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 _______________________________________________ Liveaboard mailing list [email protected] To adjust your membership settings over the web http://www.liveaboardnow.org/mailman/listinfo/liveaboard To subscribe send an email to [email protected] To unsubscribe send an email to [email protected] The archives are at http://www.liveaboardnow.org/pipermail/liveaboard/ To search the archives http://www.mail-archive.com/[email protected] The Mailman Users Guide can be found here http://www.gnu.org/software/mailman/mailman-member/index.html
