How can a gas tube rated to breakdown at 75V protect a 12V bus?
The TVS diode shown in the link below is intended to protect 12V loads.
http://krdev.littelfuse.com/products/TVS+Diodes/Axial+Lead/3KP/3KP15A.html
Fuses opening and relays de-energizing are extremely long duration events (as
in milli seconds)
they cannot protect against fast voltage transients that occur in fractions of
a micro second.
This is my story and I am sticking to it :-)
Don't mean to offend anybody.
Cheers
Ahmet
> Date: Tue, 9 Feb 2010 15:12:39 -0500
> From: [email protected]
> To: [email protected]
> Subject: Re: [Liveaboard] Wind generators
>
>
>
> PTC auto re-settable fuse, Newark Electronics PN 04H8118 Mfr PN
> Tyco, SMD075
>
> 15V DC 5W zener diode, semiconductor PN 1N5253B Newark PN 09F4308
>
> The equivalent surplus gas tube surge suppressor, Littlefuse PN
> CG75L Newark PN 44F3903
>
>
> http://www.newark.com/littelfuse/cg75l/gas-discharge-tube/dp/44F3903?in_merch=Popular%20Gas%20Discharge%20Tubes%20%28GDT%29&MER=PPSO_N_P_GasDischargeTibes%28GDT%29_None
>
> http://get.adobe.com/reader/thankyou/xpi/?installer=Reader_9.3_English_for_Windows&a=McAfee_Security_Scan_Plus&a=ARH&a=Acrobat.com&a=Air_Installer
>
>
>
>
>
>
> First let me say a bit about the wind genny surge suppressor.
>
> The circuit is constructed like this;
> The two (or even four) gas tubes are wired together in parallel and
> placed so that one side is connected to the positive supply wire (or
> charging wire in the case of a wind genny) and the other side is
> connected to the negative wire.
>
> Down stream from the gas tubes or that is toward the device, the two
> zeners should be connected so that their cathode (marked by a bar or
> stripe at one end) points toward the POSITIVE incoming supply.
>
> Essentially, that's it for the wind genny, just a very basic and robust
> surge suppressor.
>
> The way it works is simple. Gas tubes do nothing UNTIL their 'trigger'
> voltage is reached, then they act like a dead short, conducting power
> straight to ground. They can take massive amounts of power and very high
> voltages again and again and not degrade.
>
> Gas tubes have a pretty high voltage rating before they begin to conduct
> though, something like 60-90 volts or so is about the lowest so
> something further is needed for 12 volt systems protection.
>
> That something else is a zener diode.
>
> Zeners behave in a similar manner to a gas tube, that is when connected
> so that their cathode is toward positive, they will not conduct until a
> certian rated voltage is reached, then they will conduct and 'clamp'
> the voltage to the value they are rated at, so if you have a 15V zener
> it will not do anything much until 15V is reached, then it will conduct
> any power above what is making that 15 volts.
>
> Now with both gas tubes and zeners together, the zeners begin to conduct
> first and then if the voltage rises high enough the gas tubes kick in
> and take the brunt of the blow, protecting the zeners so that they can
> hold the voltage to a lower level.
>
> I DID NOT use MOV's (metal oxide varistor) which are commonly used in
> surge protectors because they DO degrade with each successive "hit".
> Gas tubes do the same thing as MOV's, just cost more but are far more
> robust.
>
>
>
> Now I will talk about a trade off;
>
> The wind genny surge suppressor will not have a PTC auto fuse device
> installed but instead will rely on the use of previously installed
> circuit breakers or a traditional fuse which MUST BE IN PLACE to use
> this device so that if a short circuit occurs they will trip or blow and
> protect the boats wiring!
>
> It is a good idea to place a fuse in series with these zeners that are
> used with the wind genny surge suppressor so that it can prevent a short
> circuit from happening if the zeners do blow out. I would use about a
> two amp fuse here. Put it where it can be easily checked and remember
> that if the device takes a hit and does blow a fuse here you loose some
> protection. If you use no fuse and the zeners take too big a hit and
> short, the wind gennys fuse or breaker will likely open and you will
> get no power from the wind genny after that. If the breakers open and
> the zeners are shorted the wind genny will then see that dead short and
> be unable to spin very fast, and at least that is a good thing. The
> zeners are cheap and can be easily replace after their sacrifice and
> hopefully all will then be well.
>
> If you read all of this and decide you see a clever modification to this
> design to do away with the fuse in series with the zeners, well, you
> really cannot use the PTC auto fuse device here in series with the
> zeners as used below in other surge protection circuits, because if you
> took a hit and the zeners conducted, the PTC would create an
> instantaneous voltage drop which would add to the 15V of the zeners and
> create an over voltage at the device being protected, so with this
> design for this application if you use a fuse in series with the zeners
> it must be a 'normal' one.
>
>
> The trade off is if you use no fuse, and take a big hit, it is likely to
> fry the zeners and then the circuit is opened and you loose the wind
> genny power until you replace the zeners and re-set the circuit.
>
> If you do use a fuse and take a big hit it may blow the fuse and then
> you loose some protection. Yes, the wind genny is still on line, but if
> you take another hit after that, it will be more likely to damage the
> wind genny.
>
> The surge suppressor for small devices is just the two zeners wired as
> before, but on the positive supply to the device, in series with it and
> UPSTREAM from them the PTC auto fuse is connected. That is the zeners
> are wired in CLOSER to the device so that following the positive wire
> you come first to the series connected PTC auto fuse and then to the
> zeners.
>
> This device can handle up to half an amp of current continuously before
> it might 'trip'. If you want more current handling capability, you can
> wire two or more such units in parrallel. DO NOT try to use a bigger
> value PTC and more zeners in parrallel, as more than two will result in
> 'current hogging' and cause some of them to burn out. If you want a
> single larger current capacity device you must use larger rated zeners
> along with a larger rated PTC device.
>
> It works like this; if a transient occours, the zeners simply conduct it
> straight to ground. If it continous, the PTC heats up and goes into a
> high resistance state, dropping excessive voltage across it and
> protecting the zeners from too much current so that they can hold the
> voltage to 15 VDC. Once the transient is over, the zeners stop
> conducting, and the PTC cools and re-sets returning to a normal state if
> it was in a high resistance state. The device is simple to make, cheap,
> very robust and fast acting. It acts as a transient suppressor, over
> voltage protector, and automatically re-settable fuse, all in one.
>
> It has two dis-advantages; one, it will create 2/10's to 4/10s voltage
> drop as a result of the PTC being in series, and two, it cannot handle a
> lot of current unless you use more than one of these devices connected
> in parallel. You may get around these disadvantages by using the
> modified type discussed below.
>
>
>
> The modified device can handle however much power you want. It consists
> of a 12V relay of whatever current rating is needed, a single 15V 5W
> zener, and a fast blow 250 ma fuse. The relays coil is provided with a
> positive input from boat power coming or supplied through a controller
> such as a breaker for the device needing protection, and a negative
> connection. The negative has in series to the relay coil a 250 ma slow
> blow fuse. (In series with relay coil ONLY, that is to say the negative
> connection going through the relays contacts to the device needing
> protection do NOT have this fuse in series with them).
> Also on this negative connection to the coil AFTER the fuse but
> connected to the coil negative input is another wire connected in series
> with the 15 V zener diode and then to the POSITIVE supply to the device
> being protected. (Remember to connect the zener with the bar end toward
> POSITIVE).
> The points of the relay are connected in series with the device needing
> protection.
>
>
> The way it works is that when the device for the circuit being protected
> is supplied power, the relay coil is energized and the points close and
> the device being protected is energized. When a transient occurs, the
> zener absorbs it and the device is protected. If the transient is too
> high or repetitive or an over voltage situation occurs, the zener
> conducts enough to blow the fuse and the relay then looses coil power
> and opens protecting the device.
> It is cheap, very reliable, easy to make, and robust.
>
> The disadvantage is that if an over voltage or transient blows the fuse
> the device shuts off and the fuse must be replaced before restarting.
> Also it is not as fast acting as the other devices as the fuse must blow
> and the points must open for full protection, even though the zener will
> clamp well in the meantime.
>
> I hope I have explained this all sufficiently, if not feel free to pose
> any questions.
>
> I can send schematics to anyone who wants them. -Ken
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