Static discharge

[picster]

I'm working on a project in which a hardwired input from the "real world" may be subject to a static discharge when it's originally connected to the PIC/circuit board. Has anyone worked through this in a safe manner? Even if I use and optoisolator, it would share the common power supply and thus could still cause a problem.

Any suggestions on buffering the input safely would be appreciated.

picster

[Gusse]

I'm working on a project in which a hardwired input from the "real world" may be subject to a static discharge when it's originally connected to the PIC/circuit board. Has anyone worked through this in a safe manner? Even if I use and optoisolator, it would share the common power supply and thus could still cause a problem.

Any suggestions on buffering the input safely would be appreciated.

picster

Hi Picster,

To protect HW, use some ESD protection components (varistor, TVS, etc..) and then add some small serial resistor (~20-40 ohm) in series in front of all inputs you need to protect. Use low Vbr ESD components, ~7-14V are OK. It will ensure that residual voltage will not burn PIC input.

To protect SW, use timeout(s) in input fuction(s) to filter out disturbances. If input still get triggered due to ESD then make sure that SW will accept only reasonable input strings.

If you have any interrupts, make sure that INT sources have filtering and SW can recover even if INT was due to ESD disturbances.

What kind of application you are building? And to which kind of environment?

BR,
-Gusse-

[Charles Linquis]

Since military electronics is my 'thing', I can help some here.

If you really want to protect your PIC:

Use only 4 or more layers in your PCB with one solid layer of GND, and one layer for Vcc.

Use an on-board regulator (either linear or simple switcher) and feed your board 12V or so.
Protect the input to the on-board power supply with a 20V (for example) MOV. Use a series 1Amp rectifier at the input to protect against reverse polarity. Use a 10uF 35V cap to GND between the rectifier and the regulator.

Put a 7V MOV across the PIC power supply.
Put a reverse-biased diode (1N4148) across the PIC supply.
Use a couple of 10uf tatalum and 1uF and .01uF multi-layer ceramics right next to the PIC for supply bypassing.

Put your Xtal very near the PIC. The same with the 22pf caps.

Keep all wires going to MCLR very short. Use a 4.7K pull-up to Vcc. Put a 100 pf cap from MCLR to GND (If you use more than that, your programmer may not work).

Use a Schmitt-trigger gate (74HC14 or equiv) on all inputs. Put a 2K resistor in series and .01uF cap to GND on the inputs of the gates (unless you need high speed).

Buffer all outputs with ULN2003's or 74C7007's or diode-clamped and current-limited 2N7000's.

I know you might not need all this, but doing things this way will guarantee operation in the harshest environments. I have pictures of circuits running at -73C and +105C, and taking indefinite numbers of 15KV (human body model) hits. We have not failed a military test yet, and we have been through lots and lots of them.

[picster]

Hi Picster,

To protect HW, use some ESD protection components (varistor, TVS, etc..) and then add some small serial resistor (~20-40 ohm) in series in front of all inputs you need to protect. Use low Vbr ESD components, ~7-14V are OK. It will ensure that residual voltage will not burn PIC input.

To protect SW, use timeout(s) in input fuction(s) to filter out disturbances. If input still get triggered due to ESD then make sure that SW will accept only reasonable input strings.

If you have any interrupts, make sure that INT sources have filtering and SW can recover even if INT was due to ESD disturbances.

What kind of application you are building? And to which kind of environment?

BR,
-Gusse-

The app is a monitoring device, to log switches (sensors) in a mechanical room environment. I figure a twisted pair should avert much of the problem, but when the circuit is first connected to the wire going to the field switches, there may be some static electricity discharged by the installer. It's battery operated, so I don't have to worry about power supply spikes generated by other equipment on the mains at least.

Picster

[Gusse]

Analog or digital sensors?

In any case, build a input filter (RC, LC, etc...) that have tuned bandwidth to match max. data rate/frequency. This with some SW filtering should be enough.

Twisted pair is cheap, easy and well working. Screened twisted cable is even better. Smaller the loop, the better. Small loops reduce common to normal mode conversion of external interferences (i.e ESD fields). And no piggy tails...

It might be a good idea to add ESD protection also to sensor end(s).

And some good luck is always needed even you might be a pro :p

BR,
-Gusse-