PCB Circuit Protection

Thread Starter

mcardoso

Joined May 19, 2020
226
Hi all,

I'm interested in learning more about what "good practice" is for protecting circuits on PCBs. In particular I am interested in:

-Fusing on PCB
-ESD protection
-Reverse Polarity protection
-Power circuitry protection (MOVs, etc.)
EDIT: -Is every PCB input/output protected individually? Or just the power input?

I bet most people don't consider this for hobby boards (I sure haven't), but I am sure it is present in commercial products.

As a related question... I think of PCBs in a commercial environment as a throw away element. I doubt many companies will rework failed boards returned by customers. What good does adding a SMD PCB fuse do? If it blows, the whole board/product is dead and thrown away anyways.

Thanks so much, not finding much literature on best practices when searching line.
 

Deleted member 115935

Joined Dec 31, 1969
0
Re PCB fusing, that's normally as a last resort.
first line is the robustness of the circuit , and the ability of it to dis connect.
second line is a self resetting fuse, ( PTC )
if all of these fail, a real fuse might be included, but very rarely .

As for inputs and outputs, and protection,
The are various regulations that all equipment must pass that has a CE mark,
one of these regulation is the ability to withstand ESD on all inputs / outputs

Reverse polarity protection is common,
from the simple, a series diode, to the also simple, parallel diode and a PTC fuse,
to the auto correct options.

This gives you an idea on the complexity of meeting the CE / IEC markings

https://assets.nexperia.com/documen...document_book_ESDApplicationHandbook_2018.pdf

can I suggest that you do a search for

pcb esd protection design
 

nsaspook

Joined Aug 27, 2009
13,079
Hi all,

I'm interested in learning more about what "good practice" is for protecting circuits on PCBs. In particular I am interested in:

-Fusing on PCB
-ESD protection
-Reverse Polarity protection
-Power circuitry protection (MOVs, etc.)
EDIT: -Is every PCB input/output protected individually? Or just the power input?

I bet most people don't consider this for hobby boards (I sure haven't), but I am sure it is present in commercial products.

As a related question... I think of PCBs in a commercial environment as a throw away element. I doubt many companies will rework failed boards returned by customers. What good does adding a SMD PCB fuse do? If it blows, the whole board/product is dead and thrown away anyways.

Thanks so much, not finding much literature on best practices when searching line.
Most people do consider this for all boards, what's actually implemented is another story that depends on how the board will be used just like in commercial products.
This small board has parallel Zener diode, PTC fuse power protection and MAX203EEWP RS-232 Interface IC +/-15kV ESD-Protected, +5V RS-232 Transceivers but the most important safety factor is the 'Glory' connector. :D
IMG_20200619_090824.jpg

I can guarantee well designed commercial/industrial PCBs are not a throwaway product. They cost too much just to toss for something like a bad fuse or opto-isolator.
 

Thread Starter

mcardoso

Joined May 19, 2020
226
OK. I'm trying to implement some of these suggestions into a revision B of my circuit.

There are two power circuits. One of them is an Industrial SMPS (5V 2A) and the other is a servo drive internal power supply (5V 300mA).

The SMPS is short circuit protected and I have to assume the servo drive is as well.

The loads are 1) The PCB (17 CMOS IC's, an LED, and a clock oscillator. 2) an external encoder (unknown specifications, looks to draw 200mA @ 5V)

The servo drive powers the logic (CMOS ICs) and the SMPS powers the encoder.

I'd like to add reverse polarity protection on both power inputs and potentially fusing to protect the servo drive. My concern is that the schottky diode for reverse polarity protection will drop the voltage by 3-400mV and a PTC fuse will add 6 ohms of resistance. I think the two of these combined will drop the voltage too low below 5V for my application.

Am I safe to skip the fuse if the traces are large enough for the full 2A fault current? Is there a better option than the schottky diode that will drop the votage?
 

Thread Starter

mcardoso

Joined May 19, 2020
226
Also wanted to ask. I was looking into ESD protection for the circuit. I can add TVS diodes on all I/O pins, however I was noticing that the 74AC logic family has the following note on the data sheets:

"Exceeds 2-kV ESD Protection Per MIL-STD-883, Method 3015"

All the chips connected to I/O are of this logic family. Does this get me out of having to add TVS diodes?
 

Thread Starter

mcardoso

Joined May 19, 2020
226
I just wanted to follow up on the couple of posts I have added above. I am trying to read more into this, but not finding the common sense design advice I think I am looking for. Basically what is necessary without going overboard.

Any insight would be greatly appreciated :)
 

Deleted member 115935

Joined Dec 31, 1969
0
This is a long bit of string. at the end of the day, one can add an infinite amount of protection, as you add more the benefits for each increase gives less improvement. Add to that , no matter what you do, some one will manage to blow something in a way that's totally un expected.

A circuit I build for an industrial environment would have

a) ESD / Tranzorbs / MOV as best on very IO
b) opto isolated IO where possible.
c) Power supply input protection against under / over voltage and reveres voltage
d) as much other protection as I can get away with...

Its only the IO that you need to protect, be that power or signal, stuff on the board is assumed to be working.

Power inputs, at low currents, a diode is fine, as an example, a SM74611 will drop arund 30 mV at 8 Amps.
or a cheaper option, a LSM115JE3 will drop 200mV at 1 Amp.
 

MrChips

Joined Oct 2, 2009
30,706
We can create and implement a comprehensive quality control flow of materials. It depends on how far you want to go and how strictly you will adhere to the rules. This is just the tip of the iceberg even without consideration of circuit design and layout.

1) Do you purchase components from reputable electronics suppliers?
2) Are components shipped in anti-static containers?
3) Once components are received are they handled, cataloged and stored in anti-static storage units?
4) Are components used for production separated from those for R&D and prototypes?
5) Do assembly workstations used for production meet ESD protection guidelines?

Once you can answer YES to all of the above then we can begin to talk about actual circuit protection.
 

Thread Starter

mcardoso

Joined May 19, 2020
226
This is a long bit of string. at the end of the day, one can add an infinite amount of protection, as you add more the benefits for each increase gives less improvement. Add to that , no matter what you do, some one will manage to blow something in a way that's totally un expected.
I figured that was the case. I should clarify that this PCB is for personal and non-commercial use only. I need it to hopefully get an old industrial robot working again in my basement. I don't need anything super fancy and I'd prefer it not get too expensive, however I also don't want to fry it by touching it or potentially wiring the power backwards.

I've never designed a digital logic circuit before this, so I guess I don't have a great feel for how easy it is to destroy these chips. I'm using 74AC CMOS logic which I understand is much less sensitive than the old 4000 series stuff used to be.

A circuit I build for an industrial environment would have

a) ESD / Tranzorbs / MOV as best on very IO
b) opto isolated IO where possible.
c) Power supply input protection against under / over voltage and reveres voltage
d) as much other protection as I can get away with...
Power inputs, at low currents, a diode is fine, as an example, a SM74611 will drop arund 30 mV at 8 Amps.
or a cheaper option, a LSM115JE3 will drop 200mV at 1 Amp.
Thanks for the part number! This will not be pulling anything close to 1A so that should be plenty sufficient.

What kind of ESD device would you suggest for RS-422 differential pair inputs/outputs or single ended outputs coming right off of a CD74AC175? Even one example part number might help me compare specs with the stuff I picked out to see if it is even in the ballpark. All the logic is 5V.

I think I understand the reverse polarity protection, but what about the over/under voltage cutoff? Is there a power management IC to do that or just discrete devices?

We can create and implement a comprehensive quality control flow of materials. It depends on how far you want to go and how strictly you will adhere to the rules. This is just the tip of the iceberg even without consideration of circuit design and layout.

1) Do you purchase components from reputable electronics suppliers?
2) Are components shipped in anti-static containers?
3) Once components are received are they handled, cataloged and stored in anti-static storage units?
4) Are components used for production separated from those for R&D and prototypes?
5) Do assembly workstations used for production meet ESD protection guidelines?

Once you can answer YES to all of the above then we can begin to talk about actual circuit protection.
This is for personal use and I'm good about some of this, not great about others. I'm buying from Digikey, receiving and storing in ESD bags. When it comes time to assemble, I do not have an ESD mat or a grounding strap. My working bench is MDF and I try to ground myself to a piece of grounded conduit once I sit down to avoid ESD. I use ESD tweezers, don't know if that helps or not...

Just don't want to fry my board or damage the servo drives.


Thank you guys for following up with me!
 

MrChips

Joined Oct 2, 2009
30,706
You can't protect your electronics from every possible mishap.

1) A fuse on the AC LINE input prevents your house from burning down. It is not to protect your electronics.
2) You can use power line surge protectors to mitigate against RF and other power line spikes.
3) Apply flyback diodes on all inductive loads such as solenoids, relays, etc.
4) A simple bridge rectifier will accept reverse connections of power supplies, etc.
5) Simple resistor/diode circuits can clamp input voltages to safe levels if overvoltage can be anticipated.
6) Most digital ICs already have input protection diodes. There is no need to add more in most circumstances.
7) Line receivers and drivers are designed to function in severe electrical environments.
8) If you anticipate operating in extremely harsh electrical environments you can provide significant protection by using opto-isolators in input and output cabling. Hence it depends on how far you want to go with providing protection.
9) Catastrophic failures aside, many would-be designers overlook simple design requirements such as proper power rail decoupling capacitors, or conducting a proper heat dissipation analysis.

The list can go on forever.
 

Deleted member 115935

Joined Dec 31, 1969
0
Just to say, the AC175 is not a RS-422 transceiver,

You should use something like the SP485
 

Thread Starter

mcardoso

Joined May 19, 2020
226
Just to say, the AC175 is not a RS-422 transceiver,

You should use something like the SP485
Sorry, I was unclear. The input signals are (3) differential RS-422 pairs received by an AM26C32. The outputs are (3) differential RS-422 pairs transmitted by an AM26C31 and (3) 5V single ended push-pull outputs transmitted by the outputs of a CD74AC175.

These are the I/O I am hoping to protect from getting fried while I handle the board and wire it into the robot.

image012.jpg

image011_LI.jpg
 

ChetanShah

Joined Sep 19, 2020
5
Zener diodes are one of the most commonly used circuit protective device, thanks to their ability to clamp voltage. If used in forward-biased mode, they will clamp voltages to around 0.6 V like any other silicon diode; however, unlike silicon diodes, when used in reverse bias mode they will clamp a voltage a specific value.

For example, a 5V1 Zener diode will clamp voltages in reverse bias mode to 5.1 V, such that if the voltage across the diode exceeds 5.1 V, the voltage cannot get any higher. These diodes are often used in conjunction with a series limiting resistor so that the current through the Zener cannot exceed its limit. The series limiting resistor can also protect the circuit from current spikes. It should be noted, though, that series limiting resistors can affect the speed performance of a circuit and are more applicable to high impedance inputs.

11.jpg
 
Last edited:
Top