Why does it matter which GND you use when checking voltage at the board?

funbobby

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Pole Position. 2 ARii's. When measuring voltage at the board, if I use a GND and a 5v test point on the board I'm getting 4.8v, but if I leave the positive lead on the board and move the GND lead to the ARii, it reads 5.1v. I understand that volts on the AR may be different than what's getting to the board, but since all of the GND's are shared, I don't understand why it matters where I put the negative lead when testing. I always thought of GND as either connected or not. Does this point towards a GND issue between the AR and the board? Or is my way of thinking about it way off? Thanks!
 
Pole Position. 2 ARii's. When measuring voltage at the board, if I use a GND and a 5v test point on the board I'm getting 4.8v, but if I leave the positive lead on the board and move the GND lead to the ARii, it reads 5.1v. I understand that volts on the AR may be different than what's getting to the board, but since all of the GND's are shared, I don't understand why it matters where I put the negative lead when testing. I always thought of GND as either connected or not. Does this point towards a GND issue between the AR and the board? Or is my way of thinking about it way off? Thanks!

There's resistance between the different test points, and PP sucks a lot of current.

V=IR
 
There's resistance between the different test points, and PP sucks a lot of current.

V=IR
So the TRUE measure of the voltage at the board is only achieved using + and - ON the board?
Years ago I did the 'bullet proofing' mods so I have additional pos and neg going to the board so it does not rely on the edge connector. Would it be a good idea to add a grounding block with ground tied together from multiple points?
 
So the TRUE measure of the voltage at the board is only achieved using + and - ON the board?

Yep.

Years ago I did the 'bullet proofing' mods so I have additional pos and neg going to the board so it does not rely on the edge connector. Would it be a good idea to add a grounding block with ground tied together from multiple points?

On pole position in particular, I always splice extra wires into the +5 and GNDs of the harness near the edge connector and add quick disconnects to go to the test points on the board to eliminate edge connector issues.
 
voltage is a concept. it is the difference between two different voltages and what you choose as a reference. If you had a 5 volt power test point and a seven volt test point you would read 2 volts between them and considering which way you measured you would have a positive or negative voltage. you consider the positive side to always be so many volts higher then the other test point. If you tie the positive of a 5 volt to gnd the negative part of the 5 volt supply is now 5 volts less then the positive that you have wired to gnd reference. These problems of resistance and different gnd voltage references are very apparent in gottlieb system 1 pinball machines in which all the daisy chained gnds starting at the power supply could lead to voltages differences in the gnd terminal at driver board of around 2 volts which could cause the unbiased transistors to lock on at start up which is why there were so many ground mods to the machines. They daisy chained them and did not use the large gnd plane everything was mounted too because it was designed by an aerospace company whose main focus was not to have rf interference cause problems is space because the large gnd plane would be an antenna and pick up rf intereference
 
This has already been said by others, but now in another persons' words.

Voltage is the potential difference between two points, providing there is a current path between those points. You cannot measure a voltage (potential difference) between two points that are isolated from each other. Most game pcbs use a common ground, but it is important to remember this point when servicing more complex circuits.

You need to consider that the ground wires have resistive losses just like the wires for the positive voltage. Measuring from a voltage source (power supply) as one point (ground) to the positive voltage at the end of a wiring harness or on the pcb for the other point bypasses the voltage drop through the resistance of the ground wiring.

To correctly measure the voltage on a game pcb you usually measure both voltage points on the pcb side of the edge connector (or plug) and again (measuring both voltage points) at the opposite end of the pcb from where the power enters. Repeat this on each pcb if there are multiple pcbs. Doing this will tell you the highest voltage (where it enters the pcb) and lowest voltage on each pcb. Then you can adjust for voltage within safe limits for the items being powered, being careful not to provide more voltage than is safe for the components being powered.

I typed more to explain the V=IR losses in relation to the wires and connectors but then removed it to keep things simple (for now).
 
This has already been said by others, but now in another persons' words.

Voltage is the potential difference between two points, providing there is a current path between those points. You cannot measure a voltage (potential difference) between two points that are isolated from each other. Most game pcbs use a common ground, but it is important to remember this point when servicing more complex circuits.

You need to consider that the ground wires have resistive losses just like the wires for the positive voltage. Measuring from a voltage source (power supply) as one point (ground) to the positive voltage at the end of a wiring harness or on the pcb for the other point bypasses the voltage drop through the resistance of the ground wiring.

To correctly measure the voltage on a game pcb you usually measure both voltage points on the pcb side of the edge connector (or plug) and again (measuring both voltage points) at the opposite end of the pcb from where the power enters. Repeat this on each pcb if there are multiple pcbs. Doing this will tell you the highest voltage (where it enters the pcb) and lowest voltage on each pcb. Then you can adjust for voltage within safe limits for the items being powered, being careful not to provide more voltage than is safe for the components being powered.

I typed more to explain the V=IR losses in relation to the wires and connectors but then removed it to keep things simple (for now).
Thanks for your reply! It is both clear and helpful.
 
Always use a ground test point on the board you're measuring the voltage on. Any one on the board should do, as there shouldn't be any practical difference, as long as it's on the same board.

That will tell you what voltage the chips on that board 'see', which is what ultimately matters.
 
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