Hi all

I have a set-up (made with help from you guys) where an IRLZ44
MOSFET is switching around 40V and is turned on by a controller
with a 5V signal. I made a mistake(!) and forgot to connect the
ground between the MOSFET circuit and the PSU, which resulted
in the IRLZ44 shorting and sending 40V into the controller. It smelled
for DAYS, and was of course fried. So, I'm thinking about adding a
fuse to take the chill instead of the controller if I mess up again.

So my question is, what value should it be, and where is the best
place for it? On the gnd wire from the MOSFET circuit going to the
controller?

Thanks a lot!

 

Hi all

I have a set-up (made with help from you guys) where an IRLZ44
MOSFET is switching around 40V and is turned on by a controller
with a 5V signal. I made a mistake(!) and forgot to connect the
ground between the MOSFET circuit and the PSU, which resulted
in the IRLZ44 shorting and sending 40V into the controller. It smelled
for DAYS, and was of course fried. So, I'm thinking about adding a
fuse to take the chill instead of the controller if I mess up again.

So my question is, what value should it be, and where is the best
place for it? On the gnd wire from the MOSFET circuit going to the
controller?

Thanks a lot!

Most of the time the fuse rating is based on the used current with a little head room in order to prevent the fuse from blowing because of a surge current ..

 

Fuses - oh fuses... this is probably the single most agonizing part that I use. It depends a lot on your philosophy. Yes - size the part for the at least the nominal current plus some additional headroom. But also consider inrush currents and how that can effect the life of the fuse and if it will degrade the fuse over time.

Also fuses have some resistance, so consider the power loss as well.

 

Thanks!

So, in case of a normal controller, the voltage is 5V and the amperage
should be about 30mA at max, yes? Does this mean I can use a fuse
that is 250V 100mA and keep my controller pretty safe?

 

A fuse will not work to do what you want. It takes a certain amount of time for a fuse to 'blow'. For a electronic part to fail can take a lot less time than the time a fuse takes. Fuses are usually only used on power to a circuit.

A diode in the output from the controller would probably work though. The diode would stop any reverse flow of electric into the controller. A fast acting Schottky diode would be best.

 

A fuse will not work to do what you want. It takes a certain amount of time for a fuse to 'blow'. For a electronic part to fail can take a lot less time than the time a fuse takes. Fuses are usually only used on power to a circuit.

A diode in the output from the controller would probably work though. The diode would stop any reverse flow of electric into the controller. A fast acting Schottky diode would be best.

correct..

And for reference a fuse is "typically" sized at 125% of max continuous current. So 100 Amp max continuous gets a 125A fuse.

 

For semi protection, use a rectifier fuse.
Fast sweep through!.
M.

 

Shortbus, that sounds reasonable ! Any advice regarding the value of
the fast diode? If a small diode is used, won't it just die and go
open on reverse voltage? (like a fuse)

Thanks!

 

A fuse will not work to do what you want. It takes a certain amount of time for a fuse to 'blow'. For a electronic part to fail can take a lot less time than the time a fuse takes. Fuses are usually only used on power to a circuit.

A diode in the output from the controller would probably work though. The diode would stop any reverse flow of electric into the controller. A fast acting Schottky diode would be best.
__________________

 

Thanks for the unprecedented information mank.

 

A fuse will not work to do what you want. It takes a certain amount of time for a fuse to 'blow'.

Rectifier or Semiconductor fuse for fast sweep through, but not cheap!
Max.

 

....and again with the unprecedented information. What the heck is going on around here?

 

Over the years I have become aware that transistors generally protect the fuses, not the other way around. The thing a fuse prevents is extra damage from shorts propagating through the circuit. A slow blow fuse is used when you know there are going to be current rushes, such as charging filter caps in a power supply.

 

Over the years I have become aware that transistors generally protect the fuses, not the other way around..

Well maybe that's the answer, a sacrificial "fuse" transistor. You could solder in a female header that the transistor plugs into, for easy replacement.