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Topic: 1/4" jack shorting issue (Read 671 times) previous topic - next topic

rrboyd

I have a concern about this attached circuit.

When inserting a 1/4" cable into the jack, the cable tip temporarily will hit the ground. Say the tip is a +15V, will this hurt the microcontroller ground?

Thanks for the help!




DVDdoug

Quote
Say the tip is a +15V, will this hurt the microcontroller ground?
No.    But, it could hurt the +15V source if it can be damaged by shorting.

rrboyd

Ok, thanks, Doug! I think it will be fine then. These sources are meant to be patched to other grounded jacks.

I got a little paranoid, because this tip shorting sent the +15V into another op amp unity gain buffer connection (picture not uploaded) through the ground ring from the jack when plugging in. It made it lock up. I have since put protecting diodes on it though and seems good now.

Thanks again!

MarkT

The 100k input resistor will protect almost anything from 15V adequately.

A unity gain buffer might not have any input resistor (and the opamp would need to have had
15V supply as well).
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

jackrae

As a kid, about 60 years ago, when learning about electronics was usually "suck it and see" I wired the house mains to a jack plug with the stupid idea of using it as a mains lead.  The flash when I plugged it into the device I'd built was quite impressive  :o .  Obviously one more lesson learned.

I know making such a confession makes me look dumb but if lessons are to be learned then it's essential to advise how not to do things (with reasons why) as well as how to do things.  Learning how to do a task doesn't necessarily teach you why you shouldn't do it some other way. 

rrboyd


I should post both circuits. Attached. I now have diodes and a 5K resistor.

The 15V will still temporarily touch directly to ground when plugging in though.


Thanks everyone for the comments!


jackrae

Assuming the 15volt supply/signal has zero impedance (impossible) then you could damage it.  Fit a limiting resistor into the 15volt line so that it will tolerate grounding during insertion.  Assuming that any instantaneous short will be maintained as a short will keep your design damage free.

rrboyd

Assuming the 15volt supply/signal has zero impedance (impossible) then you could damage it.  Fit a limiting resistor into the 15volt line so that it will tolerate grounding during insertion.  Assuming that any instantaneous short will be maintained as a short will keep your design damage free.
Hi, jack!  The signals will always have 1K resistors, so I think they will be ok.

My concern now is with my micro-controller circuits. Say the incoming signal could be anywhere from -15V to 15V. I understand the techniques on how to protect from these on the signal part (tip connection) of the jack. But what about the ground part of the jack when the tip accidentally touches it? I never see any circuits with components on the grounds of jacks. Wouldn't shorting the -15V to 15V signal directly to ground cause all sorts of havoc with my opamp, transistor, and micro-controller input pin operations?

Thanks for the help!

terryking228

#8
Dec 14, 2016, 02:50 am Last Edit: Dec 14, 2016, 02:51 am by terryking228
Hi,
The "Other Part Of The Story": Is "What is the other part of the circuit? " The 1/4" plug is connected to what?  Is it's 'sleeve' grounded to the same ground as the equipment with the jack?  Or is it 'floating'? 

These type 1/4" plugs (male) and jacks (female) can have 2 or 3 connections, labelled T-R-S  for TIP, RING, SLEEVE. (You seem to have the version with only TIP and SLEEVE (Sometimes called "Mono").

Regards, Terry King terry@yourduino.com  - Check great prices, devices and Arduino-related boards at http://YourDuino.com
HOW-TO: http://ArduinoInfo.Info

rrboyd

Yes the cord is mono with tip and sleeve only. The cord will always come from a circuit that has 1k ohm resistor at the tip. The grounds will not be connected though until the cord is plugged in.

Will my ADC readings always jump around when that tip accidentally touches the jacks ground?

Thanks!

Archibald

#10
Dec 14, 2016, 09:48 pm Last Edit: Dec 14, 2016, 09:54 pm by Archibald
With reference to the circuit diagram you attached to post #5 (here), why not connect the wiper of the potentiometer directly to Arduino input pin 1?  The voltage on the potentiometer wiper will not go outside the range 0V to 3.3V so what are the Schottky diodes for?

In post #7 you say the input signal could be between -15V and +15V.  The OPA4342 is prone to lock up if an input goes more than 0.3V below its negative supply.  So, with your possible input signal range, you need a Schottky diode to protect against that.

If the tip of your jack plug touching the ground of your operational amplifier circuit causes trouble, there is something seriously wrong with the wiring of your circuit.

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