Grounding Problem on AC and DC circuit

[Electronic Swear]

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?

 

[John Woodgate]

I read in sci.electronics.design that Electronic Swear
ing.google.com>) about 'Grounding Problem on AC and DC circuit', on Mon,
28 Feb 2005:

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?

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?

 

[John Woodgate]

I read in sci.electronics.design that Fred Bartoli <fred._canxxxel_this_
bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote (in <422622fd$0$31344$6
[email protected]>) about 'Grounding Problem on AC and DC circuit',

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)?

 

[Fred Bartoli]

I read in sci.electronics.design that Fred Bartoli <fred._canxxxel_this_
bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote (in <422622fd$0$31344$6
[email protected]>) about 'Grounding Problem on AC and DC circuit',


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)?

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.

 

[Larty]

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!!!!

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.