Mike Nickerson wrote:
Hi Lee, I don't think that is how in rush current limiters work. I use them to protect my DC-DC converter caps so they don't get whacked with a high in rush current when the connection is first made.
You're right. My explanation was not at all clear. I was staying up to late, and not thinking/typing clearly. Let's try again.
The problem with a normal inrush limiter is that they are intended to work with a known load current. Suppose you have something that like a switchmode power supply that normally draws 5 amps, but has a set of big filter capacitors. When you first plug it in, the capacitors look like a dead short, and draw a huge current spike. This damages connectors and switches, blows fuses, etc.
Inrush limiters have a cold resistance, and a hot resistance. Let's say you pick one with a 10 ohm cold resistance, and hot resistance of 0.5 ohm at 5 amps. When you connect the power supply to 120vac, the cold resistance limits the peak current to 120v/10ohm = 12 amps, so it won't trip your 15a breaker.
But that little inrush limiter is dissipating 120v x 12a = 1440 watts! It heats up FAST! As it does, its resistance falls to 0.2 ohm. When things settle down and the supply is drawing its normal 5 amps, the inrush limiter has a voltage drop of 5a x 0.2ohm = 1 volt; you're losing 1 volt on a continuous basis. And, it is dissipating 1v x 5a = 5 watts of heat continuously.
This doesn't matter much for a 120v power supply. They're designed to be cheap (not efficient), and they already have a fan to deal with the heat.
But, suppose you put this limiter between two 12v batteries. Let's say one battery is fully charged at 13v, and the other is almost dead at 12v. With only a 1v difference, the 10 ohm cold resistance only provides 0.1 amp between the batteries. 1v at 0.1a is only 0.1w -- the inrush limiter never heats up, so its resistance never falls.
If you find an inrush limiter with a vastly lower cold resistance, it may heat up initially, but will cool right back off under your normal 5a load. That means it sits at its cold resistance all the time. You may as well use a plain old resistor, or just a length of wire chosen to limit the peak current.
Or light bulbs. Their advantage over a resistor is that a) their resistance goes up 8:1 or more initially to limit the current, b) then their cold resistance is a tiny fraction of an ohm, so they don't add much of a voltage drop between your 12v batteries.
-- "IC chip performance doubles every 18 months." -- Moore's law "The speed of software halves every 18 months." -- Gates' law -- Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org Read EVAngel's EV News at http://evdl.org/evln/ Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group/NEDRA)
