Common ground for Signal Generator and 12v battery

Thread Starter

Mark Flint

Joined Jun 11, 2017
75
I have some gaps in my understanding of grounding.
I am using a signal generator which uses a 5v switched power supply plugged into 240v mains supply. The sig-gen output is going to a mosfet gate driver. The gate driver can make use of 12A so I want to make this available from a 12v lead-acid battery.
I don't have a bench PSU with that kind of amps. Neither do I have a true sine wave inverter to power the system.
At the moment the sig-gen negative is on the gate driver ground. But if I put the positive and negative of the battery to the V+12 pin and ground of the gate driver this means the battery is now using the same ground as the mains... I think.
Is this safe for the equipment? I also have an oscilloscope grounded on the gate driver ground pin.
Thank you.
 

Reloadron

Joined Jan 15, 2015
4,576
Without seeing your exact circuit and part numbers I don't see why everything including the battery can't share the same common or ground but a schematic would go a long way in giving you a more accurate answer.

Ron
 

Thread Starter

Mark Flint

Joined Jun 11, 2017
75
Ron,Dennis,
Thanks for the response.
The 5v switching PSU for the sig-gen does not have an 3rd pin, earth - just 2 pins for live and "neutral" here in the UK. So I guess the ground clip for the sig-gen is to allow it to share whatever common ground is in the circuit.
The oscilloscope does have a ground and is powered by main 240v here. Not fully understanding ground connections I'm always wary of creating a "ground loop", or a short, and creating a lot of smoke (this has already happened to me a couple of times!). I think I need to educate myself properly on how to use my scope in regards to grounding. Any tips? I'm particularly focusing on measuring current across a shunt resistor. This might constitute a different question - but if I'm putting the probe ground clip and probe across a resistor am I grounding one side of the resistor? (I have a lot to learn)
 

KeithWalker

Joined Jul 10, 2017
451
Ron,Dennis,
Thanks for the response.
The 5v switching PSU for the sig-gen does not have an 3rd pin, earth - just 2 pins for live and "neutral" here in the UK. So I guess the ground clip for the sig-gen is to allow it to share whatever common ground is in the circuit.
The oscilloscope does have a ground and is powered by main 240v here. Not fully understanding ground connections I'm always wary of creating a "ground loop", or a short, and creating a lot of smoke (this has already happened to me a couple of times!). I think I need to educate myself properly on how to use my scope in regards to grounding. Any tips? I'm particularly focusing on measuring current across a shunt resistor. This might constitute a different question - but if I'm putting the probe ground clip and probe across a resistor am I grounding one side of the resistor? (I have a lot to learn)
If your oscilloscope power input is connected to ground, the frame of the scope and the shield of the input, or probe ground lead are all connected together. Anything you connect to the input common will also be connected to ground.
I recommend that you read the attached article. It will give you a much better understanding:
https://www.allaboutcircuits.com/technical-articles/an-introduction-to-ground/
 

Thread Starter

Mark Flint

Joined Jun 11, 2017
75
Thanks for the link KeithWalker. "Chassis" ground - in the case of an automobile or airplane is obviously not referenced to earth ground. Grounding is an interesting topic!

I saw a video which said that, as long as the circuit is isolated (such as run off an isolating transformer) then this removes the danger of ground loops. So I'm thinking that if I run my circuit off of my 30v 5A bench supply that uses an isolating transformer then I can put the ground clip of my scope probe and the probe tip across a shunt resistor without any worry of creating a cloud of smoke. Is this right?
 

KeithWalker

Joined Jul 10, 2017
451
Thanks for the link KeithWalker. "Chassis" ground - in the case of an automobile or airplane is obviously not referenced to earth ground. Grounding is an interesting topic!

I saw a video which said that, as long as the circuit is isolated (such as run off an isolating transformer) then this removes the danger of ground loops. So I'm thinking that if I run my circuit off of my 30v 5A bench supply that uses an isolating transformer then I can put the ground clip of my scope probe and the probe tip across a shunt resistor without any worry of creating a cloud of smoke. Is this right?
That is correct, but use extreme caution. The floating bench supply could build up a large electrical charge which could be dangerous to yourself and damage components. The safest way to display the voltage across an ungrounded resistor in-circuit is by using two channels of your scope in A minus B configuration. The scope input A and B probe tips are connected to the two ends of the resistor and the A and B probe common connectors are connected to the circuit ground.
 
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Thread Starter

Mark Flint

Joined Jun 11, 2017
75
The safest way to display the voltage across an ungrounded resistor in-circuit is by using two channels of your scope in A minus B configuration. The scope input A and B probe tips are connected to the two ends of the resistor and the A and B probe common connectors are connected to the circuit ground.
Yes this sound like a good idea - I'm still getting used to my analog scope... sadly it won't automatically display maths results.
 

KeithWalker

Joined Jul 10, 2017
451
Yes this sound like a good idea - I'm still getting used to my analog scope... sadly it won't automatically display maths results.
On the scope input select switch can you select A + B ? Each channel should have a reversing switch. If you reverse the B input, you will then have A - B displayed.
 
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