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ScotchTapeLord said that you're alright with 1meg 1/8w and made all the math...
It's mostly dependant on the leakage resistance of your capacitors. Without knowing that you don't really know if the resistors will be overkill or not.
On some fast math, the 36k 10W ones leave room for a 600V max voltage. The 33k leaves for 480V and the 30k one for 548V approx.
Just orientative math.
Also, take into account they're wirewound type and want it or not they have an intrinsec inductance, and will probably add an inductance aswell, that's gonna lower current on the circuit too.
Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635
Sir Fixalot wrote ...
Alrighty then! If you guys think that the 1 megohm 1/8 watt resistors will work safely as balance/charge equalizers, then I'll get some asap.
No, I was just saying that 1/8 watt would be okay if you used 1 megohm resistors, and that you could determine the wattage rating needed with simple math.
Put an ohmmeter across each capacitor that can measure 10 Megohms and wait for it to stop changing (this could take a while). If every capacitor exceeds 10 Megohms then you can use 1 M resistors. If the resistance is lower, you'll want to choose a resistor that is about 10x lower than what you read for the lowest capacitor you have. Lower resistance will equate to better balancing but more losses, which means higher wattage rating. I don't recommend charging these to their absolute highest voltage unless you have confirmed that the voltage division is acceptably even through simple resistor combination analysis.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1715
Sir Fixalot wrote ...
Klugesmith wrote ...
My recommendation, which also saves resistors: Form all 8 capacitors in parallel for a while, then measure them individually & make two groups of four to dividing total C in half, as closely as possible. Each group of 4 is wired in parallel, with one bleed resistor. The two groups are connected in series.
The reason I don't want to do that is to keep the ESR Low-I would much rather do sub-banks of 2 in series.
The ESR of the 2S4P bank is exactly the same either way: half the ESR of a single capacitor. Assuming your wiring resistance is relatively negligible, which is not hard to do.
Saz43 wrote ...
Jasonr wrote ...
BE CAREFUL!!!! it will not really matter the leakage of the caps. it is more the body style of the resistors i=v/r and then p=iv those little 1/8 guys will be smoking hot! plus if memory serves 1/8 watt is only good to 150volts?
Jason
I've used 1/8 watts up to 400V with no problem.
ScotchTapeLord wrote ...
Hm? Obviously a 1 M ohm, 1/8 watt resistor can tolerate 350V. P = V^2/R or I^2*R using V^2/R .125 Watts = V^2/(10^6 ohms) V^2 = 125000 Volts^2 V = 353.553 Volts
having 350V across it is the only way to get the wattage that high! Just do math to determine what wattage you need for your application.
I think STL is missing something. Along with the power rating, all resistors have a voltage rating (which also depends on the body size and type). For very high R values, the voltage rating is the more restrictive. For example: the power calculation formula suggests that a 10 megohm 1/8 watt resistor could withstand 1118 volts. Not a good idea. Ordinary small axial R's have voltage ratings in the hundreds (check the data sheet).
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1715
ScotchTapeLord wrote ... It's very possible I'm missing something...
But in the case of a 10 megohm 1/8 watt resistor, how can you get 1/8 of a watt of dissipation across 10 megohms besides putting 1118 volts across it?
It's a semantic thing. The power spec does not say the resistor -can- dissipate 1/8 watt. It says the resistor will not overheat if the average power is kept below 1/8 watt. That statement is true for all R values in the 1/8-watt series.
Conversely, suppose the voltage spec is 300 volts for that resistor series. It does not imply that a 1000-ohm 1/8-watt resistor can long withstand 300 volts.
[edit] Here's one datasheet. For the 1/8 watt size, "maximum working voltage" is the lesser of 250 V and sqrt(P * R). For 1/4 and 1/2 watt the corresponding limit is 350 V. Most models in that datasheet have an "overload voltage" spec twice as high.
Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635
Klugesmith wrote ...
Conversely, suppose the voltage spec is 300 volts for that resistor series. It does not imply that a 1000-ohm 1/8-watt resistor can long withstand 300 volts.
But you can have peak voltages of 300V while having 1/8 watt RMS dissipation, while in the case of the 10Meg you can't have 1/8 watt unless you meet or exceed 1118V...
But I guess I'll have to submit to your point. It's unfortunate that the wattage rating refers more to the size than actual electrical capabilities, though.
Registered Member #167
Joined: Mon Feb 13 2006, 06:41PM
Location: Waterloo, WI
Posts: 54
ScotchTapeLord, Look at the data sheet for the resistor you are choosing is all I am saying. In 1/8 watt almost everyone one I see is only good to 150-200volts. This has nothing to do with ohms law but how the resistor is built! you will notice they have a working voltage of 200 for 1/8 watt. So you must have been really lucky putting 500 volts on them. I would put 2 500k 1/4 watt in series for the extra pennies it would cost and the added safety is priceless.
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Resistor Suggestion for Cap Bank
