Forums

Series Resistor and TVS diode placement

Started by mahen January 9, 2014
If I have to put ESD diode and current limiting series resistor to protect the IO pin of MCU from ESD and over current, which is the suggested way?

MCU Pin -------------- Series R -- TVS to GND -- Connector

or

MCU Pin -------------- TVS to GND -- Series R-- Connector

With the 2nd option, doesn't the R limit the surge into TVS needing only lower wattage ESD diode compared to 1st option?

-mjnk
On a sunny day (Thu, 9 Jan 2014 04:20:32 -0800 (PST)) it happened mahen
<mahen.jnk@gmail.com> wrote in
<619de8d9-3b4b-4a9b-b2d5-3dc3aa16dda2@googlegroups.com>:

>If I have to put ESD diode and current limiting series resistor to protect the IO pin of MCU from ESD and over current, which is >the suggested way? > >MCU Pin -------------- Series R -- TVS to GND -- Connector
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..
On Thursday, January 9, 2014 1:20:32 PM UTC+1, mahen wrote:
> If I have to put ESD diode and current limiting series resistor to protect the IO pin of MCU from ESD and over current, which is the suggested way? > > > > MCU Pin -------------- Series R -- TVS to GND -- Connector > > > > or > > > > MCU Pin -------------- TVS to GND -- Series R-- Connector > > > > With the 2nd option, doesn't the R limit the surge into TVS needing only lower wattage ESD diode compared to 1st option? > >
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
On Thu, 09 Jan 2014 05:28:55 -0700, Jan Panteltje  
<pNaonStpealmtje@yahoo.com> wrote:

>>> ...snip... > 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.. > >
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.
On Thu, 09 Jan 2014 05:39:20 -0700, Klaus Kragelund  
<klauskvik@hotmail.com> wrote:

>> ...snip.... > 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
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
<robert.a.macy@gmail.com> wrote in <op.w9fbahle2cx0wh@ajm>:

>On Thu, 09 Jan 2014 05:28:55 -0700, Jan Panteltje ><pNaonStpealmtje@yahoo.com> wrote: > >>>> ...snip... >> 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.. >> >> > >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.
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
<robert.a.macy@gmail.com> wrote in <op.w9fbg9mq2cx0wh@ajm>:

>On Thu, 09 Jan 2014 05:39:20 -0700, Klaus Kragelund ><klauskvik@hotmail.com> wrote: > >>> ...snip.... >> 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 > >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.
And burn out the micro and diodes with any DC.
On Thu, 09 Jan 2014 07:35:04 -0700, Jan Panteltje  
<pNaonStpealmtje@yahoo.com> wrote:

>> ..snip... > > 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.
On Thu, 09 Jan 2014 07:34:09 -0700, Jan Panteltje  
<pNaonStpealmtje@yahoo.com> wrote:

> ..snip... > 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.
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, 6 months out to one year, instead of lasting 10 years. 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
<robert.a.macy@gmail.com> wrote in <op.w9fdlrvg2cx0wh@ajm>:

>On Thu, 09 Jan 2014 07:35:04 -0700, Jan Panteltje ><pNaonStpealmtje@yahoo.com> wrote: > >>> ..snip... >> >> 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 ??
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.
>I usually use series R and RF Bead, to get a 'well-rounded' response.
[diode]C and L will resonate somewhere...