What do the Schottky's do here? (Adafruit NeoPixel circuit) - Page 1
Forum section: electronics / beginners

author: Youkai
author_rank: Regular Contributor
timestamp: October 04, 2018, 10:35:01 pm
content: I'm working on a circuit for a project I'm in the middle of. I was instructed to put these Schottky diodes in my circuit right next to an Adafruit NeoPixel. I understand that the NeoPixel doesn't want to get a data signal when it's not powered and that these diodes are supposed to help protect it for that scenario. But I don't understand how they do anything. Relevant portion of the diagram attached. Here is what I think is going on based on my limited understanding of electronics. Please help me figure out what I'm missing.If there is power connected:Then the top diode has 5v in the Yellow and is getting 5v pulses in the Green. I assume that since Yellow is saturated with voltage then the signal goes to the data pin instead of through the diode. The bottom diode never has a positive voltage on Black so nothing ever goes through the diode.If the power is not connected:Yellow is an open circuit so no current flows through the top diode. No thing happens ever on the bottom diode.Is the capacitor in the diagram "eating" the pulses when power is disconnected? Or perhaps the 1000uf capacitor in the full diagram is somehow allowing a complete circuit and draining the signal?Other thread with full circuit diagram:https://www.eevblog.com/forum/projects/borderlands-style-jewelry-box-research-thread/msg1858662/#msg1858662

author: aheid
author_rank: Regular Contributor
timestamp: October 04, 2018, 10:55:59 pm
content: My first impression on looking at the schematic, before reading your post, was that they were input polarity protection, in case someone plugged in the 3-pin header the wrong way around.But there's a good chance I'm mistaken

author: Mr. Scram
author_rank: Super Contributor
timestamp: October 04, 2018, 11:17:58 pm
content: I'd expect a transistor switch type deal to prevent data from being clocked in, not this.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: October 04, 2018, 11:27:59 pm
content: Usually you see the "do not put in a data signal if not powered" in a datasheet as the maximum voltage an input can have with reference to Vcc. For example you'll see something like ViH max = Vcc+0.3V (voltage-input-high maximum = Vcc + 0.3 volts). What the upper diode does is drag Vcc up when there's a postive voltage on the data line, making things safe for the data signal input voltage to go up too.It would be good if a resistor could be provided for limiting the current on the data line. Sometimes that's hard or impossible though. And there are better ways to do it...

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 04, 2018, 11:31:32 pm
content: Quote from: Ian.M on September 27, 2018, 07:59:38 pmFrom the other thread.Basically I guess to protect the device by ensuring the data line does not go further than about 0.3V beyond the power rails.

author: ArthurDent
author_rank: Super Contributor
timestamp: October 05, 2018, 12:02:08 am
content: If the 5 volt power is on, the data is fed through a 470 ohm resistor and the top diode limits or clamps the chip input to 5+.3 volts and when the signal goes low it is clamped by the lower diode to zero-.3 volts. If the input to the 470 ohm goes to 6 volts, the upper diode limits the chip input to 5.3 and .7 volts (6-5.3) is dropped across the resistor. If the voltage tries to go negative the voltage is clamped at -.3 volts and if it is higher, the 470 ohm resistor will drop any voltage less than -.3 volts.If the power is off the same happens only the voltage is now clamped to +/-.3 volts. With the power off the +5 line will stay at essentially ground or zero volts.https://qph.fs.quoracdn.net/main-qimg-b1ab15f46c32aa64751ec2cf02eb5cd4

author: Youkai
author_rank: Regular Contributor
timestamp: October 05, 2018, 01:58:03 am
content: Quote from: Paul Moir on October 04, 2018, 11:27:59 pmI see so basically the upper diode is making sure that if power goes to data; then at least an equal amount of power goes to Vcc. This covers the scenario when the 5v rail is unplugged. There is a 470ohm resistor on the data line. It's at the top of the cropped image.That makes sense. Thanks guys.

author: David Hess
author_rank: Super Contributor
timestamp: October 05, 2018, 03:41:10 am
content: This is a pretty standard I/O protection circuit which is also effective against ESD.The diodes clamp the signal level so that it remains within ground and power. Schottky diodes are used because their lower forward voltage drop clamps the signal before any similar silicon diodes built into the I/O pins conduct which can sometimes cause problems.

author: saike
author_rank: Regular Contributor
timestamp: October 05, 2018, 08:40:10 am
content: I had to repair several bits of kit recently that didn't have these diodes fitted. If the data signal was already present while the power rails were connected, it sometimes killed the first led in the string stone dead so stopping the whole string from working.