[time-nuts] Clocking a PIC16F628A from a Rubidium Standard

Robert Atkinson robert8rpi at yahoo.co.uk
Tue Nov 29 02:44:22 EST 2011

I'm sort of sorry I mentioned the PIC application note....
While I think it's time for this thread to die I'd like to make a couple of comments abot it tha have not been mentioned.
1/ ANY components connected directly to the mains supply should be desigined, rated and approved for this appliction. This includes the resistors.
2/ Placing heatshrink over the fusible resistors is not a good idea. It can upset the thermal properties and also is the heatshrink fire proof or just fire resistant? It would invalidate  the creapage /clearance requirements (Mosture could possibly get underneath by capillary action)
3/ The original App note intended that ALL the line connected components (even via a resistor) should be in a permanantly closed or "tool access only" enclsoure, eithr double insulated or grounded.
While I don't think Tom's interface is an immediate danger, I would not recommend it. It certainly would not pass EU LVD or North American UL / CSA regulations (I''ve designed equipment to meet to all three regulations at once).
Note that a typical linline type Line driven laptop power supply can run quite a bit of AC leakage through the DC port due to EMC filter capacitors. On the 9V battery issue I've personally used a 9V battery driven datalogger in an opertaing theatre and they were happy with it's safety based on the 9V isolated operation (Yes it was connected  to the patient).
Robert G8RPI

From: David VanHorn <D.VanHorn at elec-solutions.com>
To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
Sent: Monday, 28 November 2011, 18:56
Subject: Re: [time-nuts] Clocking a PIC16F628A from a Rubidium Standard

> Let's not be obtuse on purpose, David.

No amount of dancing is going to turn this into a design that is safe by any reasonable definition of the word.
"Safe" in terms of "it hasn't killed anyone yet" is probably true.
"Safe" in terms of "you can trust this device not to cause a fire", no.

To quote from the page:
There are no safety worries about voltage, current or power, about frequency, shock or shorting. Worst-case current is only 60 µA so you can use thin digital wire to connect the plug to the microcontroller. One wire goes to Vss (signal ground) and the other to the microcontroller digital or analog input pin. If it doesn't work the first time, swap plug polarity. Or use capacitive coupling on both wires.

Of course, the whole idea of this AC plug is unnerving at first but when you think about it, it makes sense. Better yet, just make one and test it for yourself. You should observe it tickles far less than a 9V battery (which I measured to be about 200 µA on a wet tongue). 

Does that last line actually suggest checking this contraption BY TOUCHING IT TO YOUR TONGUE???

"No safety worries about voltage...or shorting" ??  Resistors can and do fail shorted. They can also be damaged by line voltage transients and end up at some lower value.

What version of "safe" applies here?
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