SSRs can fail shorted, I don't think this buys you much in terms of reliability and safety.
I think there is a little over-kill going on. Do you really need a fast bleeder (<1 minute to safe)? Or would a five minute bleeder be OK? My understanding is that you have two 68000uF caps in series, charged to about 130V. (If this is incorrect let me know - the thread is long and rambling and refers to other thread. I'm basing this on something written in the first message of this particular thread.) Series caps divide, so you have 34000uF. Let's say that "safe" is 20V. So you need to discharge from 130V to 20V, that is to 15% of the original voltage. Ln(0.15) is -1.9, so it takes 1.9 RC time constants to get to a safe voltage. Call it 2.0 for easy math. If you want to get there in 5 minutes, then one time constant is 2.5 minutes = 150 seconds. T = RC, solve for R, gives you R = T/C = 4411 ohms. At 130V, that would draw 29mA and burn 3.8 watts. Next lower 10% value is 3.9K, that would draw and burn 4.3 watts. I like to derate power resistors by 50%, so you want a 3.9K 10W resistor. If you do indeed have two caps in series, they ought to have balancing resistors anyway, so you could split the bleeder into two sections. Make each section 2K or 2.2K and 5 watts. Something like this can be chassis mounted and wired to the caps: http://www.digikey.com/product-detail/en/RH0052K000FE02/RHRA-2.0K-ND/1166266 Cost is $5 each, total $10 Even cheaper would be to parallel up some 1W leaded resistors: http://www.digikey.com/product-detail/en/PR01000101002JR500/PPC10KW-1CT-ND/597177 $2.06 for ten - five in parallel will make 2K 5W, put one set of five across each cap. Might be a nuisance to mount. If you have can type caps with screw terminals, use power resistors with solid axial leads, crimp/solder them to ring lugs, and mount right on top of the caps: http://www.digikey.com/product-detail/en/SQP10AJB-2K0/2.0KW-10-ND/18789 $0.64 each, total $1.28 I realize that Digikey might not be your supplier of choice due to shipping costs, but my point is that an always-on bleeder is simple, safe, and at this power level, cheap. As you get to larger energy levels (more volts or more uF) or if you need a much faster discharge time you might be able to justify a switched fast bleeder. But that will ALWAYS be less reliable and less safe than an always-on bleeder. Even if you have a switched bleeder it would be wise to have an permanent bleeder with maybe a 10 minute time constant. At 10 minutes, the power and cost are negligible, and it serves double duty as the voltage balancing resistors for your series caps. Inrush limiting is a separate issue. Separate resistor, bypassed by a contactor or relay for normal operation. Put it on the AC side so you don't have any issues with DC contact ratings. The main risk is that if you try to run with the bypass relay open you can fry your resistor. One way to avoid that is to make the "resistor" a 100W incandescent light bulb. Using bulbs for bleeders is risky because they run all the time, and when they burn out they leave the caps charged with no indication of the risk. But using a bulb for charging is different. The bulb only lights for a few seconds when you first apply power. If it is burned out the caps won't charge. If you try to run with the bypass open the light will glow to let you know. A 100W 120V bulb has a hot resistance of 144 ohms. With 34000uF the charging time constant is 4.9 seconds. As the bus charges the voltage drop across the bulb goes down, it cools down, and its resistance decreased - that is good, it make the "long tail" of the charging curve go faster. The inrush during the first cycle or so will be based on the cold resistance of the bulb, but if your switch and diodes can handle a 100W light bulb you know it can handle any size cap bank fed thru that same 100W light bulb. -- John Kasunich [email protected] ------------------------------------------------------------------------------ Site24x7 APM Insight: Get Deep Visibility into Application Performance APM + Mobile APM + RUM: Monitor 3 App instances at just $35/Month Monitor end-to-end web transactions and take corrective actions now Troubleshoot faster and improve end-user experience. Signup Now! http://pubads.g.doubleclick.net/gampad/clk?id=267308311&iu=/4140 _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
