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6' wire to PIC ADC - any precautions?

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superflux

New Member
I have 6' of 24 gauge wire that will be carrying my voltage signal to the PIC's ADC - should I use a 1uf capacitor on the ADC input pin close to the chip?

Thanks!
 

ericgibbs

Well-Known Member
Most Helpful Member
I have 6' of 24 gauge wire that will be carrying my voltage signal to the PIC's ADC - should I use a 1uf capacitor on the ADC input pin close to the chip?

Thanks!

hi,
I would add a 1K in series with the 6ft lead, close to the PIC's input pin.
I dont know what common/ground/0V line for the Vsource is from the remote [6ft end], but it would be advisable to use at least a twisted pair of wires.

Thats the Vsignal and PIC 0V.

If the 6ft wire is running close to other high current switching lines, I would consider a 5.1V or 5.6V zener from the PIC adc pin to 0V.
 

superflux

New Member
hi,
I would add a 1K in series with the 6ft lead, close to the PIC's input pin.
I dont know what common/ground/0V line for the Vsource is from the remote [6ft end], but it would be advisable to use at least a twisted pair of wires.

Thats the Vsignal and PIC 0V.

If the 6ft wire is running close to other high current switching lines, I would consider a 5.1V or 5.6V zener from the PIC adc pin to 0V.


I am using a 2k resistor, but it is closer to the beginning of the 6' line. I will move it closer to the PIC.

I have 3 24 gauge wires going to the PIC, ADC wire, Ground and a 5v supply wire - all have been twisted together and placed in a vinyl jacket.

Is the Zener some type of overload protection in this case or does it block some unwanted interference?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I am using a 2k resistor, but it is closer to the beginning of the 6' line. I will move it closer to the PIC.

I have 3 24 gauge wires going to the PIC, ADC wire, Ground and a 5v supply wire - all have been twisted together and placed in a vinyl jacket.

Is the Zener some type of overload protection in this case or does it block some unwanted interference?

It's over voltage protection, but PIC pins are already protected internally anyway.
 

ericgibbs

Well-Known Member
Most Helpful Member
I am using a 2k resistor, but it is closer to the beginning of the 6' line. I will move it closer to the PIC.

I have 3 24 gauge wires going to the PIC, ADC wire, Ground and a 5v supply wire - all have been twisted together and placed in a vinyl jacket.

Is the Zener some type of overload protection in this case or does it block some unwanted interference?

hi,
The PIC internal diodes are limited to about 20mA, I was concerned if the 6ft line picked up a low impedance source 'spike' the 5.1V zener would help to absorb the overvoltage current.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
hi,
The PIC internal diodes are limited to about 20mA, I was concerned if the 6ft line picked up a low impedance source 'spike' the 5.1V zener would help to absorb the overvoltage current.

With a 2K resistor (as he's using) you would need 25V continuous input to pass 20mA.

If you've ever fitted a PIC in the socket backwards, you'll know that the protection diodes happily pass a LOT more than 20mA without damage :p
 

superflux

New Member
So there's no reason to use a small cap on that ADC line right before the input pin? The reason I ask is that it's done on the supply voltage due to the distance between the Power Supply and PIC.

On a slightly different note, can you recommend a cap size that would reduce the flucuations that I am getting? The ADC readings are on a motorcycle from a voltage dividing circuit. Currently, I'm using a 100uf, but that was just a stab in the dark.
 

ericgibbs

Well-Known Member
Most Helpful Member
With a 2K resistor (as he's using) you would need 25V continuous input to pass 20mA.

If you've ever fitted a PIC in the socket backwards, you'll know that the protection diodes happily pass a LOT more than 20mA without damage :p

Problem is I'm not sure where the 6ft wire is routed, if its adjacent to HV switching circuits, it could pickup a lot more than 25V.

For the very small cost of a zener, I personally would fit it, again we dont know how critical it is that the PIC fails and the program stops.

Nigel, how could you ever possibly believe I would insert a PIC backwards...:rolleyes::D


EDIT: on the m/cycle which voltage are you dividing.??
Superflex, you posted while I was typing.
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
Problem is I'm not sure where the 6ft wire is routed, if its adjacent to HV switching circuits, it could pickup a lot more than 25V.

But not of a low source impedance, only a very high one.

For the very small cost of a zener, I personally would fit it, again we dont know how critical it is that the PIC fails and the program stops.

Superflex, what is the actual project.?

As always, it depends on exactly what it's doing.

Nigel, how could ever possibly believe I would insert a PIC backwards...:rolleyes::D

I've still got one with my fingerprint burnt in the top :D

And a finger with the MicroChip logo permanently displayed in it :p
 

ericgibbs

Well-Known Member
Most Helpful Member
hi Nigel, My thumb print says 74LS07...[backwards]:)

I said low impedance source, because I was concerned about the 'energy' in any possible spike, not a high impedance, very low current source.

Anyway it appears the OP's project is a m/cycle which could be rampant with electrical noise..;)

EDIT:
Superflux you may find this pdf helpful, I know its written for motor car system, but the principles are the same.
 

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