measuring voltage rails

Scribble · 18th October 2004, 09:38 AM

Quick question on voltage rails here.

I have a 500VA transformer with 25v secondaries, two bridges made from MUR860 diodes as per Peter Daniels schematic.

When measuring the rails i get a bit over 37volts but can i distinguish via a multimeter which rail is negative and which rail is positive?

Yes i do know which rail is which but im curious. Any suggestions?

Thanks for your time,
Joel

rpapps · 18th October 2004, 10:03 AM

Hi Scribble
I'm assuming your multimeter is digital.
Measure each rail.
If you get a minus sign ahead of the voltage reading then your probes are the wrong way around. Change them over and you'll have the polarity. If you don't get a minus sign, then they are connected correctly and you have the polarity.
If your meter is analog, it will deflect backwards if connected wrongly. Same process.
Hope this is clear.
Cheers
Rob

MWP · 18th October 2004, 11:28 AM

Thats a very confusing way to describe it Rob...

Try:

Leave your multimeter -ve (ground) probe connected to the center tap secondary of the transformer.

Then test the bridge outputs with the mulitmeters +ve probe.
If it shows as, being -37V, thats you -ve supply.
+ve supply will just show as "37V"

Scribble · 18th October 2004, 01:41 PM

MWP:

My transformer is a toroidal with twin secondaries so it has no center tap. What would i measure it from on this transformer?

troystg · 18th October 2004, 01:59 PM

Hi Scribble-

Pls post Peter's schematic you based it from..

Then I can mention SPECFIC test points to place the positive and grd test leads.

Scribble · 18th October 2004, 02:21 PM

Unfortunatly i no longer have the schematic on the computer but i will scan the one i have on paper tomorrow when i can access the scanner and post it on here.

Scribble · 19th October 2004, 07:12 AM

troystg · 19th October 2004, 07:53 PM

Hi Scribble-

The negative side of the top BR should be ground, and the positive side is + voltage.

The bottom BR is the opposite. The plus side of the BR is grnd and the negative side is - voltage.

Get me a picture of the amp and I can circle to test lead placement positions and repost.

Scribble · 20th October 2004, 12:03 AM

Thanks rabstg that was exactly my problem!

All fixed and working now

18th October 2004, 09:38 AM	#1
Scribble diyAudio Member Join Date: Dec 2002 Location: Western Australia	measuring voltage rails Quick question on voltage rails here. I have a 500VA transformer with 25v secondaries, two bridges made from MUR860 diodes as per Peter Daniels schematic. When measuring the rails i get a bit over 37volts but can i distinguish via a multimeter which rail is negative and which rail is positive? Yes i do know which rail is which but im curious. Any suggestions? Thanks for your time, Joel

18th October 2004, 01:59 PM	#5
troystg diyAudio Member Join Date: Apr 2004 Location: Lousy Anna	Hi Scribble- Pls post Peter's schematic you based it from.. Then I can mention SPECFIC test points to place the positive and grd test leads. __________________ Troy Thinking positive doesn't make things better, it makes you a better person.

19th October 2004, 07:53 PM	#8
troystg diyAudio Member Join Date: Apr 2004 Location: Lousy Anna	Hi Scribble- The negative side of the top BR should be ground, and the positive side is + voltage. The bottom BR is the opposite. The plus side of the BR is grnd and the negative side is - voltage. Get me a picture of the amp and I can circle to test lead placement positions and repost. __________________ Troy Thinking positive doesn't make things better, it makes you a better person.

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18th October 2004, 10:03 AM	#2
rpapps diyAudio Member Join Date: Jul 2004 Location: nowhere of interest	Hi Scribble I'm assuming your multimeter is digital. Measure each rail. If you get a minus sign ahead of the voltage reading then your probes are the wrong way around. Change them over and you'll have the polarity. If you don't get a minus sign, then they are connected correctly and you have the polarity. If your meter is analog, it will deflect backwards if connected wrongly. Same process. Hope this is clear. Cheers Rob

18th October 2004, 11:28 AM	#3
MWP diyAudio Member Join Date: Oct 2002 Location: Adelaide, South Australia	Thats a very confusing way to describe it Rob... Try: Leave your multimeter -ve (ground) probe connected to the center tap secondary of the transformer. Then test the bridge outputs with the mulitmeters +ve probe. If it shows as, being -37V, thats you -ve supply. +ve supply will just show as "37V"

18th October 2004, 01:41 PM	#4
Scribble diyAudio Member Join Date: Dec 2002 Location: Western Australia	MWP: My transformer is a toroidal with twin secondaries so it has no center tap. What would i measure it from on this transformer?

18th October 2004, 02:21 PM	#6
Scribble diyAudio Member Join Date: Dec 2002 Location: Western Australia	Unfortunatly i no longer have the schematic on the computer but i will scan the one i have on paper tomorrow when i can access the scanner and post it on here.

19th October 2004, 07:12 AM	#7
Scribble diyAudio Member Join Date: Dec 2002 Location: Western Australia

20th October 2004, 12:03 AM	#9
Scribble diyAudio Member Join Date: Dec 2002 Location: Western Australia	Thanks rabstg that was exactly my problem! All fixed and working now

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