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Ideas needed to protect hall sensors and encoder outputs. — Parallax Forums

Ideas needed to protect hall sensors and encoder outputs.

T ChapT Chap Posts: 4,059
edited 2017-11-17 03:43 in General Discussion
Hey guys I could use some ideas on this. The Prop main board has various RJ 45 ports that connect to different input sources. I like cat5 cable because installers can make up cables and it is a nice method of connecting remove devices. There is a brushless DC motor that has an encoder
and hall sensors

The encoder output is totem pole. The hall sensor output is open collector, the BLDC driver board provides 6.35V via pullups to the hall sensor outputs.

In the photo and schematic, the motor has a PCB that mounts on it that has an RJ45 that connects the motor back to the Prop main board. On the Prop main board, there are 7 other Rj45 ports for connecting other devices, and all off them vary in terms of pinout and 5v and GND will be on different pins for different ports. All the ports contain 5v, GND, and signals, and I used different pin arrangements to try to reduce noise by arranging twisted pairs on the cat5 in certain ways based on what what on that port. Unfortunately this has caused occasional problems due to people not reading the warning sticker on the motor port that says damage may occur to the motor if the cable is plugged into the wrong port! It is rare but sometimes they plug the cable in the wrong port and kill the encoder or a hall sensor. One example of a wrong port could put 5v and GND on the encoder outputs A and B.

One trick I added to the motor breakout board was 100ohm's in series with the 3 hall sensors and the 2 encoders. I think this reduced some failures but still I get a rare failure and want to try to remake the motor breakout PCB with some better protection. It is too late to change the main board pinouts. Does anyone have any ideas I could try on these signal lines ie diodes etc?

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  • Usually, I try to "key" a connectors power with an open or nc slot if the connector were flipped 180 Deg, but without further study of your situation that might not be an option. Typically I don't have that many connector options in the first place. What you could possibly do is add a jumper or shunt, that either get's mechanically locked if the wrong plug is connected or one that simply enables or disables any power until you are absolutely positive the connectors are in the right position. You have to also think of the impossible when trying to "idiot proof" your design. .... alternatively you could increase the cost of your board to a point that it becomes painful for the customer to screw up despite any obnoxious warnings.
  • There are no options like that. I need something like diodes in series on the signals. A one way scheme but with totem pole and open collector I am not sure how to do this.
  • Are you using Eagle ? ... I can maybe look at it this weekend in more detail if I can look at an Eagle file and "highlight" the board and the schematic connections.
  • I can post the eagle later. I have an idea to put a bidirectional switch TLP222a on each of the 5 output signals (3 hall, 2 encoder). The switch will not turn on unless it gets the correct 5V and GND so an incorrect port will not allow the switches to turn on. In sandy his case if an accidental 5V and GND were present on encoder outputs a and b they would not be affected. I can test this later today and see how it works inserting the switches.
  • T ChapT Chap Posts: 4,059
    edited 2017-11-18 20:42
    Here is the eagle file for the new concept using the TLP222A-2. It is a bi directional switch that may serve the purpose. As Beau mentioned using a "key" I had the idea to use these opto's to create an electronic key. Unfortunately there is a port that could be connected wrong that would route the 5V and GND in reverse. So this opto only solves for most options. I still need to force polarity on the Vcc and Vss pins to reject a reversal. I assume a simple diode in series with the Vcc and Vss to the TLP will prevent reversed power input.

    Using this type of switch will allow the cat5 from the motor to be plugged into any of 7 other ports and prevent the encoder and hall sensor outputs from getting power applied to them unless the TLP is powered on.

    One thought occurred while working on this. LED's have a lifespan. So at some point of 24/7 LED ON these will diminished and fail. Then again, the encoder has an LED. So should I concern myself with extra LED's failing at some point? The goal is the longest life possible as often the motor is not accessible with some expense which can be high. I would like to find out if there is some non opto version of this idea that would remove the added failure factor. For example put mosfets in series like the TLP222a arrangement?

    Any ideas appreciated.
  • Thanks Beau for looking at it. I am trying to understand why you put the schottkys in reverse? I would assume there would be no flow from the RJ45 through the diodes to the opto's so I am missing something. Also why use two diodes? I assume you put two to spread the current over two diodes.
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  • I assumed the board was powered from whichever plug was used.... if the hall sensors are plugged in, then VCC_A is powered and supplies VCC through D1 .... if the motor is plugged in, then VCC_B is powered and supplies VCC through D2. If this is the wrong direction and the main power is supplied through VCC then the diodes won't work and some more thought needs to go into this. If this is correct, then an additional measure would be to swap VCC and VSS on only one of the headers.
  • T ChapT Chap Posts: 4,059
    edited 2017-11-19 02:32
    Sorry it wasn't clear. Here is an updated version. I marked the RJ45 port where the power comes in from the CAT5 cable. The signals from the encoders and hall sensors return on the CAT5 cable also. This shows the reverse protection with a diode on 5Vin>VCC and GNDin>VSS. This should prevent any reverse flow on the main 5v in and GND lines. I'd still like to find an alternative to the opto- mosfet TLP part. It looks like a mosfet configuration that could be duplicated with logic level mosfets, I'm just not sure how to do this for the encoder totem pole outputs and hall sensor open collector outputs. I hope to test the concept of the TLP222 tomorrow so at least I have that option.

    I am looking at a mosfet option for protection versus the Diodes to reduce the voltage drop.
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  • This removes the diodes and uses a n channel logic level mosfet on VSS to reduce the voltage drop from the diodes.
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  • Beau SchwabeBeau Schwabe Posts: 6,475
    edited 2017-11-19 03:53
    Wait .... Can you just current limit the VCC on the Encoder and Hall connector?

    So... ENC-2 and HALL-2 are tied together (VCC for both) and are connected to the main supply VCC through a 47 Ohm resistor. This limits the current to slightly more than 100mA with a 5V supply. HALL-3 , HALL-4 , and HALL-5 have an additional 220 Ohm resistor in combination with the 47 Ohm resistor, the current is limited to slightly less than 20mA with a 5V supply.
  • T ChapT Chap Posts: 4,059
    edited 2017-11-19 15:32
    You are suggesting this will serve as reverse protection as well as accidentally connecting power and gnd to the output signals? Using no diodes or switches?
  • Beau SchwabeBeau Schwabe Posts: 6,475
    edited 2017-11-19 17:21
    T Chap wrote: »
    You are suggesting this will serve as reverse protection as well as accidentally connecting power and gnd to the output signals? Using no diodes or switches?
    .... limiting the current may be all that you need for protection.... you could also put 220 Ohm resistors on ENC-3 and ENC-4 for good measure .... 5V / (220+47) = 18.7 mA .... <-- That should keep things from cooking and still allow the inputs and outputs to communicate. The reason for the 47 Ohm is to limit the overall current to about 100mA ... 5V / 47 = 106mA

  • T ChapT Chap Posts: 4,059
    edited 2017-11-19 19:15
    That looks nice, very simple too. I am looking for a fool proof board since in some cases the motor is not accessible and would cost a lot to get access if some dummy plugs in the wrong port. Do you think something like a load switch would give any better protection? Since I have to make up boards anyway, it may be easier to put everything SMT and make a stencil and run a batch of these a lot faster than TH parts as it is now. I would connect VCC to Vin and ON/Off.

    Here is a part that is small, very low On resistance, very high Off. Looks like it would be fool proof if power was reversed to it.

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  • If the power was reversed, you would still have conduction through the bulk diode of the power mosfet. You need two power mosfets back to back to prevent the bulk diode from conducting.
  • T ChapT Chap Posts: 4,059
    edited 2017-11-19 20:24
    Another option

    Reverse current protection in ON and OFF states. An internal reverse voltage comparator disables the power-switch when the output voltage (VOUT) is driven higher than the input voltage (VIN), by VRCP, to quickly (10 µs typ) stop the flow of current towards the input side of the switch. Reverse current protection is always active, even when the power-switch is disabled. Additionally, under-voltage lockout (UVLO) protection turns the switch off if the input voltage is too low.

    provides reverse current protection.
  • The maximum voltage of that IC seems a little too close to your operating voltage and I don't know how easy the package is to deal with from a soldering perspective. I would be more inclined to go a traditional route ....

    connect D1 to D2
    connect G1 to G2 and Tie to Power (V+) with a pull up resistor(10k or so)
    Your Input and Output are between S1 and S2 and can be interchanged
    To enable the power mosfets, connect G1 and G2 to GND
  • T ChapT Chap Posts: 4,059
    edited 2017-11-19 20:50
    Do you have time to put it in eagle and pm an invoice to make it worth your time? :)

    I can solder the small BGA but that link is easier for sure.

    Also if you have a non opto option for the 5 signals that would be slick.
  • Which one do you want in Eagle? ... the SO-8 or the BGA or both? Do you just need the Eagle library or do you want it incorporated into the schematic and board?

    "Also if you have a non opto option for the 6 signals that would be slick" ... I counted 5 signals... did I miss one? ... the latest BRD file was a non opto option
  • T ChapT Chap Posts: 4,059
    edited 2017-11-19 21:23
    encoder A/B = 2 totem pole out
    hall sensor A/B/C = 3 open collector. The signals are 6.35V unless the it is on that phase then the signal goes low. The 6.35V comes from the BLDC driver board, carried into to this PCB from the CAT5.

    I like the opto switch idea since if it is plugged in the wrong port the switch is not turned on and any voltages 5V/GND present on any outputs will do no harm. But as mentioned I'd like to get away from LED and degradation over years.

    We can do the SO-8. I have eagle on two computers, the newer version is PC and not paid so whatever you do I will re do in my older system. So if you find a generic S0-8 and use that I can redo it because I can't load your files in my older eagle professional Computer. Too lazy to upgrade and I can't get my old libraries converted for some reason. Short answer is you can use any So-8 based on the pinout of the part you like, I'll fix it easily. No need to lay out the board I just need to see a schematic. Thanks

  • T ChapT Chap Posts: 4,059
    edited 2017-11-20 00:05

    The Ap2411 POWER SWITCH looks interesting. It is designed for hot swapping USB power. SO-08 options.
  • What about an AM26C31 by TI.
  • That's an interesting part that might work for the encoder totem pole outputs. I'd need to see if it would tolerate 5v and GND getting accidentally connected to the outputs while the device was off. It certainly will help when I need to provide an extended cable version using differential outputs so thanks for the idea.
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  • You might want to check out the Avago AEIC 7273 or the 7272, 3487 etc.
  • How about color coding the plugs and sockets?

    Say red for motors, green for hall sensors, something like that?

    would not prevent mistakes, but make them more obvious?

  • GE and other medical device maker sometime use different connectors such as db9 male and female on board. Kinda hard to wrong connect them. Where that is cost or space prohibitive, they also put in the box connectors with a pin not in a position and the plug has an insert in it where the missing pin would be; that would allow 10 unique connections for a 10 pin plug that would be a bit harder to incorrectly connect things.
  • Frank,
    sounds like the OP is using 8 pin modular plugs. No blocking pin space available.
  • Modular connectors as universal easy-to-crimp connectors even for motors is not a really good idea. I've seen them on various devices and the contacts are really only designed for signals anyway.

    I prefer the modular screw terminals that you can plug in, especially the more compact 3.5mm variety and they come in all configurations and make it trivial to wire a cable in the field, all you need is a screwdriver. Although they are more expensive it seems to me that trying to use modular connectors because they can be handled in the field is creating a situation where the damage they cause by wrongly connecting them cannot be handled in the field.

    Circuit protection is one thing but prevention is better than a cure. Don't let those connectors get mixed up in the first place.

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