LED indicator to say that ESD point is grounded/earthed

Thread Starter

seanspotatobusiness

Joined Sep 17, 2016
210
I want to set up a green LED to let me know that my ESD grounding point is connected to ground/earth. How can I do this? A continuity tester like the circuit below would make one earth connection positive and then connect a second earth conection to the base of a transistor. I was planning to use a PC PSU to power the detection (since it's already powering a number of other things) so this would mean connecting the 3.3 V or 5 V to one earth connection and the GND (through a 15 K resistor and the transistor) to another earth connection. Is that sensible?

To get my ground connections, I might use two connections to the copper pipe feeding a central heating radiator or maybe one such connection and a connection to an earthed mains electrical socket.

 

Thread Starter

seanspotatobusiness

Joined Sep 17, 2016
210
The positive voltage would travel through the two connections to the radiator or from one connection to the radiator and the other connection to the mains earth.
 

Thread Starter

seanspotatobusiness

Joined Sep 17, 2016
210
It would be a little like this. In the United Kingdom at least, the two probe ends would be shorted together via the radiator and the mains earth connection.

 

Wolframore

Joined Jan 21, 2019
2,610
Exactly so how does your transistor get positive voltage. Oh wait this is just the ground. That might work. Have you checked the resistance?
 
Last edited:

crutschow

Joined Mar 14, 2008
34,681
If you connected that circuit to ground, then any other device connected to that supply will also have the positive side of the supply going to ground.
Is that acceptable?
If not, then use a high-side switch as Wolframore suggested, (change NPN to PNP with the emitter going to plus and the LED, connected to the collector, going to ground)
The connect the minus supply terminal to ground.

Note that for either circuit, you need a resistor in series with the LED to limit the current to the desired value (the LED typically drops only 2-3 volts) otherwise you will likely zap it.
 

MaxHeadRoom

Joined Jul 18, 2013
28,761
Most PC. power supplies are referenced to earth ground.
How is circuit this going to affect the common use a RCD at the panel in UK?
Max.
 

Wolframore

Joined Jan 21, 2019
2,610
Lol I thought I saw a battery in his circuit. That’s the one. I would use a mosfet so there’s minimal current through house ground.

UK love positive grounds. I have an old MG with one. It’s so messed up people convert them to negative ground.
 

Thread Starter

seanspotatobusiness

Joined Sep 17, 2016
210
Below is the high side circuit schematic:

View attachment 175371
Thanks. I tried this circuit with an A1020 PNP transistor that I salvaged from some place and the green LED lights up even when the probe has a 10M resistor instead of the perscribed 10K resistor. Is that expected? Wouldn't it be better if a lower resistance was needed to get the LED to light? I know anti-ESD grounding devices already have a built-in 1M resistor, so maybe a high-resistance path to ground is actually acceptable?

Lol I thought I saw a battery in his circuit. That’s the one. I would use a mosfet so there’s minimal current through house ground.

UK love positive grounds. I have an old MG with one. It’s so messed up people convert them to negative ground.
Why does a MOSFET reduce the current through the house ground? Would using a higher base resistor do the same thing? My LED lights even using a 10M resistor on the base.

Kind of but way simpler, with just a single LED.
 

Wolframore

Joined Jan 21, 2019
2,610
Why does a MOSFET reduce the current through the house ground? Would using a higher base resistor do the same thing? My LED lights even using a 10M resistor on the base.
Can you post a picture of that setup? Something doesn’t make sense. The hfe for that transistor is about 250 at best.

A mosfet is a voltage controlled device as opposed to a current controlled device. It is the difference in potential between the gate voltage and source voltage that activates the channel on an enhancement mosfet. A logic level P channel will activate with as little as 2 or 3 voltage difference. Because the gate is insulated it acts more like a capacitor and the current is in the uA. In fact without a way to drain it the mosfet will stay activated for what seems like forever. They are very efficient and can pass a lot of current once activated.
 

Thread Starter

seanspotatobusiness

Joined Sep 17, 2016
210
Can you post a picture of that setup? Something doesn’t make sense. The hfe for that transistor is about 250 at best.

A mosfet is a voltage controlled device as opposed to a current controlled device. It is the difference in potential between the gate voltage and source voltage that activates the channel on an enhancement mosfet. A logic level P channel will activate with as little as 2 or 3 voltage difference. Because the gate is insulated it acts more like a capacitor and the current is in the uA. In fact without a way to drain it the mosfet will stay activated for what seems like forever. They are very efficient and can pass a lot of current once activated.
Thanks. This is what the circuit looks like. It's getting 3.3 V and is using the green cathode of an RGB LED. The white jumper cable is holding a 11M resistor to the negative power supply cable.




Then eliminate the transistor and just use the LED with resistor.
Would that mean more current flowing through the house ground? Does that matter?
 
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