Which resistor do I need?

Hi,

Added a few caps to my ESC, but from the past I know there was always a big spark. So I decided to add a resistor, but I don't know which one I need. I looked on BrianG's site, it said I needed a 7Ohms, 200Watt resistor. Is this right?
I'm using 10S LiPo: 37V nominal, 42V fresh of the charger. And got about 7000uF of capacitance.
And which resistor do I need? There are so many different types of these things

Thank you,

Patrick

Thought you said a 7 ohm 200 watt resistor, what else do you need to know?

Well, they are huge and pricey (starting at $50 at mouser.com). I didn't think I needed such a big and expensive one...

And it does not matter what kind of resistor it is? At mouser they've got a few to choose from:
• Carbon Composition Resistors (2,112)
• Ceramic Composition Resistors (1,093)
• Chassis Mount Resistors (12,075)
• Current Sense Resistors (12,667)
• Film Resistors (239,713)
• High Frequency/RF Resistors (618)
• Metal Foil Resistors (637)
• Metal Oxide Resistors (6,574)
• Potentiometers,Trimmers & Rheostats (31,338)
• Resistor Hardware (154)
• Resistor Kits (222)
• Resistor Networks & Arrays (32,377)
• Surface Mount Resistors (179,093)
• Through Hole Resistors (108,333)
• Wirewound Resistors (36,087)

[lutach]

This looks nice:
http://www.espritmodel.com/jeti-afc-...-5mm-150a.aspx

I'm looking for something about this I saved a while back and as soon as I find it, I'll post it here.

[lutach]

Found it.

[BrianG]

lol, a 200w resistor would be about the size of an ESC! If you look closely, that 200w value is the power dissipated on initial hookup but only lasts for around 50ms (assuming you left the total charge time at 0.25 sec); after that, the differential voltage is high enough to limit power greatly. The heat is not "instant" since any thermal changes take time, and the overall temperature will average out.

Anyway, for 7000µF capacitance and 10s lipo, a 22Ω, 5 watt resistor should be fine. That resistor value would cause the caps to be charged in around 800ms. And that 22Ω doesn't have to be exact, as long as it is fairly close.

hey guys,

very helpful thread, thanks a lot,

Quick question... what size of resistor will i require?

Running 6s and 8s on a MMMXL with CapPack from castle,

i assumed around 30Ω would be sufficient?

Thanks,

Joe

[BrianG]

From the "No Spark" page at my site:

For most ESC's with around 1,000uF of total capacitance, use a 1Ω 2watt resistor when using up tp 25v (6s lipo), or 10Ω 5watt resistor when using voltages over 25v. This value will charge the capacitors slow enough to avoid the hookup spark, yet fast enough to still allow the ESC to arm properly (under half a second). The power rating is low because the only time the resistor has to dissipate the maximum power is on initial hookup. After that point, the differential voltage between the supply and the capacitor charge decreases quickly, which reduces current and therefore power dissipation.

This is a general rule of thumb. If you are using much higher capacitance, odd voltages, etc, the calculator at the bottom of the page lets you enter specific values and then shows the results. The calc does not show the specific power value to use, but generally, average the power values for the times shown.