Hello,

I have posted a message on this forum about where to connect my earth strap when I'm building a project on my breadboard but I'm still a little
confused. Can somebody please give me a simple answer to this question: When I'm experimenting with circuits on my breadboard and all my circuits have a supply voltage less than 15 volts, can I connect my earth strap to the GND terminal on my PSU to prevent static ?

Thanks

PJ

 

If you feel it necessary it is permissible to connect the chassis of isolated equipment to earth ground, although be aware of any other point that may be referenced to GND.
If the supply is isolated, then it should be OK.

 

It is still not clear what is your "earth strap".

Perhaps some photos of your setup might help to clarify the situation.

All power supply sources (PSU) have two connections at the output, sometimes labeled +V and -V.
You connect these to your circuit in order to power the project.

PSU is either "floating" or "grounded".
If the PSU is floating there is no galvanic connection to any ground.
If the PSU is to be grounded, the -V output is connected to earth ground for positive output supply.
For negative output supply, the +V output is connected to earth ground.

A project built in a metal box would have the box labeled as Chassis Ground.
At some stage you would connect this to Earth Ground which technically is bonded to a metal stake driven into the earth.

If you are concerned with ESD protection, then the earth strap of a wrist band should be connected to Earth Ground.

Grounds are labeled with different symbols to indicate their function. They are not necessarily the same thing.

 

I picture of your PSU would probably help.



Here is an example of a typical PSU with floating output.

+ and - terminals have no reference to earth ground.

The terminal with the label GND is wired to the ground pin of the power cord. This in turn is wired to the ground of your AC mains supply connection. Never defeat this ground connection. For ESD protection, connect the ground strap to this GND post.

If it makes you happy you may insert a jumper between - post to the GND post.

If you plan on using an oscilloscope to test your circuit then you need to pay attention to how you make your connections.
I am sorry if this is more confusing. You need to show us your set up so that we can give a more specific and clear answer.

Edit:
Just to be clear, your ESD anti-static ground strap should be connected to earth ground.
Your power supply output should also be connected to earth ground.
In most cases when experimenting we want the power supply to be positive. Hence we connect the negative terminal of the PSU or battery to earth ground also. (There may be some special case where we do not want to do this. Hence it depends on the particular circuit that you are working on.)

 

Also Earth Ground is your service ground which is applied to all/most outlets in a residence.

 

Do you mean a wrist strap?
If so they should be connected to earth via a high value resistor. A resistor of 1MΩ should be included in the wrist strap.

 

Yes, if the place you connect your strap actually goes to earth.

I never wear wrist straps. I do believe that ESD can cause component damage that might not be obvious until long after the circuit is built and put into service. I live where the humidity is high year-around so charges bleed off quickly.

I regularly connect circuits under test to an earth ground associated with the electrical outlet near my bench. It keeps me from getting zapped by leakage current from switching power supplies (barefoot on tile floor over concrete).

As MaxHeadRoom has hinted, if you are careful you probably will not see any failures. If you do use a wrist strap, make sure that there is a large resistance between your wrist and earth so if you happen to touch high voltage with your other hand not enough current will go through your heart to cause death or damage.

 

If you are concerned with ESD protection, then the earth strap of a wrist band should be connected to Earth Ground.

I don't think I've ever been so confused about some electronics concepts than this.

Right, if the PSU is grounded with a +ve output voltage, I can connect the wrist band to the GND (green) terminal on the PSU because the GND terminal is connected to the earth ground via the power cord into the plug and out to to the stake driven into the earth. Yes ? Or do you mean that the strap should be physically attached to the stake itself and not via the PSU ?

 

As long as it is grounded it will be OK - via the power supply will do the job.

 

As long as it is grounded it will be OK - via the power supply will do the job.

If I connect the strap to the PSU while the PSU is switched off and wear it while I'm building a circuit, the components will be protected from ESD won't they ? When I want to make some measurements of voltage or current in a circuit I should still wear the strap shouldn't I ? and doing so will protect me and the circuit won't it ?

 

Correct.

Why don't you check the resistance of your connections with a DMM set to measure ohms?



The resistance from the button clip to the alligator clip should be about 1MΩ.

From the point where the alligator clip connects to your PSU GND to the rest of any ground point should be very low resistance, no higher than 1Ω.

 

If I connect the strap to the PSU while the PSU is switched off and wear it while I'm building a circuit, the components will be protected from ESD won't they ? When I want to make some measurements of voltage or current in a circuit I should still wear the strap shouldn't I ? and doing so will protect me and the circuit won't it ?

The power supply needs to be plugged in for that to work. On modern computer PSUs this means there will be voltages present inside the computer, so it's not such a good idea to work on in that case.

The PSU is grounded through the outlet's third pin, that's what your ESD wrist band needs to be connected. As has been mentioned several times it is critical the wristband have about 1MΩ of resistance to ground. Good commercial ESD bands will have them built in.

In labs where good safety practices are required, there is a dedicated device to test this, which is done periodically. But you don't need that, it's a convenience. Use your DMM to measure the resistance from the metal part that touches your wrist and the end that connects to ground it must be at least 1MΩ (one megohm). If it isn't do not use that wristband.

 

Correct.

Why don't you check the resistance of your connections with a DMM set to measure ohms?

View attachment 259890

The resistance from the button clip to the alligator clip should be about 1MΩ.

From the point where the alligator clip connects to your PSU GND to the rest of any ground point should be very low resistance, no higher than 1Ω.

You beat me but this much.

 

If I connect the strap to the PSU while the PSU is switched off and wear it while I'm building a circuit, the components will be protected from ESD won't they ?

As long as the work surface is somewhat conductive so it does not accumulate any static charge of its own.
What is your work surface material?

 

As long as the work surface is somewhat conductive so it does not accumulate any static charge of its own.
What is your work surface material?

It's a wooden table.

 

It's a wooden table.

That should likely be reasonably free of static.