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'[PIC] revising analog input protection, 18F4680'
I'm reviewing an ugly ADC input circuit which so far has worked very adequately. An op amp's output signal (nom. 0-5v, but could rise to 12v in fault condition) is clamped for safety by a 680R/5V1 zener diode combination. So the voltage into the PIC ADC pin is externally limited to 5v1 and the current from the opamp is externally limited to 10mA if the op amp ever hits 12v.
Possibly due to some 5v1 zener diodes going obsolete and being replaced by a close alternative, I find that some PIC ADC inputs are being dragged down by the presence of the zener diodes, as the incoming voltage approaches 5v. This seems to suggest that the current batch of diodes are turning on earlier than those previously specified.
Removing the zeners seems to make the problem go away. I'm then left with the worry that merely limiting the over-voltage ADC input current to 10mA may be risky, even though the ADC inputs are very unlikely to rise above 5v during the service life of the equipment.
a) is just a current limiting resistor of 680R (for 10mA) sufficient due diligence, for protecting the PIC ADC input clamping diodes if the source voltage rises to 12v?
b) is relying on limiting the internal protection diode current, asking for SCR latchup?
An extremely good alternative to zener diodes is a clamp regulator
such as a TL431(which I tend to recommend to all and sundry). The
lower voltage TLV431 has some advantages.
This has a much sharper "knee" than a zener and can be obtained in
0.5%, 1% & 2% accuracy versions. "Programming" requires 2 resistors.
If you are committed to an existing PCB then a small add in PCB could
be made to insert where the zener no longer is.
My constant refrain is that any overvoltage risks unexpected PIC
operation - but I'll admit that a genuine 5.2V and maybe 5.3V on a
genuine 5V supply is statistically not too too likely to cause
problems. A TL431 or TLV431 would allow you to clamp very close to 5V.
680r alone probably meets absolute max ratings (see relevant data
sheet) BUT certainly violates "normal operating conditions". You can
be utterly certain that some PICs will perform bizarrely if you inject
10 mA (or even 1 mA ) via protection diodes. As long as data sheet
guarantees non destruction and as long as your system does not mind
processor going insance when Vin > ~= 5V then no zener at all is an OK
A small Shottky diode from Vin to Vcc = 5V clamps wellish but Shottky
leakage may be annoying if you have very high temperatures. The
Schottky drop also notionally exceed sthe limits that I preach about
but often does OK in practice.
Very good is a small Schottky from the middle of a split Rin to Vcc so
you clamp to about 5.3V or less with a largish resistor between V and
pin. Also violates spec but statistically better.
On 6 September 2011 21:07, Matt Rhys-Roberts <nu-ins.com> wrote: matt
I had a similar issue some time back ... 15V source/environment, voltage divider to drop that to <5V at A/D input, 5.1v zener for protection. I too found that the 5.1V zener was altering the signal as it went higher, and it was inconsistent from one diode to another.
Up to then I was using a 4.096V reference to the A/D, so I changed that to 2.5V, and altered the voltage divider to divide by 8 (7k and 1k), and removed the zener altogether. With this, I get good resolution still, and the input can safely go to 40V.
Quoting Matt Rhys-Roberts <nu-ins.com>:matt
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Attached is a circuit that I have used many times. This one is for eight
inputs and protects against positive swings. It will work for +5 or
lower, just attach the diode anode to the same supply as the micro.
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On 9/6/2011 9:38 AM, Bob Blick wrote:
> Hi Matt,
> Attached is a circuit that I have used many times. This one is for eight
> inputs and protects against positive swings. It will work for +5 or
> lower, just attach the diode anode to the same supply as the micro.
> Cheerful regards,
I like your idea, Bob.
It might be slightly improved by using 9 transistors instead of 8 plus the diode in order to better match the Vbe. And if possible, a transistor array would probably have very close Vbe matching. None of this usually matters though
Forrest W Christian
I'm using a derivitative of bob's circuit to produce a rail to clamp to.
That is, two forward diodes and a transistor arranged as in bob's schematic to produce vclamp. The second diode is to compensate for the additional drop... I use BAT54S'es as clamps, so I just use a single 54S ignoring the center pin to get the two diodes....
Then a whole bunch of bat 54S'es used as normal clamps to my vclamp rail and ground.
On 9/6/2011 2:09 PM, Marcel Duchamp wrote:
> this usually matters though.
On Tue, Sep 6, 2011 at 2:21 PM, Forrest W Christian <imach.com> wrote: forrestc
> I'm using a derivitative of bob's circuit to produce a rail to clamp to.
> That is, two forward diodes and a transistor arranged as in bob's
> schematic to produce vclamp. The second diode is to compensate for the
> additional drop... I use BAT54S'es as clamps, so I just use a single
> 54S ignoring the center pin to get the two diodes....
You compensate a PN-junction drop with a Shottky diode?
-- Mark Rages, Engineer
Midwest Telecine LLC
Thanks for everyone's contributions to my original post. The long term solution will probably be a re-scaling of the analog signals from 0-5v to 0-4.096v, with a 4.096v precision reference on the ADC, and all signal sources coming from 5v op amps.
Meanwhile, limiting the clamp current to the barest minimum without disrupting the in-range signal (i.e. using a 2k5 resistor) does not seem to have any strange or unwanted effects during normal operation.
On Tue, Sep 6, 2011 at 5:32 AM, RussellMc <gmail.com> wrote: apptechnz
> An extremely good alternative to zener diodes is a clamp regulator
> such as a TL431(which I tend to recommend to all and sundry). The
> lower voltage TLV431 has some advantages.
> This has a much sharper "knee" than a zener and can be obtained in
> 0.5%, 1% & 2% accuracy versions. "Programming" requires 2 resistors.
> If you are committed to an existing PCB then a small add in PCB could
> be made to insert where the zener no longer is.
I really like the TL431. It's dirt cheap.
The TL431 datasheet says that it needs a minimum of 1mA cathode
current. This might mess up A/D accuracy if using a higher value
The TLV431 is more acceptable, at ~55uA cathode current. These values
probably vary slightly depending on manufacturer.
-- Martin K
> I really like the TL431. It's dirt cheap.
> The TL431 datasheet says that it needs a minimum of 1mA cathode
> current. This might mess up A/D accuracy if using a higher value
> series resistor.
> The TLV431 is more acceptable, at ~55uA cathode current. These values
> probably vary slightly depending on manufacturer.
Yes. Lower current at regulation dropout is one of the advantages I referred to.
The "required" currents are what's needed to get it around the "knee"
and into proper regulation.
If it's drawing less than that it MAY be in regulation if at about the
knee voltage BUT if it's not in regulation it will be drawing less
50 uA at 5V is the equivalent of a 100k load. Current falls semi
linearly with volateg for several volts below that so looks roughly
like 100k load from say around 2 V up.
You can get a much sharper knee by adding one extra transistor
(bipolar or FET) and using the TL431 to operate that. Only load then
below clamp voltage is the sense divider.
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