Chassis Grounding and rectifiers

Captn Dave · 9th January 2009, 05:33 PM

I think it's a good idea to ground a chassis with a 3 prong plug for safety reasons. I'm using a bridge rectifier and a regulator on the current build. It just occured to me that I might have a problem in the making.

If I solder the B- to the chassis and then go to ground with a 3 prong plug, have I not shorted out the bridge diode between the AC neutral and the B-, given that the AC neutral and ground are bonded at the circuit box?

Is that question clear? Of the four diodes in the diamond shaped bridge rectifier circuit that we are all familiar with, one of the four stands between the AC neutral and the B-. That's the one I suspect I'd be shorting (through the home wiring) by using the 3 pronged plug.

Is this correct and what are the consequences?

Thanks

Svein_B · 9th January 2009, 05:49 PM

Not to reason to worry.

Provided you are using a normal power transformer, the secondary windings are completely isolated from the primary and from ground, ie. floating.

There will thus be no shorting in your amp, nor in any of the other millions in operation with this arrangement.

SveinB.

Captn Dave · 9th January 2009, 06:08 PM

Oh yeah, I forgot to mention that part. No transformer planned; 6GM8s. I'll need an isolation transformer unless I completely isolate the B-? Doesn't sound very safe without one, does it?

Jeb-D. · 9th January 2009, 06:12 PM

You must use a transformer for isolation. Your concern sounds as if you intend to directly rectify the mains which is unsafe. Earth ground is used as a voltage reference. Current will flow from and back-to the isolation transformer. Earth ground will just pull the circuit down to ground potential so that it is not floating.

Neutral and earth ground are tied together, but before the breaker. If current flows in the earth ground, the breaker will trip.

Svein_B · 9th January 2009, 06:15 PM

NOT SAFE!!!

I suggest some pyrotechnics or firecrackers to make some sparks instead of connecting to electrical supply. Much safer.

Svein.

SY · 9th January 2009, 06:18 PM

DEADLY DEADLY DANGEROUS!!!!!!!

Tom Bavis · 9th January 2009, 08:42 PM

Aside from the EXTREME hazard, using a bridge rectifier directly from mains is just not practical, as both + and - sides of the bridge will be LIVE with respect to ground. Use an isolation transformer. If current is small, use a second 6.3V transformer in reverse from your 6.3V heater winding to give (isolated) mains voltage.

Captn Dave · 10th January 2009, 05:46 PM

OK, thanks for the remarks. Consider me chastened.

Tom, clever idea on the filament transformers. I like that.

9th January 2009, 05:33 PM	#1
Captn Dave diyAudio Member Join Date: Jun 2008	Chassis Grounding and rectifiers I think it's a good idea to ground a chassis with a 3 prong plug for safety reasons. I'm using a bridge rectifier and a regulator on the current build. It just occured to me that I might have a problem in the making. If I solder the B- to the chassis and then go to ground with a 3 prong plug, have I not shorted out the bridge diode between the AC neutral and the B-, given that the AC neutral and ground are bonded at the circuit box? Is that question clear? Of the four diodes in the diamond shaped bridge rectifier circuit that we are all familiar with, one of the four stands between the AC neutral and the B-. That's the one I suspect I'd be shorting (through the home wiring) by using the 3 pronged plug. Is this correct and what are the consequences? Thanks

9th January 2009, 06:18 PM	#6
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	DEADLY DEADLY DANGEROUS!!!!!!! __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

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9th January 2009, 05:49 PM	#2
Svein_B diyAudio Member Join Date: Oct 2005	Not to reason to worry. Provided you are using a normal power transformer, the secondary windings are completely isolated from the primary and from ground, ie. floating. There will thus be no shorting in your amp, nor in any of the other millions in operation with this arrangement. SveinB.

9th January 2009, 06:08 PM	#3
Captn Dave diyAudio Member Join Date: Jun 2008	Oh yeah, I forgot to mention that part. No transformer planned; 6GM8s. I'll need an isolation transformer unless I completely isolate the B-? Doesn't sound very safe without one, does it?

9th January 2009, 06:12 PM	#4
Jeb-D. diyAudio Member Join Date: Nov 2005 Location: SoCal	You must use a transformer for isolation. Your concern sounds as if you intend to directly rectify the mains which is unsafe. Earth ground is used as a voltage reference. Current will flow from and back-to the isolation transformer. Earth ground will just pull the circuit down to ground potential so that it is not floating. Neutral and earth ground are tied together, but before the breaker. If current flows in the earth ground, the breaker will trip.

9th January 2009, 06:15 PM	#5
Svein_B diyAudio Member Join Date: Oct 2005	NOT SAFE!!! I suggest some pyrotechnics or firecrackers to make some sparks instead of connecting to electrical supply. Much safer. Svein.

9th January 2009, 08:42 PM	#7
Tom Bavis diyAudio Member Join Date: Jun 2002 Location: Macedon NY	Aside from the EXTREME hazard, using a bridge rectifier directly from mains is just not practical, as both + and - sides of the bridge will be LIVE with respect to ground. Use an isolation transformer. If current is small, use a second 6.3V transformer in reverse from your 6.3V heater winding to give (isolated) mains voltage.

10th January 2009, 05:46 PM	#8
Captn Dave diyAudio Member Join Date: Jun 2008	OK, thanks for the remarks. Consider me chastened. Tom, clever idea on the filament transformers. I like that.

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