Issue with an isolated PS

Thread Starter

ebolisa

Joined Jun 8, 2020
12
Hi,

I need to activate a 60W light bulb when touching a metal door. So, I designed a simple touch-switch ckt using a 555 chip. For the PS I used a HI-LINK AC to DC power module and for switching the AC line, a relay module as shown in the diagram below.

On my workbench all worked fine, but when I tested the project in the field by connecting the touch-wire to the metal door, the 555 chip burned out. At power on, the relay activates but doesn't turn off after the 30 programmed seconds. The door doesn't have any other wires going to it but it's bolded to ground.

I did notice, when I connected the touch-wire to the metal door, a tingling sensation on the door. Unfortunately, I didn't have a DVM with me to check the voltage on that door as I didn't expect problems but... if I had to guess the amount of AC voltage on door, I'd say between 15 to 30 VAC.

The question is, How could that be possible if the PS module lists as isolated?

TIACaptura.PNG
 

Irving

Joined Jan 30, 2016
1,069
Well, for a start your PSU as shown is reversed polarity... I'm assuming that's a typo and not how you've wired it in the field (or maybe you did, hence the 555 fried).

What constitutes 'ground' in the field?

Does your PSU and/or wiring have a protective earth (PE) connection you've not shown on the schematic? An isolated PSU means the output is galvanically isolated from L and N, but L & N can be a few volts different from the earth (PE) connection, and 'ground' as in a physical ground rod is not the same voltage as PE, especially if PE is grounded elsewhere. This is the source of the tingle - despite isolation of many megaohms. And that can be a few volts. If L & N are reversed that can be very different. Putting your trigger wire to the door frame could have injected several 10's of volts into the 555. Its a form of ESD.
 

Thread Starter

ebolisa

Joined Jun 8, 2020
12
Well, for a start your PSU as shown is reversed polarity...
It's a typo, thanks for pointing that out.

What constitutes 'ground' in the field?
Earth ground. The door's chasis is bolded to the building.

Does your PSU and/or wiring have a protective earth (PE) connection you've not shown on the schematic?
No. As I'm using a plastic box for the project and a fused line, I didn't think I'd need an Earth ground

Putting your trigger wire to the door frame could have injected several 10's of volts into the 555. Its a form of ESD.
So, a ceramic cap in line should had prevented that?
 

andrewmm

Joined Feb 25, 2011
663
15 volts will not be enough for you to feel through your fingers, your tung yes, but not a finger.
so you door is electrified to much more than 15 volts

You door is probably floating, the resistance from the door to real earth is probably many 100 of K ohm,
thus the ground you have could be any voltage, probably many many hundreds , and only get "grounded" via your touch.

Your door is also probably a very good antenna , picking up lots of voltage from the mains in the air.

Your touch wire, is thus floating, when you touch it, you probably have a static charge of many Kv on your body, which you have directly connected to the input of a chip. A perfect way to blow the chip.

I'd guess on the bench you were lucky , and did not have such a high charge on your body.

A good idea, Your detecting the charge on your body thats charging the capacitor and triggering the 555.
but as you have seen, its far from a reliable circuit.

This gives an idea on how capacitive touch sensors like this might work,

https://www.allaboutcircuits.com/technical-articles/introduction-to-capacitive-touch-sensing/

but even this misses how to do the ESD protection,
 

sagor

Joined Mar 10, 2019
283
Try the circuit with a battery powered +5V supply (with regulator). Do you still feel the "voltage"? If so, the door is either charging itself somehow, or your relay board has a flaw (or mounting issue) and feeding line voltage back to the circuit, If you feel no "voltage" on battery power, then it is most likely your power supply and how it has grounding, if any. Measure between the GND of the output and the AC line ground, look for any voltage. Try connecting the AC line ground to the power supply ground, if it has one. Re-check the wiring overall.

Good luck.
 

Thread Starter

ebolisa

Joined Jun 8, 2020
12
I'd guess on the bench you were lucky , and did not have such a high charge on your body.
This circuit is not new. It has been around almost since the invention of the 555 itself. What's new is my bad idea of connecting the trigger input to a large piece of metal, but even so, still not sure what happened.
 

Irving

Joined Jan 30, 2016
1,069
The issue is quite common. There is a capacitive coupling, small but real, of a low 100s of pF between the AC and DC side of the the PSU. If your circuit's 0v side was grounded to PE that's where it stops. Because its not, your whole device is floating up and down relative to 'ground'. The door frame is probably not grounded that well, but compared to your body's capacitive connection to ground (a few 10s of pF) at 50Hz its quite low. So when you and touch-wire contacted the door frame, it was now moving up and down relative to ground and there was sufficient leakage to generate a current through the 555 to ground and therefore a significant voltage across the ground 'resistor'; this is the tingle you felt and it was enough to puncture the gate insulation on the trig input of the 555.

If you didn't have that capacitive connection to ground your touch switch wouldn't work when you touch it.
 

Irving

Joined Jan 30, 2016
1,069
If you do ground your 0v to PE and then connect touch wire to doorframe I doubt the frame will become touch sensitive, i think the resistance to ground will reduce the sensitivity too much. But try it...

Normally the touchplates (like i have on my wall) are well insulated from their surroundings.
 

sparky 1

Joined Nov 3, 2018
384
Using a plastic insulator might result in a stable one shot signal.

If the reliability of the circuit is good and the relay is safely isolated
it could still malfunction if the conditions on an aluminum door change the adjustment.
Possibly a clean one shot signal can be made from contact arrangement from copper tape on double sided tape.
 
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