I'm making a coilgun with four 11.1V 1300mah 95C lipo batteries in series, providing 44.4V for the coils. Current will peak at 200A for short durations of time. Is there some common way to protect the battery, say, with a fuse, in case a coil turns on for too long? I'm a beginner at electronics, so it will help to be more detailed.
\$\begingroup\$
\$\endgroup\$
4
-
1\$\begingroup\$ @JRE Beat me by seconds, again… \$\endgroup\$– winnyCommented May 30, 2022 at 10:03
-
1\$\begingroup\$ Welcome! How are you triggering them? Nitpicking: it’s not the coil turning on for too long potentially, it’s your driving circuit. \$\endgroup\$– winnyCommented May 30, 2022 at 10:05
-
1\$\begingroup\$ NB: I should use a super-capacitor for "transferring" energy. But in any case, "know" absolutely what you are doing ... because of the "concentrated" and high level of energy. \$\endgroup\$– Antonio51Commented May 30, 2022 at 10:49
-
1\$\begingroup\$ This should be of some interest powerelectronictips.com/1000-a-to-a-welder-through-a-mosfet and this electronics.stackexchange.com/questions/292252/… \$\endgroup\$– Antonio51Commented May 30, 2022 at 10:55
Add a comment
|
1 Answer
\$\begingroup\$
\$\endgroup\$
7
Your options are:
- A slow blow fuse
- A micro controlled switch
A slow blow fuse sounds like a cheap and easy solution; put a slow blow fuse in line, if the current is too high for too long, fuse goes, everyone is happy. The problem is the variation in fuses. A 200A rated fuse will conduct 200A at 25C for a long time. Increase the current to 500A and it’ll blow quickly. The problem is that if you pick up 10 of these fuses, you’ll have 10 different perfromances. Some will blow quickly, some will take a bit longer, some will blow at 300A, others at 500A. Standards for fuses are very broad, because they have to be due to have fuses work. Fuses are a great simple safety device for when looking at normally drawing 5A, but during fault you draw 100A. Then it will blow and be safe. If you are normally drawing 5A, and at fault draw 7A, you have a problem. Fuses are not that accurate. This is true regardless of the fuse technology.
Your alternative, the only one which makes sense to me, is to spend more and do some proper design. Using a simple current sense resistor and some basic circuitry you can measure the current accurately. You can put this into a microcontroller or possibly a basic timer circuit, and use that to turn off a FET which will turn off the current. This is a lot more complicated, but a lot more accurate and reliable. Put a fuse in as well, just in case you get a failure and your FET fails short (as is their want).
Side note: you are doing something very dangerous. I advise that you do not do this at all. 200A from batteries is scary, LiPos explode if handled wrong. You are making a coil gun. And you are asking basic questions about it online. All of these things make me want to say “put it down and step away”.
-
\$\begingroup\$ "Using a simple current sense resistor and some basic circuitry" 200 A sense resistors and circuitry aren't simple unless you know what you are doing. \$\endgroup\$ Commented May 30, 2022 at 10:23
-
\$\begingroup\$ @WhatRoughBeast a single 5mR resistor connecting your differential ADC to either side of it gives you a 0-1V reading at 0-200A. \$\endgroup\$ Commented May 30, 2022 at 10:47
-
\$\begingroup\$ Power of resistor ... 200 W. \$\endgroup\$ Commented May 30, 2022 at 10:51
-
\$\begingroup\$ Puffafish - Yes, and what happens in the wires leading to and from the 5 mohm resistor? Where is ground connected? Unless it is exactly at the bottom of the sense resistor, the voltage to the electronics will show a different voltage than is developed across the resistor alone. Stray, unintended current paths and tiny resistances can give you fits at 200 A. That's why I said, "unless you know what you are doing." \$\endgroup\$ Commented May 30, 2022 at 10:53
-
2\$\begingroup\$ You didn't mention differential connections the first time around. Why in the world did you think the OP would pick up on it? And trust me, I've seen Kelvin connections done wrong. \$\endgroup\$ Commented May 30, 2022 at 10:59