fuse and coil

[isingkarim]

Hi

I have made this PI metal detector (stuart variant)
http://sjeffroy.free.fr/DetectPI/detectpi.html
With the coil (~330 microH) connected, the delayed fuse blows. I have tried 315 and 500 mA fuse. If i do not connect the coil, it doesnt blow. With an oscilloscope, I can see the pulses.
I do not find any .short-circuit. What could I try ?
Thanks for any advice.

Cheers

Karim

[DOOLEY]

average current use should be less than 500mA ,
i use a 1A fast blow on my pi ( blind squirrel variant) average 250mA draw,

first thing that springs to mind is , have you got any large capacitors to "buffer" the current draw after the fuse ??

during pulse on time the mosfet or transistor can supply the coil with anything from 4 to 7 amps dependant on coil and mosfet used , the only reason the fuse doesnt blow is a big capacitor , or a few smaller ones "shunt" or "buffer" the current draw for the time on ,
capacitor then gets re-filled during time off , so evening out the current draw on the fuse.

have you got a / many capacitors between neg and pos after the fuse ??

total min uF would be about 1000uF ( I USE 4 X 1000uF just to be sure)

[isingkarim]

Thanks for your help
I have 2* 2200 microF capacitors
I will try increasing the value of the fuse and reduce the time of the pulse

Cheers

K

[DOOLEY]

that was going to be my next question , what is your on time ?
200us or less.
other thing was , is the mosfet poss shorted , so when you connect your coil it draws full amps and blows fuse was my next guess.
or even poss S and D the wrong was round would do it , conducting all the time through the internal protection diode.
note : i use the IRF840's myself , they are higher voltage than the IRF740's but handle less amps , and due to silly mishaps i have blown a few myself , easy done.

[DOOLEY]

forgot to add , your caps you mention should be enough for the job , more never hurts though , but as you have enough uF i don't think this is your problem.

[isingkarim]

humm hummm I have just checked with the oscilloscope the length of the pulse : 360 uS (when on) which is quite high ...

Karim

[DOOLEY]

360us not exes high , but may be enough to blow the 500ma fuse.
every detector is different , but we are using the same mosfet , and my coil is about 3 ohm , so were pretty similar , if i do 200us i'm using 270ma ,
so simple ratio calculation , we come out at 486ma for your 360us on time.
now if your coil is around the 2 ohm mark , that 1/3 more power used which = 729ma ,
or there abouts ,
now that's a detector thats using a fair amout of power , if you went for a 1A fuse your mosfet with no heat sink would cook,

yes , lower the on time hopefully a guru will chip in here with a suggested on duration,
my guess is try around the 160us mark,
if you still blow a fuse , there's something else a'miss.

[isingkarim]

Hi

I have even reduced the length of the pulse to 0 with the coil connected. The fuse doesnt blow anymore but I switch off the circuit because it behaves not correctly (lcd off for example)

Cheers

Karim

[isingkarim]

Forgot to say that the coil resistor is 2.8 ohms

[isingkarim]

I found why the fuse blows : when the coil is connected I have a current of 700 mA (!) and do not find why ... No coil : ~100 mA
The time between two pulses is ~ 4ms and the length of the pulse is short (~0)
Besides adding a resistor in series with the coil, I do not know what to do, what to look at
thanks for any advice