How to increase the protection of circuit - Page 1
Forum section: electronics / projects

author: amar
author_rank: Contributor
timestamp: March 31, 2013, 02:55:36 am
content: Hiin industrial circuit which contain both analog and digital circuits.how we can increase immunity of whole circuit against short circuit in any sectionlike if input/output is short !any suggestion please

author: Rerouter
author_rank: Super Contributor
timestamp: March 31, 2013, 03:23:04 am
content: a bit more info on the specifics of your circuit would help,for signal inputs, buffer with a moderate value resistance to limit the maximum current, then clamp with diodes to prevent it exceeding maximum voltage on every input,for voltage rails input give it some protection against revered polarity, and a fuse or polyfuse, to limit how much current it can take, and then put a voltage crowbar or similar after this protection so should the input voltage be exceeded it clamps itself and prevents damage,any high impudence outputs should also be clamped with diodes to the rails to protect against miswiring, (I've seen 24V DC / AC devices have 240V mains wired to the connections before)any low impedance outputs should have a fuse or polyfuse, for analog, you either modify your existing output, or buffer it so that its negative feedback is after the fuse , this way when there is a excessive voltage on the output its protected and when its under normal operation the feedback corrects for the fuse, sized accordingly to what exactly your using it for, and likely clamped rail to rail again at the output before the fuse so that excessive voltage gets shunted to your supply rails to trip the fuse,

author: amar
author_rank: Contributor
timestamp: March 31, 2013, 04:51:53 am
content: how to clamp the output with diode so that driver ic can't damage?

author: Rerouter
author_rank: Super Contributor
timestamp: March 31, 2013, 06:36:17 am
content: i've drawn something up, and i'll explain it a little more, (forgot to mention ESD)with the low impedance inputs or outputs, there is a suggestion, it would probably be wiser to tie them to only the over voltage protection so that your supply rail does not risk being dragged up and frying whatever chips you may have on there, for the positive rail clamp, while the higher impedance inputs should be fine to tie to the regulator as the minimum load should absorb it,ideally you want the thing to trip or at-least begin to trip at the normal supply rail voltage as the diode drop will offer you a margin of about 0.3-0.7V before tripping, so for a signal that's normally 5V, it would trip at say 5.3V and either blow the fuse or maintain it at that maximum (crowbar vs shunt regulator) in both cases you would want a moderate resistance of a few K between the clamping mechanism and the normal supply rail to hold the diodes off on the inputs until you get an over voltage situation,also one other thing for your inputs, in commercial products they tend to use TVS diodes across the inputs to cope with ESD spikes on the input, there very similar to the voltage clamps with the diodes, but they operate much much faster and are only there to cope with spikes in the high hundreds to mid thousands of volts,

author: C
author_rank: Super Contributor
timestamp: March 31, 2013, 01:58:10 pm
content: HiDistance: A little distance can sometimes help more than fancy circuits, but distance can also cause more problems. One trick is moving all the easy to make connections to the world, away from your circuits. In doing so you could end up with an easier to install, cheaper to replace I/O connection board that has some secondary protection for your circuits. This idea also can cause big problems. The wires used to extend the main circuit to the I/O connection board idea can act like a capacitor, an inductor, the secondary of a transformer, an antenna causing more problems.Installer Created Problems: The installer may think that the power output of a motor controller is just the same as the power used everywhere else. Think of that motor controller that is used to control a huge motor. Instead of 3 meters of wire between the two, the installer uses 1000 meters of larger wire to cover the loss. The installer just added a long antenna with lots of inductance. Making the motor controller's protection better is a good idea, but good documentation stating very strongly "3 METERS OF WIRE" could do more good.C

author: Hardcorefs
author_rank: Regular Contributor
timestamp: April 01, 2013, 07:09:19 am
content: Or you could just design the PSU properly and employ PSU control chips that allow you to startup/shutdown gracefully when you go out of spec.Crowbaring a PSU can have all sorts of consequences as regards to surrounding electronics,the power does not just 'magically' disappear when you crowbar a supply and in some situations it may cause reverse bias of substrates and CMOS latchup as the power bounces off the end of the 'blown' fuse......

author: Rerouter
author_rank: Super Contributor
timestamp: April 01, 2013, 07:35:38 am
content: that is crow-barring the input to blow a fuse... so yes the power does magically disappear on the input, then the diode across the regulator protects the regulator and power is suddenly removed from the power rail, discharging the whole circuit, to 0.3 or 0.7V depending on your diode, enough to essentially turn everything off,while my first post was a tad messy (way early morning) i tried to later clarify with the image, for low impedance inputs they should probably be connected to a common zener, or individual crowbar, that way when the voltage rating is exceeded the device is protected... this is due to almost every commercial regulator being single quadrant meaning it can only raise voltage not lower it, nor would one want there delicate 3.3V or such logic suddenly seeing 4V,equally how reliably can you make your psu for it? industrial products mean it can see anything from +5V DC to 2 phases of a 3 phase connection across the terminals (never assume the fitter will read the manual or even check what the voltage is) no pulling low on an enable pin is going to solve that... and to the reverse biasing, you also clamp your output power rail then, a schottkey or similar between ground and your rail, means you never go beneath -0.3V

author: amar
author_rank: Contributor
timestamp: April 01, 2013, 09:37:33 am
content: thank uonce short circuit happens and crowbar protect the circuitthen in this case zener diode get damage right ??

author: Rerouter
author_rank: Super Contributor
timestamp: April 01, 2013, 09:42:01 am
content: ideally you would size the zener and ptc thermistor so that once an excessive voltage occurs it can maintain it, with the thermistor heating up, going high ohms and the zener holding the voltage there by dropping enough current to keep the voltage at the limit and the thermistor hot,

author: VanitarNordic
author_rank: Frequent Contributor
timestamp: April 02, 2013, 09:59:49 pm
content: Basically your customer should define his required standards for you and your job would be reaching that requested levels in terms of immunity, EMC, temperature, vibration ....Many regulator or switching components provide short circuit protection as one of their features.

author: amar
author_rank: Contributor
timestamp: April 03, 2013, 07:26:04 am
content: Thank u so much for reply