Circuit Protection - Page 1
Forum section: electronics / beginners

author: logictom
author_rank: Unknown
timestamp: February 06, 2012, 05:29:36 pm
content: I am trying to protect a circuit from over/reverse voltage.I am running a PIC with a absolute rating of -0.3-6.5V on Vdd with respect to Vss. (16F1823)My circuit has to withstand 7.5V for an hour and still be operational.Withstand 12V for 5 minutes operate safely during and function after.Withstand -12V for five minutes and function after.Has to withstand ESD testing. (Reason for ESD diode rather than just Zener)I've been looking at clamping diodes to keep the voltage below 6.5V, would something like thatPESD3V3S2UATbe appropriate? VcMax 5.6V so this would clamp at anything above 5.6V?I was thinking a diode in series with the supply but I can't afford the voltage drop across the diode - what other alternatives are available?Thanks.

author: McMonster
author_rank: Frequent Contributor
timestamp: February 06, 2012, 05:50:43 pm
content: Quote from: logictom on February 06, 2012, 05:29:36 pmMaybe use aP-MOS as a reverse protection? And looking into the datasheet this PIC has built-in clamping diodes to both rails, so a simple resistor my be enough for under- and overvoltage protection, but it depends on what you need. You can dig out Atmel's AVR182 application note (it'll work the same for PIC) for more details and see if this'll work for your application.

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: February 06, 2012, 06:23:35 pm
content: Linear Technologies LTC4365 may be a good answer here. It protects from reverse polarity, overvoltage and undervoltage. I dida brief writeup here, with links to the data sheet and applications notes.

author: olsenn
author_rank: Frequent Contributor
timestamp: February 06, 2012, 06:51:29 pm
content: Zener diodes may help you there

author: armandas
author_rank: Frequent Contributor
timestamp: February 06, 2012, 07:09:45 pm
content: Looking at this image from Jon's website:If you replace the diode with a zener, it will protect from both overvoltage and reverse voltage. Replace the fuse with a polyswitch and it will recover after reverse voltage is removed.

author: Rerouter
author_rank: Super Contributor
timestamp: February 06, 2012, 07:36:00 pm
content: full wave rectifier then a crowbar, so you can have it run like normal off reverse voltages (albeit a drop) and the crowbar works with them

author: Rufus
author_rank: Super Contributor
timestamp: February 06, 2012, 08:03:02 pm
content: Quote from: logictom on February 06, 2012, 05:29:36 pmAnd give us a totally inadequate specification of requirements (not that it stops people making up their own requirement and suggesting solutions to meet it).What is the supply voltage requirement for the circuit you are protecting? What is the operating current requirement? What is the normal supply voltage range? What is the source impedance of the supply under abnormal conditions? "ESD Testing" is more or less meaningless.

author: olsenn
author_rank: Frequent Contributor
timestamp: February 06, 2012, 08:34:44 pm
content: This is for the absolute most basic level of protection, that in my opinion should be deployed in every circuit made. If you expect to have LARGE voltages accidentally passed through the circuit you may want to consider something like a transient voltage supressor; if you need greater control over current protection you may wish to use both a fast acting fuse in the case of very high currents and a slow acting fuse for the small over currents; if isolation is key in this application you may wish to consider optical isolators et cetera.

author: logictom
author_rank: Unknown
timestamp: February 06, 2012, 11:10:53 pm
content: Thanks for all the responses.Quote from: McMonster on February 06, 2012, 05:50:43 pmThe PIC will probably be ok but there are other components that need protection. I will have a look at some PMOS datasheets - trying to minimise parts but meet all specs.Quote from: Jon Chandler on February 06, 2012, 06:23:35 pmThanks Jon, that nicely summarises the different protection methods. Whilst offering a nice solution the LTC4365 would be contributing the majority cost to the circuit so think it's a no go on this particular circuit.Quote from: armandas on February 06, 2012, 07:09:45 pmQuote from: Rufus on February 06, 2012, 08:03:02 pmI haven't been given this information, the circuit has been designed to run at 5V and draws max 90mA.Without the above info am I correct in thinking I can't choose PTCs / Zeners because I don't know how much current the supply can source and if more than the PTC Ihold max - poof!Quote from: Rufus on February 06, 2012, 08:03:02 pmSorry that's my inexperience of not knowing what parameters are important and just trying to get an idea for what solutions are possible. The ESD testing is either IEC 810-2 or hand held discharge unit capable of 17kV, to be shocked 10x on each connector - Power, Gnd and two outputs - currently protected with the PESD5V0L2UU (if memory serves me correctly)Quote from: olsenn on February 06, 2012, 08:34:44 pmThe testing is just a requirement asked for - it shouldn't happen as the connector is keyed so unless they have an identical connector with reverse or higher voltage this shouldn't happen - it will be worst case there is a fault on the 5V rail. Fuses maybe an option but not at board level as the circuit will be potted making it a little more than tricky to replace

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: February 07, 2012, 12:45:04 am
content: Quote from: armandas on February 06, 2012, 07:09:45 pmNice change to the circuit to gain some additional protection.

author: Zero999
author_rank: Super Contributor
timestamp: February 08, 2012, 11:06:09 pm
content: Quote from: Jon Chandler on February 07, 2012, 12:45:04 amAnother improvement would be a crowbar with a slightly lower voltage than the zener but with a low pass filter so it provides a dead short if a certain voltage is exceeded for longer than a certain number of RC time constants. In this case the zener could be 6.2V and the crowbar 5.6V.

author: matt100psi
author_rank: Newbie
timestamp: June 17, 2014, 05:45:44 pm
content: Hi folks,I know this is an old topic, but I found it when attempting to educate myself about over-voltage protection. Almost all the zener diodes I've found at the Vz I need (~3V) have fairly small power ratings. Even at 10W (a hard to find part), you can run <4A across it. How is this addressed in practice when your fuse must support a higher current for normal operation?Thanks,Matt