Mains ground point and circuit ground point

[Johnson777717]

Hey folks. I need some advice if I may?

I have a transformer (120VAC 60Hz primary, 28VAC secondary) which has a ground wire which is fastened to the outter casing of the transformer. Would this be a sufficient ground point for a power supply circuit using this transformer? basically, I would like to know if it is proper for the ground to be attached to this ground wire on the transformer, rather than grounding to the chassis.

Are there any drawbacks to using a ground point on the transformer casing?

Should I just ground to the chassis and remove the ground wire from the transformer casing?

Also, I'm using a mains plug that contains a ground wire. Should I attach this ground wire to the chassis, or should I attach to the transformer ground wire?

Also, should I install a diode to keep current from flowing back through the ground wire? (1N4001? 1N4148?)

Please see sketch. I thank you for your assistance with this.

 

[Styx]

the GND lead on the casing of the XFMR is for safety if one of the windings get shorted to the case (thus making it live).

GND is GND so you can use the GND of the casing - it is common to star GND to stop earth loops - star all your GND points (only GND on secondary side)


NO diode !!!! this will circumvert the purpose of a GND lead.
I'm in UK and we have nice big chunky 3pin 13A plugs with a dedicated earth pin. If a earth leakage (due to a fault to earth) exists it will trip out the juntion box or blow a fuse.

I was under the impression that standard US sockets do not have an earth point (probably wrong).

 

[fat-tony]

I was under the impression that standard US sockets do not have an earth point (probably wrong).

That used to be true, but finding an ungrounded outlet these days is pretty rare. Still, I've seen a few really hideous adapters that take a 3-pin appliance, and plug into a 2-pin outlet.

 

[Johnson777717]

thanks for the replies folks! I appreciate your assistance. As you can tell, I've been studying AC to DC conversion and power supplies for the last few months, thus, I'm still a newbie at the sport.

Again, thanks for the advice.

 

[ante]

For stability in regulation, use negative output socket as a common ground (DC negative) point.

Ante :roll:

 

[kjennejohn]

Several years ago, a friend of mine was installing phone equipment in a new high rise office building. The chassis case got grounded to a metal member in the wall. As the AC power got plugged into a standard grounded outlet, the magic smoke was let out of the box and things got hectic. After unplugging, he started looking for the reason. Eventually he discovered a potential difference between the AC ground and the (supposedly) grounded building frame: 80V! More than enough voltage to toast an electrical circuit. I'm hazy at this point as to the fix, but I believe he installed some kind of isolation device, like a transformer.

I bet he wished he had a virtual ground that day!
kenjj

 

[Bob Scott]

I'm hazy at this point as to the fix, but I believe he installed some kind of isolation device, like a transformer.

I bet he wished he had a virtual ground that day!

Either the equipment being installed was faulty or the building had a serious wiring error. This is a safety hazard and it should have been troubleshooted and corrected.

 

[Dean Huster]

Normally, bolting the transformer to the chassis is sufficient for transformer case grounding and you don't need to worry about that extra ground wire -- cut if off. The extra wire is for those times when the transformer is mounted to a non-conductive surface -- then the ground MUST be connected to the mains ground.

The green (or green-with-yellow-stripe) wire in the power cord should always be connected to the chassis and consequently any metallic parts outside the enclosure which can be touched by the user.

Dean

 

[ericgibbs]

For stability in regulation, use negative output socket as a common ground (DC negative) point.

Ante :roll:

hi ante,
Are you suggesting using the mains neutral as an equipment grounding connection?

If yes, I would strongly advise that this should NOT be done.

 

[Hero999]

In some cases (such as control pannels) it's necessary to bond one side of the secondary to earth, this ensures that if the mains comes into contact with the transformer's output an RCD or fuse will trip rather than the 28V supply floating on top of the mains.