EEVblog Electronics Community Forum

Electronics => Beginners => Topic started by: Adam60 on February 21, 2016, 01:11:33 am

Title: Common Ground vs Earth Ground in Power Supply
Post by: Adam60 on February 21, 2016, 01:11:33 am
I have a quick question regarding voltage measurements on a power supply in a stereo receiver. If I look at the diagram, there is a pin 7 which goes to chassis ground, which I assume means Earth ground in this case. If I look at the rectifier, I would assume the point between Pin D1 and D3 to be the most negative point of the circuit.
So the question is, to arrive at the voltage values that they have given, do you use the negative side of the rectifier as a reference or do you use Pin 7.

On another note, lets say it is an SMPS. Same question although the unit is similar to a laptop adapter. Where do you reference for the COM lead of your DVOM.

http://www.kallhovde.com/pioneer/sx-1010sm.pdf (http://www.kallhovde.com/pioneer/sx-1010sm.pdf)

Page 57
Title: Re: Common Ground vs Earth Ground in Power Supply
Post by: IanB on February 21, 2016, 02:00:34 am
When voltages are given on a circuit diagram these are referenced to the common rail or "circuit ground". This is where you should put the black lead of your meter when making measurements.

Often the common rail will be connected to the chassis (but it doesn't need to be), and sometimes the common rail will also be connected to the earth safety ground (e.g. for noise rejection and signal quality), but it doesn't have to be.

Typically, anywhere on the schematic that shows the chassis ground symbol will be a suitable place to put the black meter lead.
Title: Re: Common Ground vs Earth Ground in Power Supply
Post by: Shock on February 21, 2016, 07:11:47 am
GRD (7) is connected to the negative side of a reservoir/smoothing capacitor C18 on the 13.4v rail (8.) So using that same technique see if you can deduce from the schematic if GRD is common to all those voltage rails. You can verify this using your mutimeter as well, keep in mind this circuit has high voltages.

Another way of looking at it is you should see 112v and 48v DC across the +/-56v and +/-24v rails, indicating they are tied by a common point (assuming that power supply is running and functional). Wondering if +56v and -24v have a common point? If they did take a guess at what voltage you would see between them.

For an SMPS on the output you can use the same technique to look if the negative side of the filter caps are common. When you don't have a schematic you can can "ohm out" (discharge and disconnect your circuit first) and trace that side of the circuit right to the DC plug or connectors. If you get sensible measurements between that and voltage rails you can deduce if it's common.

Some SMPS may need a load or be started before you can measure their output.
Title: Re: Common Ground vs Earth Ground in Power Supply
Post by: Adam60 on February 21, 2016, 11:41:53 pm
Thank you. Both answers seem sensible and I will approach circuits using these ideals. I know that voltage is a pressure difference netween two points, but I just wasn't sure what they would use for a reference point in an older unit like this.