I need protecting - Crowbar Protection Circuit Help !

Discussion in 'The Projects Forum' started by pscammp, Mar 3, 2016.

  1. pscammp

    Thread Starter New Member

    Feb 22, 2016
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    1
    Hello all, Here I am again looking for some good advice from you wonderful people.

    I am soon to build a ' 75Ah deep cycle marine battery' based power supply to power astrophotography equipment,
    I will be using a few Buck / Boost DC-DC converters to ensure the equipment gets the correct voltages as the main
    battery will probably drop below 12V as it discharges and could be as high as 14.5v at full charge. The converters I'm
    using are as follows:

    http://www.ebay.co.uk/itm/391241142973?_trksid=p2060353.m2749.l2649&ssPageName=STRK:MEBIDX:IT

    The output voltages for the converters will range from DC 12V to 14.5V and possibly one set to 19.5v to power a Laptop
    although the laptop supply is not definite yet. Load currents will range from 0.5A to approx 5A IF laptop was used.

    The outputs of all the converters will be appropriately fused for their individual equipment and will then go to XLR conn-
    ectors situated on the side of the enclosure where custom made leads will then supply each of the equipment with what
    they need.

    So all of above describes the basic foundation so now lets move on to what I'm here to get some assistance on..........
    Overvoltage Protection......

    The above mentioned converters, although they have their own built in fuse, dont have any kind of over voltage protection
    on the output side and two of my pieces of equipment have a value of £1000 or above. What I'd like to do is to factor in a
    Crowbar circuit on each output to protect it's load from an overvoltage situation should one of these converters go haywire.

    My initial idea is to get little black projects boxes from Maplin / RS and put the crowbar circuits in them and have these little
    boxes in-line on the supply cables a little way from it's XLR connector as there's not a lot of spare room in the enclosure.
    Does this Sound feasible ????

    Obviously there are loads and loads of different Crowbar / Active Crowbar circuits on Google using all sorts of different
    idea's and components, and, I see mention here & there or 'Overheating' (the crowbar circuit I assume). I wouldn't like
    the thought of overheating situations to begin with, a fuse blow would be nicer and safer, but as there is so many options
    I really don't know which way to go.

    In your infinite wisdom ow masters of all you convey - Would someone be able to suggest an appropriate Crowbar circuit
    which would help me out here ??????

    So many Thanks and not enough time.

    Regards
    Paul
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The easiest way to protect from transients or loss of regulation is the good old MC3423 chip and a SCR. The chip is programmable to select your desired trip voltage. Then it fires the scr until your fuse pops.

    http://pdf1.alldatasheet.com/datash...MI/MC3423/+02183UllREMIcPhYzvC+/datasheet.pdf

    If you don't want to buy any old stock from China, drop me a PM, I have a boatload of the genuine motorola chips from the 1980s.
     
    Sinus23 and pscammp like this.
  4. crutschow

    Expert

    Mar 14, 2008
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    You can also use a TL431 reference IC to generate the voltage trip point as part of a crowbar circuit, but that would be somewhat more complex than GopherT's circuit.
     
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Use a TL431 programmed zener and a Triac, set the trip voltage with the two resistors R1,R2.
    Trip voltage = 2.5V x ( 1+ R1/R2)


    The load R3 can be 1k, use a BT137 triac or similar, terminals MT1 to positive, MT2 to negative.


    https://www.google.co.uk/search?q=tl431+crowbar+circuit&prmd=isvn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjuqdbI4abLAhVJOJoKHflrAIkQ_AUIBygB&biw=360&bih=511#imgrc=xG-onZtt9b8DIM:


    Or a simple zener diode and C106d Thyristor circuit


    https://www.google.co.uk/search?q=tl431+crowbar+circuit&prmd=isvn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjA34T75KbLAhVjMZoKHflsC2YQ_AUIBygB&biw=360&bih=511&dpr=3#imgrc=keFxYp-lXekjzM:
     
    Last edited: Mar 4, 2016
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The big question is, would a fuse pop soon enough to protect sensitive/costly equipment downstream?
     
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    That's a good question. The regulator could be protected on the unregulated side with another transient protection circuit (set to appropriate voltage).

    Devices include Transient protection diode (picosecond response)
    Metal Oxide Varistor (nanoSecond response)


    As I think about this there, could be additional protection by adding an RC network with a few micro-second (approximately) time constant.
    Also, these devices should be placed close to the regulator - for additional protection, a low value resistor (sub ohm) could be added in series with a decoupling capacitor shunting to ground after that resistor. The time constant of the RC should be about the time needed to pop the fuse. Good luck.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    As long as the SCR is large enough to carry sufficient current (as determined by the source and fuse resistance) to clamp the voltage from the source until the fuse blows, then the equipment should be protected.
     
  9. pscammp

    Thread Starter New Member

    Feb 22, 2016
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    1
    Hello all,
    Sorry I have not been back here sooner but I have to admit I completely forgot about this post LOLOL

    Really sorry and thanks for all the advice and help !

    Anyway, project is well under way and all electronics are installed and wired up in my case. Almost fully functional except for 3 of 6 of the DC-DC converters NG on arrival and installation. One blew the 10A fuse immediately the power was supplied to it's input, the other two seem to be pulling more watt's than the 'good three' at idle and getting a little warm too. Supplier has sent me three more so hopefully I'll be fully up and running very soon, this happens now and again so they said.

    My god, what a choice you all have given me, which one do I choose ?

    GopherT - Like the idea of an IC but I'd need a full circuit diagram of how to hook the IC up with any supporting components, not to mention component ratings and part numbers etc. I tried to PM you Yesterday but cant find any way on this forum which will allow me to do so LOLOL Your profile said it was restricted ! Would it be easy to set individual voltages for each of the 6 Crowbars using the IC option ?

    Regards
    Paul
     
  10. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    If I had thousands of dollars in equipment I sure wouldn't power it with the cheapest/low quality dc/dc one can get..
    You've already got a 50% failure rate..
    I'd buy quality dc/dc converters that will have all the functionality you need built in and sleep well at night..
     
  11. pscammp

    Thread Starter New Member

    Feb 22, 2016
    23
    1
    mcgyvr,
    Although I agree with you I also have to live within my means and make the best of the
    situation. Certainly not the cheapest DC-DC's the ones I got have a very good reputation
    and reviews generally and their physical size, reviews and price helped me decide.

    Bare in mind that they would need to be as small as reasonably possible for portability,
    would you be in a position to suggest such a device you would consider appropriate so
    I can consider in the future ?

    Regards
    Paul
     
  12. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Hopefully you add your review and state that you had a 50% DOA rate so far.. ;)
    Sadly a lot of reviews now are fake/paid for.. by the very sellers of the junk..
    Or ok right out of the box but don't last long due to overheating,etc.... very few will come back after 2-3-6 months later to state its dead/update their review..

    But I understand.. Not sure what your "crowbar" addons are going to add to the total cost..

    If I get a chance I will see if I can find something "cost effective but still quality"...
    digikey/mouser/newark would be my first stop..
     
  13. pscammp

    Thread Starter New Member

    Feb 22, 2016
    23
    1
    Thanks Mcgyvr,
    I know its a mine field out there which is why this forum is a very valuable resource for me. The DC's I have were only about £12 each, not a massive outlay should they fail in time just so long as I have protection built into the system as best I can my equipment should survive.

    The DC - DC's I have at the moment measure 78mm (L) x 45mm (W) x 15mm (H), Height is not really an issue, couldn't go much wider as I may be pushed for room as you'll see in this pic of current progress, the DC's have the No 9 on the pic as if I needed to tell you.

    https://dl.dropboxusercontent.com/u/4938813/Progress.jpg

    The best I can do for the moment is add the crowbars (which will be quite cheap), even If I had quality units like you suggest I would still have a crowbar in line to the equipment just for piece of mind. This will at least give me over voltage protection should the worse happen for my current situation. The fused input to the DC's and the fused output side too should give me cover there and once I'm fully up and running an assessment of the current being drawn on each circuit under real world conditions may also allow me to tighten the fuse ratings a bit more to add a little more protection.

    I'm no electronics expert, this is just hobby stuff for me, but learning, by mistakes sometimes, is the best I can do and make the best of the situation I have and heed the good advice I can get here.

    If you do see a quality DC converter at a reasonable price that's input will allow max 14.4v down to around 11.9v min (Leisure batteries don't like to be discharged below this level) and be capable of outputting in the range of 12V - 20v and able to support a 6A max output then I'm all ears.

    Many Regards
    Paul
     
  14. pscammp

    Thread Starter New Member

    Feb 22, 2016
    23
    1
    OK so the decision has been made.....

    I'm going to use the following circuit:

    https://dl.dropboxusercontent.com/u/4938813/Crowbar_Circuit.jpg

    Taken from the web site:

    https://en.wikibooks.org/wiki/Practical_Electronics/Crowbar_circuit

    I have 1W zener diodes on the way which are all 1.0v above their respective voltage outputs
    from my box. I also have thyristor's on their way of the following type:

    http://uk.farnell.com/on-semiconductor/2n6509g/thyristor-25a-800v-to-220/dp/9557202

    Voltage output's from my DC-DC's will be:
    12v USB Hub (3A Fuse)
    14v Mount (4A Fuse)
    12v Camera (4A Fuse)
    12v Focuser (1A Fuse)
    19.5v Laptop (6A Fuse)
    12v (SPARE) (3A Fuse)

    So ref my outputs above and components on the wiring diagram:

    C1 Capacitor Does this value 100nf look ok - What type of cap will it need to be ?
    C2 " Value ok ? Again, what type ?
    R1 Resistor 1K look ok ? and should I go for a particular type and wattage for my needs ?

    ref the Schottky diode SD1, I was going to get the 15A version of this:

    http://www.ebay.co.uk/itm/Low-Volta...hash=item3ac2247a20:m:mrG9v6GHomILaZSWEl8q9Fw

    Theoretically the 15A / 45v version should do the trick.....Yes ?

    or will I get away with 10A ones much cheaper do:

    http://www.ebay.co.uk/itm/10SQ050-1...045342?hash=item28152e439e:g:E0oAAOSwgApW~rJj

    Thanks in advance for any help

    Regards
    Paul
     
    Last edited: Apr 27, 2016
  15. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    If you use a TL431 reference along with a PNP transistor to fire the thyristor you wil get much better accuracy of the trip voltage.
     
  16. pscammp

    Thread Starter New Member

    Feb 22, 2016
    23
    1
    kubeek,

    can you suggest a circuit link I can see with the way it would be linked up ?

    Paul
     
  17. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Something like this, choose R1 and R2 to have 2.5 in the midpoint at the trip voltage.
     
  18. ian field

    Distinguished Member

    Oct 27, 2012
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    The TL431 is easy in the link posted by ronv, but traditionally a crowbar on DC uses an SCR - the TL431 can get a little tricky in that configuration.

    Traditionally - an off the shelf standard voltage zener to pull the gate is usually close enough for most applications.
     
  19. ian field

    Distinguished Member

    Oct 27, 2012
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    You could leave out R5 because as soon as the transistor starts to conduct; the SCR will short out its supply.

    I'd put about 1k from gate to GND to minimise noise and false triggering.
     
  20. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    If the source has high enough resistance so the few doesn´t blow, then it limits the current through the gate so it doesn´t destroy the thyristor, especially with higher fuse rating and higher voltage.
    Right, forgot to draw that one. Also a resistor between the gate and ground is a good idea to prevent triggering due to leakage of the pnp.
     
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