Circuit protection help - Page 1
Forum section: electronics / beginners

author: GeorgeJr
author_rank: Newbie
timestamp: August 31, 2021, 12:48:42 pm
content: I have a very basic circuit board.12-14v DC input and ground wires coming in.Standard 1 amp diode for reverse polarity protection on the 12v sideBuck converter to drop the 12v to 5v. (Even after the issue stable 5v is found)I have a 470 uf capacitor on the 5 v side near the Nano clone microprocessor to buffer anything from the Servothen goes into my Arduino Nano 5V pinOff the Nano I have a receiver powered off the 3.3 v extremely low ma draw at 3.3 vAlso a parallel connection to a 1602 LCD ~ 200 ma at 5vAnd also a parallel 5v connected to a remote serv0 30 ft away with 18 awg wire from the ground, 5v and signal pin on the nano to a hobby style servo.So the Servo and LCD 5v are all parallel connection and don't require the Nano to get their power.On a small percent of items I ship out I'm finding a burnt smell on the Nano, the LCD and Receiver boards are fried and if the servo if connected is sometimes fried as well (burnt coil). I haven't been able to duplicate this on my own. When I ship replacement parts after plugging in they are in business with no more issues from what I have found.Question:Anything you can think of that would cause enough amps to fry this? I had originally thought this was customers powering the 5v side with 12v, but had one installed by a shop and they ran into this issue (never connected the servo as I had requested they leave that off till successfully powered up).Should I look at putting a low amp resettable fuse going to the Nano/LCD at say 500ma and another one going to the servo that is sized accordingly to its needed amps plus say 20%?I'm getting electric discharge bags to start shipping product in to see if that eliminates the issue but I have a hard time assuming this would be the case as the ground wire is connected before the power switch is turned on (mechanical switch).Other ideas / good practice?Unit is in a plastic case and the servo pins are protected with a rubber boot. Sealed in a plastic bag when shipped with other parts in case that could help ID the problem.Thanks for any help / suggestions. Long time viewer first time question.

author: Jwillis
author_rank: Super Contributor
timestamp: August 31, 2021, 07:19:01 pm
content: Any electrical device , whether it;s an LCD , Servo , Nano , etc , when working within manufactured specifications will only draw as much current(Amps) as it requires . Only Over loading (draws more current ) or over voltage of the device will cause failure.You can add over loading protection to the output side (peripherals) of your device and over voltage protection to the input (power) of the device . You already have polarity protection which is a good idea .My first guess is the failed devices were over loaded .But you can never predict what a user will do , so you need to make certain that your instructions include warranty limitations . Engineering can only do so much .

author: GeorgeJr
author_rank: Newbie
timestamp: September 01, 2021, 05:44:56 am
content: I had not thought a solution for voltage protection but from your comment I just read up on Zener Diode. That would be very easy to incorporate into my current board across the capacitor. New boards I’ll get them with thru hole for it separately.Because this would only trip for above 5.1 v how should I size the wattage for this. 1N5231 Zener Diode is what I’m looking at now.This would be a easy solution to implement and see if it reduces the issue.Thanks for the quick response

author: WattsThat
author_rank: Frequent Contributor
timestamp: September 01, 2021, 06:21:13 am
content: Post a schematic. Please. Words are not sufficient. Then tell us where this is being used. This is a very important part of the problem that you’ve neglected to provide. You’re also not stating which parts are failing, again, a large piece of the puzzle that is missing. If this an automotive or marine application, you’re not addressing the 40v spikes common in these systems and a 5.1v zener is pointless.The Nano has a crap design power section with a very low capacity Schottky diode in series with the 5v supply for reverse polarity protection. The result is high forward voltage drop and they’re a common failure point.If the board is a CH340G based clone, you cannot draw power from the 3.3v pin on the Nano as it is created by the CH340G and it is not a power supply, it’s an internal usb voltage reference for the chip.Thirty feet of wire with the supply and ground connected to servo sounds like a potential problem in a noisy environment. Shielded cable with twisted pairs being used?

author: Jwillis
author_rank: Super Contributor
timestamp: September 02, 2021, 04:05:05 am
content: Quote from: GeorgeJr on September 01, 2021, 05:44:56 amThe wattage of the zener is dependent on the current required . Essentially your creating a regulator to keep a constant voltage . But beware . Just using a zener diode is not good as voltage protection . The input voltage is unpredictable and as the input voltage rises so must the dissipation of the zener increase . So if your current draw is 200mA at 5V with a 6V input the zener will need to be over 1W . If the input voltage is increased to 12V the zener will need to dissipate near 10W . This also applies the the current limiting resistor of your zener regulator . This makes them unpredictable for voltage protection .A better idea is to use a dedicated voltage protection circuit like the one below . When the voltage is less than the preset level of 5.1V, the base terminal of Q1 is high and as it is a PNP transistor, it turns OFF. And, when it is in off condition the base terminal of Q2 will be LOW and allows the current to flow through it.When the voltage exceeds the preset value of 5.1V the Zener diode starts conducting, which connects the base of Q2 to ground and turns ON Q2. When Q2 turns ON the base terminal of the Q1 gets HIGH and Q1 turns ON which means Q1 becomes an Open Switch. Q1 does not allow the current to flow through it and protects the Load from over voltage.