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Over voltage protection.

Discussion in 'General Electronics Discussion' started by sureshot, Feb 25, 2016.

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  1. sureshot

    sureshot

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    Jul 7, 2012
    A quick question here.. How affective in terms of speed, and function is a fuse and zener diode protection against over voltage. This compared to adding an SCR and it then being a crow bar circuit. So my question is, will a fuse and zener be suitable for psu circuit over voltage protection ? Thanks for reading any help appreciated.
     
    hashim ibn othman likes this.
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    A fuse is slooooooow compared to an electronic protection.
    Which on to use depends also on the nature of the overvoltage:
    • short high voltage pulse (e.g. surge, ESD)
    • long duration medium overvoltage (e.g. excessive mains)
    In a typical setup the overvolateg protection will be electronically by a parallel circuit (SCR crowbar, suppressor diodes etc.).
    The fuse will be in series with the overvoltage protection.
    • Short pulses will be limited by the parallel circuit without the fuse acting (did I say a fuse is slow?).
    • Long duration overvolateg will activate the parallel protection circuit, too. This time, however, the current drawn by the parallel circuit will lead the fuse to blow due the longer time it is active.
    Remember that any overvoltage protection by a parallel circuit works by making use of the series resistance in the wiring leading to the circuit. When the parallel overvoltage protection is activated, it draws a high current which drops the overvoltage across the series resistance of the wiring. The lower the wiring resistance, the more current you need to achieve a suitable voltage drop. You may have to add extra resistance to achieve the desired voltage drop.

    Note that in any case even a fast protection circuit will pass some overvoltage in the form of a pulse before it becomes active. Typically you add a low pas filter after the protection circuit to keep these short pulses away from the rest of the circuit. The mains filter used anyway in power supplies ist mostly sufficient, so make sure the filter is located after the protection circuit.
     
    Lovetas, Gryd3 and Tha fios agaibh like this.
  3. sureshot

    sureshot

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    Thanks for your reply, think I've got it, on the input (mains) I was going to use an adiqate fuse, and MOV and NTC for soft start. This question was for over voltage on the output. So 12 Volts DC powering radio equipment, in the event of a fault situation not to exceed, or over OVP kick in before getting above 14.4 Volts DC and typically a bit less if possible, thank you again for your help here.
     
  4. cjdelphi

    cjdelphi

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    You'd normally have a MOV hot side / 110/240ac but on the 12v side, place a 12v TVS Diode across your 12vdc out
     
  5. sureshot

    sureshot

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    Jul 7, 2012
    Ok yes the MOV across the mains input, and some filtering, and yes suppression diode on the secondary output. Thanks !
     
  6. sureshot

    sureshot

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    Jul 7, 2012
    Think I will add a crow bar, rather than just a zener and fuse combination.
     
    hevans1944 likes this.
  7. hevans1944

    hevans1944 Hop - AC8NS

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    Make sure the crowbar sinks enough current to blow the fuse or to trip a reset-able circuit breaker.
     
  8. sureshot

    sureshot

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    Jul 7, 2012
    Thanks, yes I will, I'll have to look at resettable verses sacrificial fuse, resettable sounds interesting out of the two.
     
  9. Tha fios agaibh

    Tha fios agaibh

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    Although a fuse is slooooow, a resettable breaker is even sloooooooower.
    Another downside is they are not as precise as a fuse.
     
  10. sureshot

    sureshot

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    Jul 7, 2012
    Ah ok might not be such a good idea then, thanks for the info, quick blow fuse and crow bar circuit it is then.
     
  11. cjdelphi

    cjdelphi

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    Oct 26, 2011
    A TVS diode is /the fastest/ way to protect sensitive circuitry, the TVS diode will react in picoseconds...
     
  12. hevans1944

    hevans1944 Hop - AC8NS

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    Doesn't really matter how fast or slow the fuse or circuit breaker is if you are using a crow-bar for protection.

    If you are crow-barring the protected circuit, the crow-bar current will have already reduced the supply voltage to a safe value, and it continues to do so, no matter how long it takes for a fuse (fast-acting or slo-blo) to open or a circuit breaker to trip.

    You just need to make sure that during the time interval from when the crow-bar trips to when the fuse blows or circuit breaker trips, the excessive current through the crow-bar (typically an SCR is used for this) doesn't cause any up-stream circuit damage to the power supply. There is always excessive current, beyond normally-operating circuit current, when the crow-bar trips. You just need to make sure the current isn't too excessive (perhaps insert a current-limiting resistance from power supply output in series with the crow-bar), yet is still large enough to eventually cause a fuse to blow, or a circuit breaker to trip, and turn everything off.

    Of course, if the "fault" current created by the crow-bar doesn't overly stress any components, it doesn't matter whether this fault current blows a fuse or trips a circuit breaker... ever! The crow-bar has done its thing, the protected circuit has power removed from it by the crow-bar, and now everything awaits for a reset to normal operation by something or someone turning the crow-bar conducting element off again. It is not uncommon to have crow-bar circuits that automagically reset themselves after a period of time without having fuses blow or circuit breakers trip.

    It all depends on what you are trying to do.
     
    Tarija and Tha fios agaibh like this.
  13. sureshot

    sureshot

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    Thank you for this in depth description, I think I will read up a bit more on the operational elements, and what's going on in the crow bar circuit under different circumstances. My knowledge is limited, so a good read up first before finding a suitable circuit to build.

    With a series resistor in the protection I don't won't to limit the working usable current, typically 20Amps maximum of a 30Amp rated circuit. The plan is to build the psu circuit so it runs no more than 2/3 of its maximum capability.
     
  14. hevans1944

    hevans1944 Hop - AC8NS

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    Agreed. Always do some research before diving over the cliff to discover the hidden rocks under the water. This is called "due diligence" in lawyer-speak. You might even decide not to jump, and hire someone else to jump instead.:D

    Power supplies are normally used anywhere from a minimum load (which could be no load at all) all the way up to their rated maximum load without derating if properly cooled. Why? Because the expense of constructing a power supply depends on how much maximum (or rated) power it is expected to provide. No one wants to pay for power capability that will never be used. Still, it is wise to leave some "headroom" if you expect to increase the load at a later date, for example by adding another server to a server farm.

    The difference in cost between a 20 A rated supply and a 30 A rated supply is not likely to be much, but there will be a difference. If you never expect to need 30 A, then you should build or purchase the 20 A PSU. IMO nothing is gained by operating a 30 A PSU at 2/3 of its maximum rated capacity. That said, for home-brew projects it is often a case of using what you have, and what you have may be waaaay more than what you need. Go ahead and use what you already have.

    I acquired a humongous power transformer with large stud-mounted full-wave bridge rectifiers set into a large convection-cooled heat sink. The whole thing weighs about seventy pounds or so and produces about 15 V DC, unfiltered, at several hundred amperes without breaking a sweat. I mounted it on a wheeled cart, along with a Variac and 50-0-50 A meter for use as a lead-acid battery charger. It is fitted with heavy-gauge, very flexible, welding cables and large alligator clips salvaged from a car jumper set. So one day I decided to find out if it had enough current output to start my car. It did, but I forgot to remove the zero-center current meter from the circuit. Poof! Lost the internal meter shunt. One of these days I plan to disassemble the meter (the movement itself seems to have somehow survived!) and replace the internal shunt, but for now I just use it to re-charge car and motorcycle batteries, sans Variac and meter.

    The point of that story? Well, low-current, 1A car battery chargers sell for less than thirty bucks everywhere, and they do the job of re-charging a "dead" automobile battery overnight. See photo below.
    upload_2016-2-26_14-10-15.png
    Why would I spend money to purchase one of these when my 70 pound kluge with its 300 A output can do the same thing?

    Good luck with your PSU design.
     
  15. sureshot

    sureshot

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    Jul 7, 2012
    Thanks ! Its built for fun really, and to power radio gear, typically all of it. Yet its the amplifier that draws all the power at 100 watts. For future proof and the option to use 250 watts, I don't actually get the RF output rated by the manufacturer, so the 100 watt unit gives me around 80 watts or there abouts. I might never use 250 watt amplifier, but the power would be there if I wanted it. In years gone by I've used 400 watts and purchased a huge smps psu especially to power it. But ive been converting a few ATX units, and old server psu's with mixed results, the power is always good from these. But as its for RF use the noise can become a problem at high currents, the ripple noise.

    So having built a couple of low current linear power supply's, nothing exceeding 5 Amps, I thought its about time to build a high current unit. And searches lead me to the voltage regulator emitter follower circuit, its parts count and simplicity looked interesting for what it was. Then my question was.. Does it work ? After building two versions, a single and double transistor version, I discovered it works quit well (the double transistor version in use 24/7) This is still fine 1 month in now.

    So its time for the six transistors version now. My searches lead me to many circuits, some using dedicated IC's that look very interesting, and having looked inside retail linear supply's over the years I recognised the circuit in these units.

    Yet I was taken with simplicity, parts count, and cost. After trying the two transistor build, and its performance over all, I think it could do the job. After measuring voltage, current, and heat, it voltage drop under load improved with the addition of the second transistor, by a good margin. So looking at this so far, and having tried it, I'm confident its worth a try to build a six transistor version.

    From the original TIP2955 transistors, I started looking at more powerful alternatives, that lead me to the MJ11015 and the MJ11033 transistors. I've ordered 6 x MJ11015 transistors now, the MJ11033 transistors where a bit to expensive for additional power I would never use. So back to this thread, that's what lead me to the protection circuit, the mains input poses no real problems, but over voltage on the secondary final output would equal an expensive fault in an over voltage situation. I once had a retail linear go high voltage, instead of 13.80 Volts on its rated output, it developed a fault and put 23.00 Volts into my radio gear destroying it.

    After a good look a figured a potentiometer wiper lost contact withcits track, this sent the voltage high, I've since learned if the designer had tied the other leg of the potentiometer to ground it might not have destroyed my radio gear. So that's it really, I got into doing different things with modifying reclaimed smps units and building simple linear psu units.

    That's a big transformer and bridge rectifier you have there ! So now a bit more research on the crow bar circuit, and sourcing a decent enclosure. I am a bit of a perfectionist and can't stand ruff and ready final results, its got to be as good as I can build it inside and out. Thanks again for the help.
     
  16. hevans1944

    hevans1944 Hop - AC8NS

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    Yep. But since it was free, I had to find something I could use it for, just to justify the labor of lugging it home. I had some slotted angle-iron and plenty of bolts and nuts handy, so I cobbled together a "frame" for it and added four smallish wheels and a bungee cord to drag it around the garage. Most of the time it sits in a corner gathering cob webs, but when I need it... well it sure comes in handy during long cold winters that take their toll on lead-acid batteries.

    P1030241.JPG

    It's impossible to see the transformer because the "welding cables" are wrapped around the base. And I just now noticed they aren't welding cables but really just heavy power cord cables. I think I took my 4 AWG welding cables to work one day and someone walked off with them. <sigh> Well, I don't have a welder at the moment, so no need for welder cables. I can sometimes find them at Mendelson's for a good price. I really like welding cables: they have a LOT of very small strands twisted together in a fashion that makes for a very flexible cable, even in sub-zero temperatures.
     
    sureshot likes this.
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