MOTOR
O-+--(---|________|-------AGND |
| | D |
| | ________ _______ /
| |_| |______| | G ||
| | BRIDGE | 12Vdc|CONTROL|-----||<-
|_____| AND |__ | | || |
| STEP | | | | \|
| DOWN | | |_______| S |
|________| GND | |
| AGND
GND
Click to expand...
This may be a possibility.
________
O-----+--| |-----------------------+
220vac | | diode | 311Vdc |
| | bridge | MOTOR
O-+--(---|________|-------AGND |
| | D |
| | ________ _______ /
| |_| |_______|___, | G ||
| | BRIDGE |Const-I| | |-----||<-
|_____| AND |__ | \_|_ | || |
| STEP | | 12V/_\ | \|
| DOWN | | |___|___| S |
|________| N/C | |
+------------+
AGND

If the Bridge+Stepdown generates a constant current
it can be locally regulated in the Control block
with a 12V zener diode, and the whole of the Control
block referenced to AGND.

The dc constant current will (eventually) return to
the Bridge+Stepdown via the high current diode bridge.

--
Tony Williams.
 
"Tony Williams" <tonyw@ledelec.demon.co.uk> a écrit dans le message de
news:4d45341d79tonyw@ledelec.demon.co.uk...
In article <42249d1d$0$31353$636a15ce@news.free.fr>,
Fred Bartoli

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.

The low voltage bridge might be being fed via a
capacitive dropper?
Click to expand...
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 www.tech-chat.de)

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


--
Thanks,
Fred.
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top