Protecting 12v to 5v converter

Discussion in 'Automotive Electronics' started by geekoftheweek, Apr 27, 2016.

  1. geekoftheweek

    Thread Starter Member

    Oct 6, 2013
    42
    2
    I'm more or less wanting to know if my thinking is right...

    I'm trying to put together some protection for a 12 to 5 volt power converter that I'll be using to power some gadgets in my truck.

    12v-5v-converter.jpg

    The converter I'm planning to use is a Murata Power Solutions converter that can be found at
    http://www.digikey.com/product-search/en?keywords=811-2692-nd

    I've never dealt with tvs diodes before so I picked one with a breakdown voltage of 22V and clamp at 32V. The particular one is at http://www.digikey.com/product-search/en?keywords=497-4998-1-nd

    For R1 I have chosen a 5 ohm 30 watt resistor. I found on the datasheet for the converter that at it's lowest input voltage of 7 it will draw 1.16 amps at maximum output. I used 7 volts and 2 amps of drop across R1 to come up with 3.5 ohms... then converted to 14 watts. The second least costly resistor on digikey happened to be the 5 ohm 30 watt.

    The capacitor I'm not too sure. I'm basing this design off of another thread that explained this set up, but didn't show it. I don't remember what was recommended for it. I was thinking around 22000 mF if I remember correctly.

    My logic may be a bit off and I'm wondering if I'm over sizing things. The converter datasheet recommends a 2 amp fuse for protection on the input side so I'll have a 2 amp ATC fuse somewhere before all of this yet.

    I also just remembered a Schottky diode was mentioned... so I was thinking 150V 3A like the one here http://www.digikey.com/product-detail/en/stmicroelectronics/STPS3150RL/497-3216-1-ND/654568. If I remember that should come first right?

    Thanks for your thoughts!!
     
    Last edited: Apr 27, 2016
  2. ScottWang

    Moderator

    Aug 23, 2012
    4,856
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    Since the input range of DC to DC is 7~36V, so the R1 and D1 is no need.
    How much current will be drawing of your 5V load?
     
  3. Papabravo

    Expert

    Feb 24, 2006
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  4. geekoftheweek

    Thread Starter Member

    Oct 6, 2013
    42
    2
    Thanks Papabravo! The links helped clear up a few things. You mentioned the very reason I'm even asking this in the first place... I know enough that I need to find a way to deal with them, but not enough to know how to go about it. I like the LT4356 mentioned in the stackexchange page, but would like to go simpler if possible.

    What kind of rule should I follow to determine tvs parameters? I know I want to limit to 30 v to be on the safe side and would like to keep the other voltage specs as close to 30 v as possible... more or less so the tvs just has to handle the big spikes when they happen. Do I actually need a resistor and how would be the best way to size one if needed? I kind of more or less aimed for worst case with the calculations I've already done. I would assume the resistor itself won't serve much more than a way to keep from cooking the tvs and Schottky since it won't be enough resistance to actually affect the converter power supply if I'm thinking correctly... and since the converter is designed with a pretty wide input range it would work at whatever voltage is available after the resistor right?

    Scott... I haven't sat down to calculate what kind of load I'll be pulling. At the moment I'd be surprised if it's more than 200 mA with what I have working so far, but I'm planning on expanding. Rough guess is probably just under 1 amp by the time it's all done.
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,178
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    I'm not an automotive engineer and I've never had to actually do this. The reason I know about it is that I came close to having to do it once to write the proposal, but somebody else won the contract. I believe there is a great deal of information on this subject in the SAE (Society of Automotive Engineers) archives from the 1980's and 1990's. This is the era where the car makers strove to reduce the amount of "home run wiring" by replacing it with network cabling and lot's little microprocessors all over the vehicle. Each one of them had to be protected. It's no good asking the vehicle makers; for them it is all proprietary.
     
  6. geekoftheweek

    Thread Starter Member

    Oct 6, 2013
    42
    2
    Thanks again Papabravo. I'll be checking that out. A couple years back I was forced to make a career change which landed me a job as a mechanic. The last six months or so I've done quite a bit of research into the particular vehicles I work with... I never realized just how computerized things have become. My electronics and computer hobbies have helped in so many ways already.

    I was kind of hoping for a one size fits all type answer, but believe with the background I have so far along with your suggestions I'll be able to come up with what I'm after. I've been using a hacked cell phone charger to power this for the last two years and I haven't had any problems yet with this particular charger... I just kind of wanted to build something cleaner looking and repeatable. (I've been told this particular model charger is discontinued and I've had problems with the ADC's holding steady with other chargers)
     
  7. RichardO

    Well-Known Member

    May 4, 2013
    1,238
    385
    Check data sheets for old National Semiconductor voltage regulators intended for automotive applications. (I don't have any numbers at my finger tips right now).
    They data sheets talk about the importance of using the right kinds of capacitors for the temperature extremes in an automobile. For example, electrolytic caps can freeze and loose their capacitance. This can cause problems with some regulators -- especially low dropout ones.
     
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