tvs as continuous over/under voltage protection - Page 1
Forum section: electronics / beginners

author: JWoodrell
author_rank: Contributor
timestamp: October 31, 2014, 06:05:24 am
content: hey guys i'm working on updating a project that I am making much smaller, and my current thought I am looking at using a tvs protection chip, a "rclamp3354s" from semtech as part of an input protection circuit.http://www.semtech.com/images/datasheet/rclamp3354s.pdfthe external signal is 24V and the micro controller input is 3.6v (2mA max). it is input only (not bidirectional) it is configurable by the customer to be "active high" with the internal pull down, or "active low" with the pull up, depending on the config file they load.the old layout worked but doesn't offer any esd, or any negative voltage protection other than the tiny internal set built into the micro-controller.so i was looking at using the TVS chip as the over/undervoltage circuit but i can't find anything in the datasheet on continuous current rather than pulse current. would this work OK, or am I looking at the wrong solution. is there a better option to do this from 24v down to 3v.there are 4 input channels i have todo which is the other reason i was looking at is is it had the 4 TVS channels in it.thanks guys.

author: doctormord
author_rank: Regular Contributor
timestamp: October 31, 2014, 06:21:51 am
content: I normally use "esd5v3l1b". Your voltage divider 10k/1k should connect to ground, with 1k increased to 1k5. ESD Diode wired in parallel.Or use a dual 5V6 dual z-diode package to clamp to between VCC/GND. (Standard diode clamp)

author: Rufus
author_rank: Super Contributor
timestamp: October 31, 2014, 02:56:04 pm
content: Quote from: JWoodrell on October 31, 2014, 06:05:24 amThat doesn't make much sense or sounds potentially pretty rubbish.Are you saying the input can be driven with +24v for an active signal or 0v for an active signal and in the 0v case the signal is going to be pulled up internally by the processor with another 11k in series or 10k and a diode in the current design?

author: JWoodrell
author_rank: Contributor
timestamp: October 31, 2014, 06:22:44 pm
content: this is the current version that i have which does the job so far.I don't know where the customer is going to install the unit. it is an industrial alarm that plays audio files from the SD card at about 105 dB so they can have whatever alarm sound / signals they want. it chooses from 15 files based on 4 inputs (2's compliment). so i made it adaptable.I was just looking for ways to improve things, with stronger ESD and steering diodes outside the ones internal to the processor

author: Rufus
author_rank: Super Contributor
timestamp: October 31, 2014, 08:33:48 pm
content: 50uA of wetting current and less than a volt of noise margin in an industrial environment is pretty crappy.Especially when you have a rectifier so a few volts of noise or rf on the wire would be enough to give you a 0v on signal.I bet the processor is not specified to operate with 0.2mA clamp diode current in the +24v on case either.

author: JWoodrell
author_rank: Contributor
timestamp: October 31, 2014, 09:05:03 pm
content: I don't fully understand your first sentence.<disclaimer>now take all my following comments as coming from someone figuring this out as I go. I reserve the right to be wrong about things, that's why I ask.</disclaimer>the noise on the line would have to be sustained enough to drain down the 10uF cap which is charged to 3.3V through thethe data sheet lists +-2ma diode current on any device pin. which isn't all that much, but i think its built to be OK with 0.2ma.with this being said, I would like to learn how to do things better.thanks for taking the time.

author: Rufus
author_rank: Super Contributor
timestamp: November 01, 2014, 12:30:49 am
content: Quote from: JWoodrell on October 31, 2014, 09:05:03 pmhttp://en.wikipedia.org/wiki/Wetting_currentYou should be aiming to draw or push at least a few mA on the signal lines unless you specify your customer not use mechanical contact switching or gold plated contacts if they do.The high input impedance also means it doesn't take much leakage to look like a closed contact. Without working it out maybe 400k between +24v and your input would look like an on signal. You could get 400k from dirty and/or damp connectors and wiring.Quote from: JWoodrell on October 31, 2014, 09:05:03 pmYes but 'noise' can be DC. If the 0v your customer is switching to is 1v higher than your 0v the input won't turn on. You also have a diode in the circuit which means ac noise or rf pickup pumps down the voltage on the capacitor instead of the capacitor averaging it out.Quote from: JWoodrell on October 31, 2014, 09:05:03 pm2mA is an absolute maximum rating meaning less than 2mA will not cause permanent damage to the device. It definitely does not mean the device will function and meet all other specifications with up to 2mA of clamp current flowing. You will have to look closely at the data sheet and possible application notes to see if there is any specification for maximum pin injection current. You probably won't find one which means the maximum is zero. You will probably get away with some injection current especially if you don't much care about the operation of analog functions in the chip. It will be your risk and the lower the current the better. I would be looking to 'wing it' with a few uA not 200.You are kinda screwed really. There is no way to provide industrial strength input driven high or input driven low selection by fiddling with internal pull up/down resistors in the chip.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 01, 2014, 06:58:40 pm
content: A standard way to get wetting current AND minimize the effect of contact bounce is to simply place a capacitor across the contacts.Tim

author: JWoodrell
author_rank: Contributor
timestamp: November 02, 2014, 05:27:09 am
content: ok working with your suggestions, i decided since i had a few extra pins on the microcontroller, that I would split the "hi" input and the "low" input. so my program will just choose which input to watch.I decided to use an optocoupler which allowed me to get over the high impedance input problem (400k triggering the input) but since i need it to react to be driven hi, and driven low i hooked two opto channels to the "input" biased the input 12v, and hooked the otherside of the led to 12v as well. so it has to be pulled high or low to trigger the LEDs.using an eternal 1k pullup means it will take a large set of noise to trigger high or low, (approximately 8V or less will trigger low, and 16v for hi) so there is a +- 4v zone in the middle that won't trigger either hi or low.as far as wetting current it is 7 mA now. 5mA from the LED, and 2mA into the bias resistor.I am going to have to work in 2 opto chips (4 channels each) and a 12v regulator, but i think i can make it work.how does this look towards being more "industrial" it is what I could come up with working in the simulator the last 2 days, but I think i was able to address the issues.thanks guys.

author: David Hess
author_rank: Super Contributor
timestamp: November 02, 2014, 06:27:27 pm
content: Look up the power dissipation for the typical sot-23 packaged part and work out the current from that.

author: doctormord
author_rank: Regular Contributor
timestamp: November 02, 2014, 07:19:18 pm
content: Don't like it.

author: Rufus
author_rank: Super Contributor
timestamp: November 02, 2014, 07:47:46 pm
content: Quote from: JWoodrell on November 02, 2014, 05:27:09 amLooking better. I assumed you didn't want to make such significant circuit changes. If that is acceptable then there are lot of options. You need to define exactly how your inputs are going to be driven.If it is always what is usually called NPN switching to 0v or PNP switching to +24v or contacts doing the same then you can make it simpler. An input which is driven to 0v and +24v makes it more complicated.If inputs are only driven one way then there is no need to select between the 2 optos. One or the other will be on when the input is driven either way and you don't care which. You could use a single AC coupler like an SFH6206-2. I have used that in a design with just a resistor in series with the LEDs. The other side of the LEDS of multiple inputs was common and presented to the user. It was up to them to tie it to +24v or 0v depending on how they wanted to drive the inputs. You are kinda connecting the common to mid rail so it works either way.See anything wrong with the attached? The capacitor bypasses serious high frequency noise or RF on the floating line and is maybe not needed. Single spikes of noise you should filter out in software - it doesn't seem like you need any high speed response to the inputs.

author: JWoodrell
author_rank: Contributor
timestamp: November 03, 2014, 05:09:43 am
content: at least the initial use of them, the customer said they will be using these IO modulesthe Balluff - BNI004Ahttp://balluff.com/balluff/MUS/en/products/product_detail.jsp#/210042which lists a switched PNP in the datasheet.however i know other equipment in their plant (about 130 units of the original design) is a switched negative signal.both of those applications are not 24V AND 0V... however i don't know for sure that it won't see that setup. I like the simplicity of yours, but i think I'll keep the two optos per input to have the capability of dealing with 0Vand24V if it runs into it.so I end up with 4 copies of this, for the 4 input circuitsthanks for working with me on this, I learned new stuff

author: Rufus
author_rank: Super Contributor
timestamp: November 03, 2014, 03:05:38 pm
content: Quote from: JWoodrell on November 03, 2014, 05:09:43 amThat gives me a feel for the application and yes they are straight forward PNP outputs.Quote from: JWoodrell on November 03, 2014, 05:09:43 amThe more I think about it the more I think you shouldn't bias the inputs half way to be driven in either direction.Outputs driving both directions would be very unusual, however, outputs being used to drive more that one load isn't so unusual and another load may pull the output in the opposite direction. When you bias your input half way you will tend to turn on other loads on the output. I have seen PNP output proximity switches with integral LED indicators. The indicators were just an LED and resistor between the output and ground so if the load on those outputs was to 12v rather than 0v it still works but the indicator LEDs never go out.Maybe you customer wants to drive a visual indicator from the same outputs driving your equipment? The modules you linked have LED indicators for the outputs - maybe those LEDs will never go out.I would use those ac optos with a singled resistor and present the common side to the user to be connected to 0v or 24v depending on their outputs being PNP or NPN. That also has the significant advantage of completely isolating the input side of your equipment meaning you are not relying on a connection between your and their power supplies.The scheme you have needs a 0v connection between supplies anyway, or maybe you are requiring a 24v supply they give you to be common with the supply for the control signals and you are sharing the 0v for power and signal use.

author: JWoodrell
author_rank: Contributor
timestamp: November 03, 2014, 08:39:58 pm
content: I hadn't thought of back feeding like that, but it makes sense. so to keep the switch ability in software, I took it down to the ac opto to free up pins, so now I have a pair of optical SSR relays to connect and disconnect the pullup and pulldown.there will only be the two SSR relays so all 4 inputs will have to be set together as far as pullup or pull down... but that is acceptable i guess.as far as isolating power supplies it doesn't really come into play. they are feeding the 24v to my unit that it runs off of, the only internal power regulation is an LDO down to 3.6v to run the control circuits. my unit is plugged into a M12 5 pin cable that feeds it the 24v, gnd, and 2 input lines per cable (there are 2 cables if they want to run 4 input)

author: Rufus
author_rank: Super Contributor
timestamp: November 03, 2014, 11:41:18 pm
content: It seems complicated but should work. The capacitor would be better placed across the LEDs so it bypasses them rather than trying to get rid of ac/rf by coupling it to ground.

author: Rufus
author_rank: Super Contributor
timestamp: November 05, 2014, 03:02:00 am
content: Thought of something. You need to be careful with that circuit because there is cross coupling between inputs via the 4k7 resistors which are not pulled up or down. Better with a single resistor in series with each opto, but, then you have the risk of shorting the supply if you turn on both SSRs.