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    Short circuit protection in PCB design

    Dear All,

    I'd like to implement short circuit protection for my designs and have some doubts.. For example now I'm designing a vehicle installed meter with PIC microcontroller. From 12V to 5V I use Murata OKI-78SR-5 regulator and from that supply PIC and 5V to pressure sensor. I need short circuit protection for that 5V supply to the sensor.
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    I tested this circuit and noticed that it draws too much current when on. I modified it so that changed R2 to 10k, R5 to 10k and removed D2 as I don't need any indication if shorted. Circuit worked on breadboard with BC337 and BC327 transistors. My guestion is, how do you calculate the base resistor for this kind of situations when there are many components interacting with each other? I know how to calculate it for single transistor. In my tests I had only one led representing the load. I'd like to be sure that this modification is working in calculations as well. I liked this solution as it did auto recover when short was removed.

    The other thing, I will add TVS diode between supply and ground, before the murata and diode to positive side for protecting reverse polarity. Are there other things to consider?

    Thank you in advance.

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    Re: Short circuit protection in PCB design

    Hi,

    I canīt see how this can work as "short circuit protection" it rather is a short circuit indicator and shows when 5V voltage is too low.

    ****
    Every design should start with specifications.
    * How do you want it to operate? (OFF on overcurrent and stay OFF? When do you expect it to swrtch ON? Analog current limiting? ...)
    * Whatīs the max. sensor current?
    * Whatīs the sensor operating voltage range?
    * timing of the overcurrent system.
    Mind: many active sensors have capacitors installed at their supply. These may draw high current for a short time to be charged.

    Klaus
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    Re: Short circuit protection in PCB design

    Try considering L200 instead of 7805.
    https://uk.farnell.com/stmicroelectr...094204?st=l200
    I used these regulators for supplying 4-20mA transmitters with current limited to 30mA (in case of short circuit).
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    Re: Short circuit protection in PCB design

    Ok, so I should ditch that so called short circuit protection schema.. I wonder why it actually worked on the breadboard. I understood that it diverts the current through the 10k resistor instead when shorted. Anyways, is there any easier solution than using relay like in GreatScott example:
    https://www.instructables.com/id/DIY...nt-Protection/

    Maybe modifying that so that the transistor gate is controlled by PIC instead of button and by delay try to turn it on n times until leave it off if short is still present and indicate it to the user. I don't want the whole circuit render inoperable only this pressure sender line. That sender specs are 6-24VDC and 10-184ohm. Previously I have used the same sender in different application with 5V and in pic end voltage divider and it worked well.

    That murata regulator actually has short circuit protection if the circuitry itself gets shorted, but that's not what I'm after..



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    Re: Short circuit protection in PCB design

    Hi,

    There are many ways to do what you want, I think you originally didn't want it to shut down but you wanted the "short circuit protection" to divert excess current away from the sensor line - is that correct? If so, you could mess around with either of these circuits which are/were in the ST LM78xx datasheet in the applications section:

    Click image for larger version. 

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    An overcurrent shutdown circuit would be quite different, and off the top of my head I'd consider a PMOS pass device after the regulator rather than a relay; how you control the MOSFET gate is another matter I haven't thought through - maybe a comparator (with a reference voltage) driven by an op amp used to sense the current passing to the sensor, I think that is - downside - more parts but - upside - less messy than relying on random BJT Vbe turn-on voltages and less temperature-dependent. PICs have comparators in them, maybe, to save on one part?

    Look at - I think they're called - powerpath ICs and things like that if you want a small, ready-made and time-saving solution.

    - - - Updated - - -

    ...Figure 13 divides the current proportionally between regulator and transistor, e.g. 10% through former and 90% through latter - maybe that could be used to shunt excess current to ground via a resistor (rather than pass it to the load), not sure if it would still work, 'though, you'd need to test that theory.

    Figure 14 does the same as 13 and it turns off Q1 when current across Rsc creates sufficient voltage drop to turn on Q2. Afraid I can't remember what it does with the regulator. Hopefully another member could explain that part.

    - - - Updated - - -

    14 looks like it does hiccup mode functionality until the problem/short circuit current either goes away or power is removed, maybe not what you're looking for... The more the rusty mental cogs whirr, the more I think that that is a current limiting circuit.


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    Re: Short circuit protection in PCB design

    Not hiccup mode, all that happens is enough current passes through Rsc that it turns Q2 on. As the saturation voltage of Q2 is lower than Vbe of Q1 it shuts down most of the current through Q1 and leaves the regulator alone to pass current. It is up to the designer to ensure the residual current through the regulator and the heat it generates is safe.

    When designing short circuit protection there are two basic ways to do it, by current limiting and by foldback limiting. Current limiting is just that, it stops damage by only allowing enough current to flow that nothing serious can happen. A limiter is normally ahead of the regulator so the voltage remains constant up to the limiting point than falls under overload because the regulator is starved of input voltage.

    Foldback is a shut-down mechanism, it senses the output voltage or current and if it exceeds a threshold figure, it cuts the power off completely or partially. It implies there should be a re-start mechanism to restore normal operation if the overload is removed. Hiccup mode is one method of that, cyclically turning the power on again and rechecking if the fault condition is still there.

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    Re: Short circuit protection in PCB design

    I decided to use the op amp and power resistor circuit from GreatScott example. Then that signal to PIC input. PIC will cut the current by small 5V relay. Actually I programmed it so that it will bang it on 5 times in 10 seconds interval and if the short persists, it will turn it off permanently and indicate it through led.

    Then I got another quick guestion. LM358P is rated for 0-70deg C. Low temp limit kind of kills the idea of using it outside wintertime. Will it stop working in minus degrees or is the voltage drift just severe in those conditions? Any DIP package recommendations for similar spec OP amp with low temp limit -30C?



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    Re: Short circuit protection in PCB design

    LM358 will work fine at low temperatures but may go out of specification. I'm pretty sure it is a 'reject' LM258, or else the LM258 is a selected LM358 as they share the same internal design and specifications except temperature range. It means a batch test of LM358 may have shown at least one was out of spec but it doen't mean they all are or that they suddenly fail at zero degrees.

    Use at your own risk but it will probably be OK.

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    Re: Short circuit protection in PCB design

    Hi,

    If you use a simulation tool, it might be worth doing a temperature analysis with the LM358 to see if the simulation shows it doing anything undesirable/unacceptable. I think some parts do show their true behaviour and some don't so you have to use your judgement/common sense about the validity of the graphical results of such simulated tests.

    Do you have energy to waste there or does every milliAmp count? As you're using a PIC, you could add a subcircuit (and little code routine) that senses the temperature and does something as grubbily cheap as turning on a transistor and warming up a resistor or two placed strategically close to the LM358 under e.g. -5šC. No idea if in practice that is quite hard to implement .r.e. controlling how much resistor warms up over time, you'd hope not with a PWM-style signal to a MOSFET base.


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    Re: Short circuit protection in PCB design

    I came across PTC multi fuse. That seems like super simple and automatically resetting and no other components needed. Is that viable option? Only thing I noticed it has around ~2ohm resistance, but that's ok. Are there any downsides of those components, other than the resistance?



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    Re: Short circuit protection in PCB design

    Hi,

    Are there any downsides of those components, other than the resistance?
    How can we know? Did you give your requirements/specifications? We donīt know nothing about your application.

    Klaus
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  12. #12
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    Re: Short circuit protection in PCB design

    Hi,

    Depends on what is considered a downside. Slow to return to low resistance state (a few seconds, apparently), having high/considerable leakage current when "blocking", and staying at 4 times initial resistance after tripping could be viewed as downsides.

    This Wikipedia PPTC entry is concise.



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