Fuse(s) for my coil driver circuit?

Thread Starter

Nicholas

Joined Mar 24, 2005
139
Hi again guys!

Once again :), this is the circuit(see below), but there are two of them side-by-side for driving two coils(or the like).
The coils will not be the same, one might draw 10 amps, the other 5 amps. They will be different for sure.

What should I do about fusing the circuit? and should I fuse it at all?

Thanks,

Nick
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#12

Joined Nov 30, 2010
18,210
What could go wrong?
The mosfet shorts and the coil stays on? That won't pop a fuse.
The coil (or the wiring to it) shorts and the mosfet gets in danger? Yep. A fuse would help that.

Regard' My method for making decisions.
 

#12

Joined Nov 30, 2010
18,210
I dunno. Are mosfets cheaper than fuses?
I think so. If you are the only person who has to use it, fine. If some non-electronics person needs to use it, install a fuse that can be replaced...or don't. Everybody else is making un-repairable boards to force new purchases. Why be different?
 

hp1729

Joined Nov 23, 2015
2,304

Thread Starter

Nicholas

Joined Mar 24, 2005
139
What could go wrong?
The mosfet shorts and the coil stays on? That won't pop a fuse.
The coil (or the wiring to it) shorts and the mosfet gets in danger? Yep. A fuse would help that.
.
Wouldn't a coil that stays on increase in heat and thus increase in resistance...and then draw more amps?

Thanks:)
 

hp1729

Joined Nov 23, 2015
2,304
Wouldn't a coil that stays on increase in heat and thus increase in resistance...and then draw more amps?

Thanks:)
An increase in resistance would draw less current. Getting hot it might decrease in resistance and draw more current, yes. Typical for inductors.
 
Hi all,
One thing to remember, fuses are for safety, not to save components. Todays' semiconductors often require over current of a few milliseconds or less to die a violent death. Typical fuses are thermal devices that often take much longer than that to heat up and open.

Make a list of the possible failure modes that could compromise safety. Look at the SYSTEM that includes the coil circuits. Have an understanding of what COULD happen and the probability. Then you must weigh the cost vs. the probability to decide where and what kind of protection to use.
 

#12

Joined Nov 30, 2010
18,210
Wouldn't a coil that stays on increase in heat and thus increase in resistance...and then draw more amps?
Totally backwards thinking.
A coil that is designed to stay energized just sits there doing its job. If it is designed for pulse activation, it overheats and the resistance of the copper increases. Increased resistance to current flow means less current flow.
 

wayneh

Joined Sep 9, 2010
16,398
That's a good list of basics. Point 4 should point out more forcefully, in my opinion, that fuse ratings are easily misunderstood. For instance, if your circuit might be damaged by 1A for a few seconds, you might think a regular 1A fuse, not a "slo blo", should be about right. But if you look at the specs for the fuse and read the current/time chart, you'll see that a generic 1A fuse takes a long time to blow at 1A. It'll blow much faster at 2A, but will still take far longer than you might expect. It's eye-opening the first time you carefully read a fuse chart.
 

MaxHeadRoom

Joined Jul 18, 2013
20,925
Hi all,
One thing to remember, fuses are for safety, not to save components. Todays' semiconductors often require over current of a few milliseconds or less to die a violent death. Typical fuses are thermal devices that often take much longer than that to heat up and open.

.
Although there exists Rectifier or Semiconductor fuses which are intended to save Semiconductor circuits and a have extremely fast sweep through when blowing.
Max.
 
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