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Thread: Electronics Question: how to prevent from frying my board

              
   
   
  1. #1
    Senior Member __Tango's Avatar
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    Electronics Question: how to prevent from frying my board

    Hey folks,

    this should probably go on an newbie electronics forum as this really isn't elmoto specific...but anyway here goes. My traction pack is 120VDC, I have an arduino and a hall sensor that needs 8VDC with very little current draw (100-200mA). I was previously using a Vicor DC-DC to convert the 120VDC to 24VDC (that powered my kelly controller on an isolated circuit), and then used a buck power converter chip (TI LM2825-ADJ) to get it to 8VDC.

    I've changed my controller to a sevcon so i don't have the 24V DC-DC anymore, and the controller runs off of the traction pack and is NOT isolated. I want to be able to do 2 things:

    * use a TL783 voltage regulator to convert the 120VDC to 8VDC to power the arduino and hall sensor.
    * use a simple voltage divider to read the battery voltage on one of the arduino analog IO pins (0-5V).

    I get the basic concepts on how to do this, but are there protective components i should put in the system so if there's a voltage spike of some sort, it doesn't fry anything. I was planning on putting a 1A fast acting fuse on the 120VDC input. I'm not sure about anything else.

    Thanks.
    EVs: 2007 Zapino Scooter, Honda VF500 Conversion (electriceptor.wordpress.com), Red Nissan LEAF SL (Arrived!)

  2. #2
    Senior Member Hugues's Avatar
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    Hey

    I'm not responding to your question but...
    Why you don't use a dc-dc converter anymore ?
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  3. #3
    Senior Member __Tango's Avatar
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    Quote Originally Posted by Hugues View Post
    Hey

    I'm not responding to your question but...
    Why you don't use a dc-dc converter anymore ?
    haha. yeah, good question. The short version is I'm wondering if it's possible (and what i'd need to do) to power the device with a $3 (USD) chip vs. a $120 DC-DC.

    The long story is that it started with me rebuilding my canbus logger by getting it to talk to the sevon vs. the kelly controller. I'm currently running the arduino and friends from the 24V that the controller spits out to run the Sevcon "ClearView" display tool. Then there's a buck power conversion chip that converts the 24V to 8V that I run the arduino and hall sensor off of. The sevcon 24V circuit only supports something like 50 or 100mA, and i'm worried about pulling too much current. I'm also temporarily running a ClearView as well as my arduino and sometimes the arduino or ClearView resets while riding, and i think this may be due to too much power draw.

    As with any other coding or building project, i've had some serious feature creep too...I've upgraded the LCD to one with a keypad so i could build a menu system. I added a WiFly (WiFi) card so it can upload data remotely, i'm learning Eagle and building a custom arduino PCB/shield to make all the components fit cleanly and designed and 3d printed a custom box to hold all of the components. I figured since i'm doing all of this, why not also add the ability to measure the pack voltage using a simple voltage divider. Since now this means i'll have pack voltage coming into the box (or near the box) why not just try to do the power conversion to 8V directly.

    even if i don't power the whole setup with the pack voltage, I still want to try to add the voltage divider to measure pack voltage, so my original question about protection circuitry still holds...
    EVs: 2007 Zapino Scooter, Honda VF500 Conversion (electriceptor.wordpress.com), Red Nissan LEAF SL (Arrived!)

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    Moderator Nuts & Volts's Avatar
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    My thinking is that if you have an isolated 12V (DC/DC or 12V battery) then you should be ok. I would then fuse the resistor circuit somewhere between the HV tap and then another fuse between the resistor tap (middle of the resistors) and the Arduino pin. This will protect again HV being shorted (if both resistors fail) and keep HV off of the Arduino? I'm not sure a fuse will be fast enough. The fuse across the HV should protect things if a short were to occur and your batteries won't care about the short for the 1sec it may see it.

    After googling for awhile I don't see any good way of measuring the HV with full isolation cheaply. Is there anyway to isolate the 0-5V output of the voltage divider without affecting the signal? Makes sure the Arduino board is always safe, but still leaves potential for a HV short which is less likely to damage anything? And you maintain full isolation between 12V and HV.

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    Seņor Member podolefsky's Avatar
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    One possibility would be to use an optoisolator as a voltage-controlled resistor. Elithion has an example of using one to take the 0-5V signal from their BMS to control a 0-5V throttle input. Not that different from what you want to do. Main problem is they're not that linear - the resistors Elithion spec get you close, but not perfect. You'd need to create a lookup table to convert output values to actual voltage readings.

    http://lithiumate.elithion.com/php/curtis_inverter.php


    H11F1M optoisolaor datasheet:

    https://www.fairchildsemi.com/ds/H1/H11F3M.pdf


    Otherwise, you could go to a lower value fuse, which will be more sensitive. If your voltage divider is 5k, then with 120V you have <0.03A. I think you can find regular size fuses as low at 0.125A.
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    Senior Member ARC EV Racing's Avatar
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    For measuring the pack voltage via the adc you'll need to have a common reference, so direct measurement won't work if your arduino is isolated. Using an opto as an isolated voltage controlled resistor is interesting though.

    I've experimented with HV linear regulators in this application and my advice is to wear eye protection. Those things go off like a bomb when you stress them with too much voltage or inrush. At 120v you're pretty close to the 125v max of the reg so shrapnel is a possibility.

    You could use an NTC thermistor for inrush protection but you it won't protect your circuit if you power cycle quickly, it has to cool down to do its job next time power is applied.

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    Empulse R #24 frodus's Avatar
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    What I've done on a project, is only isolated the RS232 that I wanted to use, and kept everything else with ref to pack negative. Just make sure you aren't somehow linking the 12V isolated side to the HV pack somehow. A small fuse should help to make sure you don't pull much current through the voltage sense, but you should be mostly ok.
    Last edited by frodus; 18 January 2014 at 1249.
    Travis

  8. #8
    Senior Member __Tango's Avatar
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    Quote Originally Posted by ARC EV Racing View Post
    For measuring the pack voltage via the adc you'll need to have a common reference, so direct measurement won't work if your arduino is isolated.
    Can you explain more?
    EVs: 2007 Zapino Scooter, Honda VF500 Conversion (electriceptor.wordpress.com), Red Nissan LEAF SL (Arrived!)

  9. #9
    Moderator Nuts & Volts's Avatar
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    Quote Originally Posted by __Tango View Post
    Can you explain more?
    **I think** if your Arduino (ADC) is powered by an isolated (from HV pack) source then the measurement voltage is simply floating. You need a common reference (pack ground needs to be tied to Arduino ground) so that the Arduino actually sees the 0-5V and not just a floating voltage. Hope that makes sense.
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  10. #10
    Senior Member ARC EV Racing's Avatar
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    Quote Originally Posted by Nuts & Volts View Post
    **I think** if your Arduino (ADC) is powered by an isolated (from HV pack) source then the measurement voltage is simply floating. You need a common reference (pack ground needs to be tied to Arduino ground) so that the Arduino actually sees the 0-5V and not just a floating voltage. Hope that makes sense.
    Yep that's what I was getting at. Actually I probably shouldn't have used reference when talking ADCs, that's what scales the bit count. You need your grounds to be common as N&V said.

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