Series Resistor and TVS diode placement

On a sunny day (Thu, 9 Jan 2014 04:20:32 -0800 (PST)) it happened mahen wrote in :

This one, but perhaps add an extra R in case somebody connects it directy to a power source: MCU Pin --- Series R1 -- zener to gnd -- series R2 ---- Connector

I used zeners as it limits the voltage to both +zener voltage (use for example a 5.6V zener for 5V micro), and -0.7V. Or transzorbs.

It also depends on speed required, and power environment, 24V, 110V, 230V etc..

With the second option all your surge current is running in the series R and it would need to be either of very small resistor value or large power handling value to cope with the surge energy

This way:

MCU Pin ---- Series R ------External ESD diode ------ Series R -- TVS to GND -- Connector

TVS takes blunt of the surge pulse. Resistor limits current into series ESD diode. Second resistor limits current input MCU pin to avoid latchup (normally you need to stay below 1mA depending on the die technology)

Cheers

Klaus

Jim, be careful of using zeners as circuit protection. They turn on EXTREMELY slowly and during that time can let a lot through.

For grins, take two 5.6V zeners and put in series 'facing each other' as feedback on a high speed OpAmp. Then drive the thing and you won't get a square wave out, you'll get an extremely 'spikey' square wave out. We're talking audio frequencies, too. I've seen 20kHz spikes.

Do again with tranzorbs and you can really see the difference.

Or, you can do what we used to do, slightly turn the zener on ahead of time, that helps. but takes a fast switching diode into the turned on zener. Today, too many parts, then, the only way.

How about RF beads instead of R's, they're almost the same physical size. ?

Then you get the equivalent of over 1k to 10k ohm series resistance, but at the operating frequencies of the component, very low impedance, so you don't end up compromising waveform shape, too much.

On a sunny day (Thu, 09 Jan 2014 07:16:55 -0700) it happened RobertMacy wrote in :

I have used zeners in big data networks that way, no problem. Zeners have quite a bit of capacitance too.

If u want 100% protection use optocouplers.

On a sunny day (Thu, 09 Jan 2014 07:20:59 -0700) it happened RobertMacy wrote in :

And burn out the micro and diodes with any DC.

Right. is that before or after the esd diode burns up? or when the series resistor burns up, what are these 0402's 1/64 w ??

I usually use series R and RF Bead, to get a 'well-rounded' response.

What you can 'get away with' and what happens are two different things. For example, I don't use an anti-static wrist band, nor am very careful about handling components, no problem. Sure, no problem ...today! but probably shortened their little lives so they'll fail in anywhere from 2,

And, optocouplers? not quite the panacea one would expect. kind of work for AC mains isolation and some on voltage protection, except capacitive coupling high voltage can still 'punch' through. and worst of all magnetic pulses [even the pulses caused by HV discharge from HV protection], just go right around an optocoupler. EMP's can do a LOT of damage. All a manner of degree.

For total protection, I like to think in terms of analog filtering components. the uP has C and limit [low Z], therefore place a high-Z in series, then place a low-Z to GND, and then a high-Z out to the unknown origin. [this is a judgment call, because using a low-Z here can result in a spike of current injected into the GND plane and/or inject a current pulse into adjacent circuitry] And, be sure to design for at least 3GHz spectrum and you should be ok.

On a sunny day (Thu, 09 Jan 2014 08:06:53 -0700) it happened RobertMacy wrote in :

The idea is that the _zener_ shorts. For a short touch to for example a power line, chances are things will still work. Maybe you can use a fusible resistor, as I mentioned it all depends on the power environment 24V, 110V, 230V.

On a sunny day (Thu, 09 Jan 2014 08:18:56 -0700) it happened RobertMacy wrote in :

Tell it to the PLC guys who use optos by the thousands. I have used optos too, where spikes made other solutions impossible.

Come on, there are all sort of optocouplers, also for HV and low capacitance. But for 500$ I will say u are 100% right

sigh make that 600

:-)

NO!, not if done CORRECTLY. Also, RF Beads are NOT L, they're very lossy, very low Q inductors.

Again, DESIGN the solution. Don't 'throw' parts at the problem and wait until it "worked this time."

On a sunny day (Thu, 09 Jan 2014 08:51:12 -0700) it happened RobertMacy wrote in :

Thank you. Accolade indeed.

Jan, if you ever want some of the LTspice models and designs for 'pure' bypass filtering on uC chips done for n ADC mounted in and about Bluetooth, let me know. When I say pure, I mean the bypass looks resistive in AND out, all that happens is 'rerouting' the energy to GND. That way, you don't get that pesky ringing on the power terminals caused by the impedance of the bypass at some spectrum suddenly going up to 10, and even 100 ohms reactance! Had to abandon many of the manufacturer's recommended bypass components to do it, but gives BETTER performance.

On a sunny day (Thu, 09 Jan 2014 09:10:08 -0700) it happened RobertMacy wrote in :

Thank you, cool. Yes it sometimes needs to be looked at a bit closer, but some basic guidelines like optos works in 99,99% of the normal industrial cases, especially as one also has to cope with ground loops, and I can tell you that there can be a lot of volts ground difference in for example a factory, not to mention machines starting up increasing that momentarily etc. Optos are very safe,

I suspect that's the turn-on of the forward biased diode. Try the same thing with a couple of 2N4007s in inverse parallel, and you'll see spikes too. Zeners are usually fast.

Cheers

Phil Hobbs

Disagree. The resistor between the TVS and I/O pin is better for the reason stated (current limiting) and the capacitance of the TVS also helps (assuming the R and C don't cause functional problems).

So do unidirectional TVS diodes. They're a faster than zeners and will take a lot more abuse. Low voltage zeners are horrible.

Of course. The R gets problematic at high currents. ;-)

Thanks Jan. may be a dump Q. Series R2 b/w zener and connector already limiting the current into the pin, correct? Why series R1 again?

They use optos for galvanic isolation, to combat common mode noise, since they handle signals in industrial environments with a lot of ground bounce etc.

For DM noise, use the tranzorb, resistor, diode, resistor combo

Cheers

Klaus

On a sunny day (Thu, 9 Jan 2014 22:29:53 -0800 (PST)) it happened mahen wrote in :

If the zener limits to a bit[1] above supply, and bit[1] below ground, then R1 limits the current into the on chip[2] diodes in the chip[2]. [1] as in 'a little bit' [2] as in 'silicon chip', better 'integrated circuit'.

:-)