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Half wave rectifying and grounding

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Inducktion

Wed Apr 11 2012, 05:17PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Hey all;

It is my understanding that when you use a full wave rectifier, you cannot ground the circuit as it automatically shorts out on the negative part of the sine wave...

When you use a half wave rectifier, is this still an issue? I don't see how it would be, as it would be blocked by the diode and thus prevented from shorting out. The only thing I could see being an issue however is if the neutral and hot in the mains plug were switched...

I'm trying to figure out how to make 170 vDC for my nixie tube clock, by the way. I was thinking about doing a boost converter but I don't know how well that'll work out. It needs to be able to push around 20 mA at 170 vDC; running 4 nixie tubes at once for the hours and minutes. No seconds. :p

sngecko

Wed Apr 11 2012, 05:25PM

Registered Member #3447 Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97

You clearly can ground a full bridge rectifier following a isolating transformer. If you wanted to do this, you could use an isolation transformer to isolate the neutral/return from earth ground. I'm guessing that you're wanting to half-wave rectify mains directly, right?

I love that amber Nixie glow.

Ben Solon

Wed Apr 11 2012, 05:40PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

nope(sngecko). a scope(many of them) are internaly grounded. now, depending on how your house is wired, you can connect ac nuetral and groung together, as the only potential between the two wires is noise. altough because yhe scope is grounded, you should be able to avoid using the ground clip anyways.

i cant help you beyond here though, because i have only used the old analog type scopes a few times. i have a handheld digital. but you can only take off the neg halfe of a sine wave by hooking ground to ac hot, and this will take off of the positive half as well!

edit: i was refering to a full bridge rectifier, but the same principle effects a half wave rect.

Patrick

Wed Apr 11 2012, 08:07PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

ben123324 wrote ...

... as the only potential between the two wires is noise....

Careful here... there is a voltage drop proportional to the return current carried by (and across) the white (neutral) wire. The green and white wires are connected at one location in the panel/box for the house, it may be a small defference, but when using line AC, the details can be important.

Fires, arc flash, explosions, and liquid copper are all signs of an "Oops, i did that wrong!" kind of realization.

Isolating transformers are advisable where ever possible/practical.

sngecko

Thu Apr 12 2012, 01:41PM

Registered Member #3447 Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97

Ben-
I've built a linear 20kV, 200mA PSU (Mains=>Variac=>PT=>Bridge=>Capacitor) with an earth grounded negative pole on the bridge rectifier. No problems there. I must be misunderstanding what the OP and Ben meant.

Steve Conner

Thu Apr 12 2012, 03:21PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Well, if PT means power transformer, the transformer provides the isolation.

The OP is talking about an off-line power supply with no transformer: the kind of thing that (deservedly) died with the All-American Five table radio.

If (like the All-American Five) you insulate the guts of the clock so nobody can touch them, then you can happily rectify the mains any way you want. In the half-wave rectifier scenario, you'd "ground" your circuit to the neutral wire. Using the ground wire would trip GFIs, because the whole operating current of the circuit would be earth leakage.

sngecko

Thu Apr 12 2012, 04:02PM

Registered Member #3447 Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97

Thanks Steve! It all makes sense now. (Well, not quantum gravity, but everything else!)

By "PT", I meant potential transformer or instrument transformer (120:1, 1500VA), but it amounts to the same thing.

Forty

Thu Apr 12 2012, 07:16PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

So with some form of isolation transformer, the "-" output of the full bridge rectifier can be grounded to the earth/mains ground on the input side of the transformer?
and without isolation, it should not be grounded

Found myself recently faced with a similar problem after rectifying a step down transformer and wondering what to do with the "-" output, and whether or not it should be grounded, or if it was actually at negative potential relative to earth.

When working with that same transformer, I also discovered that with one multimeter lead in the earth ground wall socket, and the other in my hand, I measured about 22vac. Does that indicate 1.) bad wiring somewhere, 2.) bad multimeter, 3.) I have super powers, or 4.) nothing.

and for Mr. Duck
Nixie clocks are pretty sweet, but I've never had more than 1 or 2 tubes so I couldn't try making one.
Sounds like you don't need a whole lot of juice to run them, have you thought about just modding some sort of small smps by reversing or rewinding the transformer? even a cellphone charger might work.

Two small and ~identical mains transformers could be wired back to back with their secondaries connected to make an isolation transformer (I'm picturing those stepdown transformers found on microwave oven circuit boards.) might even be able to tap off in between them and rectify+smooth+regulate the low voltage to run your clock brains.

Haven't gone much into boost converter territory, so I can't say much about those.

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