On Sunday 19 May 2019 03:43:45 am Nicklas Karlsson wrote: > On Fri, 17 May 2019 20:23:57 +1000 > > Erik Christiansen <dva...@internode.on.net> wrote: > > On 16.05.19 21:18, Nicklas Karlsson wrote: > > > > ... > > > > I'd put the battery bank in an outbuilding. If something shorts > > > > out and the battery bank melts down, the little building may > > > > burn but my house won't. > > > > > > There certainly is a lot of power from a byttery and it could be a > > > little bit hard to turn off. You put a fuse at battery cable? > > > > If local regulations do not require a HRC (High Rupture Capacity) > > fuse between the battery bank and any connection to it, then it's an > > invitation to disaster. A dc arc is much more difficult to > > extinguish, because there are no zero crossings, and molten metal > > can end up flowing on the floor. > > I got the feeling DC arc is much hard to extinguish but found no good > information about it. Then reading about how welding should be done > and EDM I learned then air i ionized resistance get lower, in practice > I know it is often hard to ignite the weld while it work for some > distance then lit. I suspect and get the feeling then welding not the > least from the ignition problem some distance could be kept then lit > distance to keep air ionized depend on current, happen to anything > about this? > > > Small HRC fuses are generally in ceramic tubes, and are sand-filled, > > to quench the arc when the element fuses. The one I have on the desk > > beside me is a 100 A with a 100 kA (100,000 A) arc rupture capacity, > > but it's 27x44x49 mm, and is overkill. Just 10 times AH rating seems > > to be normal, i.e. 2000 A rupture capacity for a 200 AH bank. (The > > fault current will be limited by battery internal resistance.) > > > > > I also think DC relays have to be a little bit different than AC > > > relays, used an ordinary AC relay for a few few days connected to > > > a DC voltage and even though two switches is connected in series > > > it sometimes make a buzzing sound then turned off. > > > > Most relays and switches are marked with both AC and DC current > > ratings. The DC rating is always much lower, due to the difficulty > > of rupturing a DC arc. Rather than bother with > > maintenance/replacement due to contact deterioration after many > > switching cycles, I prefer to switch DC loads with MOSFETs - they're > > easy to parallel, as the inherently share. > > Agree, as is evindent furher up in message. > > For safety I learned relays fail open circuit while MOSFETs fail short > circuited, I also experienced MOSFET short circuit failure. Happen to > know anything more about this? > It is for this reason that higher current dc loads, which will often be inductive are often of the t-bar design. The armature, when released will rise and hit the crossbar contact with enough force to break the microweld, but even that isn't trusted, so I've seen circuits that used a 4 pole contactor with all 4 poles in series.
I'm sure you've all seen the familiar flywheel diode placed across the relay coil to prevent the inductive spike when turning off the relay, from rising more than a volt or two above the supply voltage, it is absorbing the relay coils currant during the turn off period, preventing the transistor from being over voltaged by the inductance and punched thru for a permanent short. This also delays the relays release by a few milliseconds. If controlled by a relay, the circuit should be expected to have a few milliseconds of lag anyway. Modern mosfets can often withstand 1000 volts or more for short periods of time, so a lot of research has gone into mosfet _drivers_ to do 2 things, 1st being to switch it as fast as possible to reduce its heat dissipation during the transition, and 2nd to get the opposite side of a switching circuit turned on just slow enough you don't have a short because both sides are on at the same time, but you must get the opposite side turned on with a minimum of dead time between them to short circuit the voltage stress on the one just turned off. 5ns can make or destroy in this case. The diff is that it works for decades, or you break the mirror and let all the smoke out. > Regard Nicklas Karlsson > > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
