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?



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.

Put a 1K resistor across the Base /Emitter to help low resistance probes.

You can use a Continuity tester for this...

 

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.

I swapped the PNP transistor for a P-channel power MOSFET and the circuit still lights up with an 11M resistor on the gate.

Put a 1K resistor across the Base /Emitter to help low resistance probes.

You can use a Continuity tester for this...

A continuity testing circuit is what I started with in the OP. I don't want to use the continuity tester on my DMM if that's what you're suggesting. I want an always-on green LED, just because.

 

Have you tried just an LED and resistor, no transistor?

 

Would have been a better trick to short the pins under the breadboard

 

Have you tried just an LED and resistor, no transistor?

Yes, that seems to be better (not so over-sensitive) but is it sketchy in any way? Are there any caveats to doing this? I will be using the PC PSU to power some Rasberry Pis, a network switch, a label printer and maybe some other things.

 

Yes, that seems to be better (not so over-sensitive) but is it sketchy in any way? Are there any caveats to doing this? I will be using the PC PSU to power some Rasberry Pis, a network switch, a label printer and maybe some other things.

I see no problem with that small current.

 

It seems I am the unofficial safety Cassandra hereabouts. I will mention that your indicator shows something interesting but just to be clear, it only relates to the “safety” of your electronics, not yourself.

To test the safety of an ESD wristband, you must be sure the megohm resistor is intact and protecting you from being part of a Darwin Award special presentation should the live mains find its way to ground.

Always make a point of periodically checking the resistance from the strap to ground, and expect 1MΩ.

 

Thanks guys. My wrist strap has a built in 1M resistor and I am wary of doing anything with mains voltage (it's a rare thing for me), especially if grounded.

 

So does this light tell you if your ground strap is attached correctly? Meaning it's got good connection to the ground point and to your wrist? That would be cool. maybe a capacitive sensor? It could work.

 

So does this light tell you if your ground strap is attached correctly? Meaning it's got good connection to the ground point and to your wrist? That would be cool. maybe a capacitive sensor? It could work.

It is an interesting problem. I was wondering whether a second wire could be used to check for the appropriate resistance...

 

OK @Yaakov let's create this device... we can have @joeqsmith test it to bazillion KV and go into business together.

I always thought of the ground strap as protection for ESD sensitive components from my electrifying personality during winter wearing a sweater. I think I can still kill myself wearing one.

 

There are commercial continuous monitors, but I think they are only checking ground, not the resistance. You'd have to make a lead with a sense wire incorporated. The 1MΩ resistance is somewhere in the lead, if not something about the whole lead. But if you had a sense wire that ran to the wrist you could check the resistance from there to the ground.

 

ESD was a huge concern in the Marine Corps some of it having to do with the Navy providing us with taxi service, those of us doing electronics had to be aware of immense Static charges that occur on ships... They say it took us hundreds of years to learn to manage this safely. Imagine the static charge created by releasing the anchor for instance...

 

So does this light tell you if your ground strap is attached correctly? Meaning it's got good connection to the ground point and to your wrist? That would be cool. maybe a capacitive sensor? It could work.

No, it only tells me that the ground point is connected to ground.

ESD was a huge concern in the Marine Corps some of it having to do with the Navy providing us with taxi service, those of us doing electronics had to be aware of immense Static charges that occur on ships... They say it took us hundreds of years to learn to manage this safely. Imagine the static charge created by releasing the anchor for instance...

Wouldn't the sea water be conductive enough?

 

Wouldn't the sea water be conductive enough?

I believe it was in the reels and mechanisms not on the actual anchor and chain. I'm sure this is now mitigated. There were many other examples including the steam pistons and any cabling...