Using a MAX485 and Step-Up Step-Down circuit

Discussion in 'The Projects Forum' started by MrSmoofy, Sep 24, 2015.

  1. MrSmoofy

    Thread Starter Member

    Jul 28, 2014
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    I'm not looking for someone to completely write a complete circuit/component for me as I'd like to learn but I seem to be having a problem wrapping my head around what I'm trying to do so help pointing me in the right direction so I can order some components and start pluging things together to learn is what I'm looking for.

    Would like to bread board this. Once I build some prototypes and learn what I'm doing when I'll build a schematic with surface mount components to make it smaller

    I am pretty sure the MAX485 is what I want to use for what I'm trying to do. Would be 2 one as a transmitter and one as a receiver

    http://datasheets.maximintegrated.com/en/ds/MAX1487-MAX491.pdf

    I know that the input voltage could be as low as 4v and as high as 36v. So I was looking at http://www.diodes.com/_files/datasheets/AZ34063U.pdf (see page 10) as a step-down/step up

    It gives examples but I'm not 100% sure that works for me because the voltage in could be as low a 4v which it would then need to step that up to 5v and the high could be 36v so that would need to step it down to 5v to go to the MAX485

    The input side of this board will be 3 wires into probably a screw type terminal like what I think is called a EURO connector which would be +V (4v to 36v) -V and a SPI data signal. The output of the MAX485 would be to a RJ45 connector and use 1 pair of the twisted wires. Another pair will send down +V and third pair would send -V over a cat5 cable to another one of these as a receiver.

    I started a list on digikey http://www.digikey.com/short/th7rvc I could be wrong on the parts since I'm not 100% sure on using the step-down/step-up component

    So first I think we should start with getting the 4v to 36v up or down to 5v On page 10 it shows the example for 24v in and 5v out but I'm not clear that if the V in can be anything I want or if that example will only work with 24v in.
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    It is not clear to me what you are trying to do with this signal. Is it a digital signal with two states or is it an analog signal? Can it ever be at ground?
     
  3. MrSmoofy

    Thread Starter Member

    Jul 28, 2014
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    The signal is an SPI signal from Smart Pixels like those that use WS2811
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    A signal that can range from 4V DC to 36V DC is not very useful as an SPI signal. For a receiver I suggest you use an analog comparator with an open collector output. I need to see a schematic of your setup to know more.

    I have no idea what Smart Pixels are and I'm not familiar with a WS2811. Can you provide a link?
     
  5. MrSmoofy

    Thread Starter Member

    Jul 28, 2014
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    Sorry for the confusion the power will be 4v to 36v the data is 5v. Using my 0ho e right now so hard to paste a link
     
  6. Papabravo

    Expert

    Feb 24, 2006
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    Don't know what an "Oho e" is. When you get around to describing and documenting what you want, maybe I'll be motivated to spend more time on your problem.
     
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The RS-485 hardware protocol uses nominal TTL voltage levels. A high is anything above 2.0 V and a low is anything below 0.8 V. To run on 4 V power without going through a buck/boost converter, use another RS-485 chip. Digi-Key has 284 part numbers in stock that will run on 4 V or less. Many of those are the same chip in different packages and temperature grades, but still they have a lot to choose from.

    ak
     
  8. MrSmoofy

    Thread Starter Member

    Jul 28, 2014
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    Sorry it was late and I was tired been working a lot of hours this week and that was typed from my Phone "Oho e" not sure why the attitude though.

    I understand how RS-485 works there seems to be confusion on the power and data here.

    I'm not sure why where the data comes from is relavent but it comes from a controller board that drives a strip of LED lights that use WS2811 chips and 5050 LED's The Strip is powered by a power supply that will range from 5v to 36v. The reason I said 4v is because at the end of the LED strip (depending on length) there is voltage drop.

    So at the end of the strip there are 3 wires +V, -V and Data which is SPI. These 3 would connect to this new board I'm trying to come up with.

    The MAX485 takes a 5V supply and the Data in and transmits it over twisted pair to another MAX485.

    So my thought was to use a Step-Up/Step-Down like http://www.diodes.com/_files/datasheets/AZ34063U.pdf (see page 10)

    It would take whatever the voltage is off of the end of the strip which could be as low as 4v and as high as 36v and step it up or down to 5V for the MAX485.

    The data signal degrades over about 20ft so I want to use this to extend the signal to a further distance. so at the end of 1 strip one of these would be plugged in and using a RJ45 cable could run for a longer distance to another strip of LED lights.

    So I am just trying to understand how to make the 4v to 36v into 5v for the +V on the MAX485

    Does that clear things up?
     
  9. Papabravo

    Expert

    Feb 24, 2006
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    We get a great number of people asking for help while providing very minimal information. I considered your first response to my questions non-responsive. Your second answer was far superior, so apparently my attitude got through to you.

    Generally speaking it is easier to construct a DC-DC Converter with an output voltage that is higher or lower than the input voltage, but not both. SPI as an interface requires more than just a data signal. The minimum configuration consists of a data signal and a clock signal. You can't do it with a data signal alone. Also since most SPI devices transfer data in both directions, a one-way data path might be difficult to debug and monitor. An RS-485 device can work with a supply that is lower than +5VDC. If so then you would only have to consider a "buck" converter (output lower than input voltage) for this application. Would these design alterations be feasible?
     
  10. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Do you really want to go through the hassle of a switching regulator for 1/10th of a watt?

    To me, the simple and reliable solution is to go with a linear regulator rather than a buck or buck/boost, and solve the device power range problem with a different device. An RS-485 driver draws less than 10 mA static current, and RS-485 signal currents are less than that. So using 20 mA as an ultra-margined estimate of the regulator output current, worst case power dissipation in an LDO linear regulator is 620 mW. A 3.3V RS-485 driver will make the correct output signal voltage with margin to spare, just like a 5 V part.

    ak
     
  11. MrSmoofy

    Thread Starter Member

    Jul 28, 2014
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    Since the MAX485 can operate at under 5v then yes only need to worry about a step down. I believe the max is 12v. My only concern was at 12v would it shorten it's life and get hot?

    Yes some LED Strips do use a clock signal, but the ones I'm working with do not. but you bring up a good point that I should probably make this work with those as well

    3 wire vs 4 wire LED Strips and Nodes
    Example of 3 wire 12v LED Strip http://www.holidaycoro.com/Smart-Pixel-LED-RGB-Strip-30-LEDs-m-50-Pixels-m-p/700.htm
    Example of 3 wire Nodes http://www.holidaycoro.com/Smart-Pixel-LED-RGB-8mm-12mm-Nodes-12v-DC-p/711.htm

    Don't have links handy at the moment for 4 wire of the same lights but like you said the 4 wire versions have a clock and data signal.

    Taking into consideration 4 wire versions that have clock and data, would you just use 2 MAX485's to send both?
     
  12. Papabravo

    Expert

    Feb 24, 2006
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    A "485" device is a transceiver. It contains bot a transmitter and a receiver. The clock signal requires only a transmitter. One more point, and I cannot stress this enough; if there is only one signal line, there are lots of things it can be, but SPI is NOT one of them. There is no give on this point.
     
    Last edited: Sep 25, 2015
  13. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Also, while RS-485 parts can be used for other purposes, the RS-485 signalling protocol is a pseudo-balanced system. It takes two signal wires (plus ground) to transmit, receive, or both. The receiver will not respond if only one of its differential inputs is wiggling.

    ak
     
  14. Papabravo

    Expert

    Feb 24, 2006
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    It might respond if the other input is fixed inside the wiggle range of the wiggling input.
     
  15. MrSmoofy

    Thread Starter Member

    Jul 28, 2014
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    I understand the 485 can both transmit and receive that's is the idea is that it can be either a transmitter or a receiver. So you would have 2 boards one as a trasmitter and one as a receiver.

    Here is an example of what I envision with all the parts as examples of what one configuration would be.

    1. PixLite controller (https://www.advateklights.com/pixlite-control/)
    This is coummuicated with E1.13 (DMX over Eithernet) from software.
    2. Meanwell 5v Power Supply (powers the PixLite board and LED lights) (possible second power supply)
    3. two strips of WS2811 Smart LED Pixel Strips
    4. two new boards which is what I'm trying to come up with

    PC with software connects to PixLite board over eithernet.
    Pixlite board is powered by Power supply
    One LED strip (3 wires) connects to Pixlite board. +v -v and data
    At the end of the LED strip there are 3 wires that come out +v -v and data. +v will have started with 5v but will have voltage drop
    +v -v go through voltage regulator to drop voltage down to what MAX485 can use. This is in the event somthing more than a 12v power supply is used.
    +v -v and data connect to MAX485 and is set to transmit.
    2 twisted pair from RJ45 cable are used for the MAX485. Another twisted pair is for +v and another for -v
    This is connected eventually via RJ45 jack on board so board has srew terminal wire connector to connect end of led strip to this board.
    out of the board is RJ45 jack and for example up to 200 ft away to another of the same board that is set to receive from the RJ45 and then take the +v -v and data out of another screw terminal into another LED strip

    Does this help at all?

    I want to make this board as small and cheap as possible. I thought I had a general idea of what I was doing but now I feel compelty lost.

    The board would be a transmitter or a receiver. So it would have the MAX485, the components to step down 36v down to 12v or less, 4 wire screw terminal (if it handles 4 wire led strips) and a RJ45 connector. Will also want to include LEDS for power, tx and rx

    So this is why my original question was about just the stepup/down to get 5v for the MAX485 but if MAX485 can use less then 5 then all I need to be concerned about is step down when it's more then 36

    Based on what I have read I should be able to use the MAX485 how I intended but I wasn't thinking about 4 wire LED strips that do have a clock signal. Which is why I asked would I then just use 2 MAX485's on the board 1 for the data and 1 for the clock to send to another board receiving it with 2 MAX485's?
     
  16. Papabravo

    Expert

    Feb 24, 2006
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    I was unable to locate the details of the SPI connection to the strips. The answer is that you can use the transmitters on one board and the receivers on the other board. In the SPI specification, all four signals, DATAIN, DATAOUT, CLK, CS are unidirectional. Some devices allow DATAOUT to be tri-stated so they can be connected together. In this case you can use CS to enable one of the 485 transmitters if you need to do this.

    One other thing to check is that SPI, which is designed for connections between peripherals on a PCB, can rise to some very high data rates in excess of 10 MHz. clock rates. I don't think RS-485 devices will work at such elevated rates.

    Before you cast anything in stone you might want to post a schematic so we can see if anything got lost in translation.
     
  17. MrSmoofy

    Thread Starter Member

    Jul 28, 2014
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    I've ordered some parts from Digikey to bread board it out and see what I can make it do. I'm still confused on the clock line on the 4 wire type LED's though. The MAX485 has 1 data in so wouldn't I just use 2 of these to send the data and the clock down the 200ft RJ45 to the reciving pair of MAX485 chips or am I not understanding how that works. I don't have any 4 wire LEDs anyway (I don't think) but would like to make this work for both types to cover a larger group of DIY LED light enthusiasts.

    My understanding of the 3 wire LED strip was that the data wire contained a string of data packets. With RGB values.

    The WS2811 chips on these strips do a "Take one pass it along deal" so they take the first block of data and retransmit the rest of it down the line.

    So if you had 3 LED's that all all white you would have 1:255,255,255 2:255,255,255 3:255,255,255 and this is sent repeatedly otherwise the LED's would turn off.
     
  18. JWHassler

    Member

    Sep 25, 2013
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    The WS2811 chips claim that each device re-shapes the single data-line for the next: I don't think you need a driver-chip.
    However, you may have a distance variation that makes that necessary.

    As for the voltage boost/buck, it sounds like spending a lot to save a little
    Just run the power-supply to both ends and/or re-feed it anywhere the voltage sags
     
  19. MrSmoofy

    Thread Starter Member

    Jul 28, 2014
    112
    5
    Yes the WS2811 chips do reshape the signal however this board I'm wanting to build would go between 2 strips with a large distance between them. So Yes the power would be at the first strip and second strip. The Boot/Buck was for powering the MAX485 chip where the voltage out of the LED Strip could be between 4volts and 36 volts and I believe the max voltage for the MAX485 is 12v
     
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