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Power transient filter to PIC Micro Board

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UroBoros

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Hai,

Sorry if this is a subject already have discussion, (Good chance for that) but I was not able to find answer for my issue with search here.
For my Pic Micro circuits I usually Power from small SMPS and in some cases transformer and bridge. Usually I bring 12v to Board, provide a filter of arounf 1000Uf and Then a 7805 with .1uf at both ends. then close to PIC Power pin I provide 220uF and .1Uf In parallel. and in MCLR I provide 4k7 pull up and .1uF to Gnd. This usually works without much problem.
Now I have to do a PIC Board which will be placed in the proximity of High current motors and brushed grinders etc which can cause noise and spikes in power lines. So I think I need a more robust power supply transient protection and filtering in the power lines. Like to know what will be the best approach in this case.
What comes to my mind is a pi filter with a inductor. I searched and found circuits and and formulas for cut off frequency etc. But what should be the cut off frequency and what should be the core for inductor. Since usually inductor value seems high, then ferrite core may be needed.

Can I get some info without too much calculations but with required basic info to tailor for my needs. My PIC circuits may maximum take below 500mA.
Thanks
 

What you have already should be sufficient but I would be a little worried about the capacitor on the MCLR pin. The value is low enough not to do permanent damage but in general it's better to either add another resistor directly in series with the MCLR pin or to leave the capacitor out and use the start-up timer instead. The reason is the capacitor can retain charge after VDD has dropped and apply a reverse polarity at the MCLR input.

If you do want additional filtering, I would be inclined to add an inductor at the 12V input with the existing circuits after it.

You are far more likely to suffer from inductive/capacitive coupling on the data lines but the best way to filter those depends on your application. When I've designed in extreme noisy environments I've added 5.1V Zener diodes across the PIC inputs and 100 Ohm resistors in series with the signal feeding it. On slow signals I've also added small (100pF) capacitors to VSS to act as HF filters. Obviously they have some impact on the signal so that method may not be appropriate in your case. Make sure you have a good ground in your circuit, a bad layout can make a small noise problem into a major headache!

Brian.
 
Thank you very much for the detailed Replay.
but in general it's better to either add another resistor directly in series with the MCLR pin or to leave the capacitor out and use the start-up timer instead. The reason is the capacitor can retain charge after VDD has dropped and apply a reverse polarity at the MCLR input.
I have seen such an arrangement in Microchip Notes but I was thinking that, it is needed only if we use high value caps in MCLR. If it is a standard procedure (Now I understood the purpose of that resistor)I will add it since it is not a big issue to add such a thing to make all safe to the possible extend.
But regarding the start up timer I have a doubt. My understanding is startup timer gives a delay in PIC starting for few millisecond even if there is no rising ramp in MCLR pin because of R and C. But my understanding was the capacitor has another use of bypassing reset causing pulses that may appear in MCLR from Power lines. (correct me if I am wrong). So avoiding the capacitor will make PIC more susceptible to unwanted reset.?

If you do want additional filtering, I would be inclined to add an inductor at the 12V input with the existing circuits after it.
what is the optimum value and preferred core? Ready made conductors with current capacity are not easily available for me. better to make one if possible.

You are far more likely to suffer from inductive/capacitive coupling on the data lines but the best way to filter those depends on your application. When I've designed in extreme noisy environments I've added 5.1V Zener diodes across the PIC inputs and 100 Ohm resistors in series with the signal feeding it. On slow signals I've also added small (100pF) capacitors to VSS to act as HF filters. Obviously they have some impact on the signal so that method may not be appropriate in your case. Make sure you have a good ground in your circuit, a bad layout can make a small noise problem into a major headache!
Brian.
This is a very useful information for me. My inputs can manage that lag. I was also thinking about a method to protect I/Os . But now only I got a plausible filter cum clamping combination. Got the point. 100Ohm and 100pf wont cause a problem in most of my input needs. (actually I used to assert the pin status by looping through a pin check loop, a suitable number of times - the software way, but adding this can make it more sturdy)
Thanks again
 

The above RC component values used in the MCLR generate a reset delay of less than 1ms. I tend to believe that more intense disturbances in the power supply, once being able to reset the uC, may not come alone, so that choosing larger delay values (eg adding there a 10uF or 100uF capacitor) would keep uC off for as long as things stabilize on the VCC bus. Of course, an antiparallel diode with the 4K7 resistor would also be beneficial in order to reset it immediately with the power drop, without the risk of having charge on the MCLR pin during the VCC drop.
 
Of course, an anti-parallel diode with the 4K7 resistor would also be beneficial in order to reset it immediately with the power drop, without the risk of having charge on the MCLR pin during the VCC drop.
This seems a good solution to discharge the capacitor immediately.
Thank you
 

A word of caution - and explanation of the isolating resistor:

Unless you are using one of the PIC32 or 33 series, if you are using ICSP and add the parallel diode it will prevent the device being programmed by dumping VPP into the VDD line. The resistor on the other hand actually helps by isolating capacitance from the VPP voltage. It's worth taking the programming requirements into account when looking at filters on the VPP/PGC/PGD and PGM pins.

If your PIC has internal reset, you could consider using it instead of the external reset to save a few components, also see if 'brown out' detection is enabled as there is a potential problem of VDD dropping and crashing the PIC while the capacitor on MCLR keeps it high and prevents reset occurring.

Brian.
 
A word of caution - and explanation of the isolating resistor:
Brian.

Thanks for pointing it out. The resistor solution is what Microchip documents are suggesting. In my case I can use both methods if needed. Because normally what I do is The Capacitor in the MCLR pin I do not solder while developing. The proto PCB will be having Footprints for them, But the Real ICE and PiCkit3 sometimes (Not always! but That is against documentation) object the MCLR capacitor. The PGC and PGD pins, I usually use for less important I/O purposes which is not very critical for debugging. Since those pins are internally pulled down, even if a Transistor through a base resistor is live on the PGC , PGD lines, PiCkit 3 and Real ICE usuallyl work for ICSP and Debugging. (I/O function wont work)

btw any suggessions for the series inductor?
Thanks
 

Sorry, I forgot the inductor value question.

It depends on the nature of interference but a 'ball park' value would be around 50uH. It's purpose is to delay the rise of transient edges rather than to be resonant so the value isn't critical. Larger values will make better filters but at the expense of higher resistance.

Brian.
 
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