Input protection - Page 1
Forum section: electronics / projects

author: fitzgerald
author_rank: Contributor
timestamp: August 02, 2015, 07:17:32 pm
content: Hi,I designed the circuit attached and I am looking for comments, especially on the input protection.R3 is a pull-up resistor for an external switch that should short the input to ground. R4 is a first series resistor with clamping diodes D3 to both power rails. R5/C2 form an additional low pass filter. U3 is a Schmitt trigger that cleans up the signal.Some questions:- Instead of a pair of clamping diodes you often see a bidirectional TVS diode. Is there a particular reason to select one over the other?- Is it recommended to use PTC's for R3 and/or R4? (in case someone accidentally connects it to a power source- How do you calculate what is 'good enough'? (in particular if you need a more bandwidh)thanks

author: SeanB
author_rank: Super Contributor
timestamp: August 02, 2015, 07:50:20 pm
content: R4 R5 could easily go up to 10k or more, and lower the value of C1 along with using a NPO capacitor in this position so the debounce time does not depend on supply voltage. That way the clamp diodes do not see too much current, and the higher resistance reduces dissipation and clamp current to the supply rail. A TVS on the input, along with another on the 5V rail, will help to avoid voltage spikes killing the circuit.

author: fitzgerald
author_rank: Contributor
timestamp: August 02, 2015, 08:16:59 pm
content: Increasing R4/R5 to 10k will lower the peak current, but not much the dissipation. As I understood, the ESD event can be seen as capacitor (~100 pF) being discharged. Increasing R will increase the RC time constant of the discharge: the peak power/current will be less, but duration a bit longer with similar energy being dissipated. For that reason I am hesitant to lower the value of C1 as it can act as a buffer for all charge being injected (10nF/100pF ~ 100x, so a 8kV ESD pulse will be reduced to a ~80V pulse)The BAT54 can handle a surge forward current of 600 mA; does that mean that if I want it to hold up to a 8 kV discharge (what is good enough?) R4 should be at least 8 kV/600 mA = 13.3 kOhm?You mention adding a TVS on the input; would that be instead of the BAT54?

author: SeanB
author_rank: Super Contributor
timestamp: August 02, 2015, 08:28:38 pm
content: Keep the BAT54, and add the TVS on the input, probably using a 100R series resistor on the input that will be a sacrificial device. The supply TVS will clamp the current injected back into the rail as well, you just need to use one that breaks down at 5V6 or so, definitely try for under the ABS MAX value of 7V at a reasonable current. You may also want to have the pull up resistor isolated from the supply rail for voltage feed back ( makes clamping diode leakage easier to handle as you can use a 10V part) by another BAT54.

author: fitzgerald
author_rank: Contributor
timestamp: August 05, 2015, 08:21:19 am
content: I have updated the schematic; see attachment.Input is now made +12V tolerant by the BAT54S. With +12V DC input the current through R4 and D3 is (12-5)/1k = 7mA; at that current the forward voltage of the BAT54S is less than 400 mV so the IC's input should be safe.Protection from ESD is done using a TVS diode with 12V operating voltage so that it doesn't fry when the pin is connected to +12V DC but clamps at ~13V at ESD zaps.Any comments?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 05, 2015, 09:22:05 am
content: I don't recommend putting TVS or caps right at the pin, because this encourages a low impedance, which tends to encourage EMI/RFI. Resonances on cables and such.Resistors are usually okay for ESD (a typical 0805 chip resistor will withstand whatever), so it's not necessary to put a TVS outside to protect them.Ferrite beads can be used to increase the RF impedance, reducing cable resonances. But this is usually only necessary for sensitive low impedance inputs, or outputs where the impedance is low to begin with. FBs will do nothing if you have resistors there -- the resistors have much higher impedance at any frequency.In either case, you've got the clamping, filtering and hysteresis, which check all the big boxes on my list.So I prefer the first circuit, but given a suitable purpose, either will do.Tim

author: B.B.Bubby
author_rank: Regular Contributor
timestamp: August 05, 2015, 09:32:13 am
content: Is there anything wrong with old school optoisolation?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 05, 2015, 06:37:44 pm
content: The signal in this case isn't connected to a different ground, so it would simply waste an opto.It is a good approach where extreme ground noise is expected, or where isolation is necessary.Tim

author: unitedatoms
author_rank: Frequent Contributor
timestamp: August 06, 2015, 04:03:15 am
content: - The TVS vs diodes choice depends on how robust is the power rail and ground rail when it is being hit by a an external source. If external source is brutal and over 0.7V beyond rail, then TVS is needed.- I will look at datasheet for logic gate and see what is acceptable input current. If there is TVS and assumption that voltage after being limited by TVS never exeeds some value, I'd calculate the R and will not use the thermistors.- The good enough calculation is a RC bandwidth, I will calculate tau=RC and divide it by 10 to be sure.