How to kill a Teensy

[spencoid]

I just discovered new way to kill a Teensy. However they are so tough, the damage was only partial. So I have a question or two about the likely damage and whether I should even consider keeping the damaged Teensy for an emergency backup.

I have a Teensy 3.2 running my gin still. It measures temperatures flow rate, liquid level etc and controls the heater and cooling pump valves and fan. All works very well. I have a simple 4 line LCD (I2C) a bunch of one wire temp sensors and am using a touch input as a liquid level sensor.

I was doing a little improvement to the still and forgot to turn off the 220 volt supply to the heater SSR. I dropped a grounded guy cable and guess where it landed? Right on the (now protected) 220 volt line. Sparked and melted the cable. The still continued to operate and run the heater. This requires that all sensor inputs are valid. The problem is that the display regularly becomes a bunch of gibberish. I have code that refreshes the LCD whenever you change display screens. The code that is running in one mode never messes up the display but others do. Does it sound like the I2C lines were damaged or the flash memory itself?

I am not interested in figuring out what code causes the display to go bad. The solution was to replace the Teensy and it all is working as it did just before the accident. My questions are:

even though the ground was well connected to the utility ground, i am sure i got one hell of a spike on it. The teensy ground is not connected to the metal of the still nor to the utility ground. My guess is that the surge went through the touch input that is connected to ground via the boiler fill. So, is this bad way to sense liquid level? Otherwise I would need to make a bridge / comparitor circuit and isolate it optically. I know that is the right way but are there simpler options? Is there some sort of simple protection device I can use for the touch input such as a large resistor and maybe a MOV?

 

[BJB]

MOVs can be dangerous. They will oft explode while 'arresting' the surge and start a fire. Study annex Q of IEC/UL60950-1, IEEE C2-2012, and UL1449.

Product safety does not care about reliability. When large current and voltage surges enter equipment, the primary design objective is to prevent the unit from failing in an unsafe manner. Safe means not being a fire or shock hazard, and not emitting toxic materials.

And surges are coupled to equipment via rather interesting mechanisms; some of which are indicated per the tests defined in IEC61000-4-5. The surge could have been common mode, where 'ground' was not ground, but an equapotential reference for the surge.

 

[spencoid]

i don't think there was enough energy to explode the MOV. i have had MOVs explode so i know what it is like. one flamed out and started a fire but this was a huge surge (actually called a fault because it was caused by 60k touching the 12 kv line) another fault caused a whole house protector to explode. this was a commercial unit with the ability to dissipate many many joules. this is the type of thing used on factories not houses. again it wsa a 5 to one fault with pretty much unlimited current. it blew the steel door off the enclosure and hit my 1984 Mercedes 300TD and dented the metal. that old car was made of real metal.

so compared to that the energy in the submerged line through the liquid would be nothing. i doubt it would explode an MOV. however would a spark gap be better or should i just use a bridge to measure the resistance and control a comparitor that i then connect to an opto? i just remembered that i have a pile of soil humidity sensors that i bought for an arduino project. they have a resistance probe and a comparitor but not opto output.

also i do not plan to have another surge like this one and if i do, well, it is just a $15 teensy

 

[Donziboy2]

You probably did not get much voltage, just a few small surges from the AC if the Teensy did not have a ground loop return.

Normally when I want to protect pins I do a resistor in series with a pair of diodes(preferably Schottky).
Resistor should be capable of limiting current to something reasonable like 5-20mA. With AC mains the resistor wont last long.



Another approach would be to use an opto on the boiler fill, provided you power it separately. With an Opto you can separate the Teensy from a grounding source.

 

[dev009]

Teensy Not Recognized by Teensy Loader. Physically disconnect the Teensy. Reboot your computer (complete power off is best) Make sure the Teensy Loader application is running. Hold the reset button down, before plugging in the USB cable.
i hope it is work thank u

 

[spencoid]

Teensy Not Recognized by Teensy Loader. Physically disconnect the Teensy. Reboot your computer (complete power off is best) Make sure the Teensy Loader application is running. Hold the reset button down, before plugging in the USB cable.
i hope it is work thank u

no problem re-programming the teensy. it worked except that the display would go nuts frequently. must have fried the I2C lines.