Surf PI V1.2 Problem

[lazarjovancic99]

I had build (homemade) metal detector but it dosen't work how it supposed to do. I can adjust pot and that works. It detects big things at max 2cm near coil. Coil is R=30cm and 27 turns and with resistance about 1.8 ohms(measured with cheap $8 multimeter in 2 wire configuration(direct from multimeter to coil)). Power supply is 12v rail of atx smps.
Here are some images: http://imgur.com/a/5WCgR I replaced TLC555 with NE555 but i think that will not cause problem. I replaced also MPSA13 with BC517. There is sound but with noise in background.

[silverdog]

It depends on your wire , 0.5mm wire, 30cm diameter gives approx 686uH far too high, surf pi needs 250uh-350uh
Look in the coil section on this forum there is a coil calculator that will help you

[bernte_one]

try to get something near the 300µH (280-320 is ok)
mayb 18-20 turns with 30cm Diameter coil depending on the wire diameter

[lazarjovancic99]

I tried now coil 16 turns, 30cm diameter, 0.6mm thickness wire, so I calculated that about 310mH in coil calculator. IRF gets hot and there is a treasure in itself buzzing coil probe
The voltage in the IC I checked and all are ok.
U1 pin 8 v12.40 +
u2 pin 7 v 4.97 +
u2 pin 4 4.75 -
U3 pin 8 v 4.97 +
U3 pin 4 4.75 -
U4 pin 14 in 4.97 +
U4 pin 7 4.75 -
U7 pin 14 in 4.97 +
U7 pin 7 4.75

[Qiaozhi]

I tried now coil 16 turns, 30cm diameter, 0.6mm thickness wire, so I calculated that about 310mH in coil calculator. IRF gets hot and there is a treasure in itself buzzing coil probe

16 turns of 0.6mm wire at a diameter of 30cm (150mm inner radius) calculates as 244uH.
Coil Calculator

[lazarjovancic99]

Flat coil or normal?
IRF is hot like hell, i cant touch it for more than second.
Can i try 18turns of 0,6mm wire?

[Qiaozhi]

Flat coil or normal?
IRF is hot like hell, i cant touch it for more than second.
Can i try 18turns of 0,6mm wire?

Normal coil (not spiral) - you'll need to use a different calculator for that.
Try 18 turns, and check your TX pulse width on a oscilloscope.

[lazarjovancic99]

I tryed with 18 turns and same thing happends again(with normal coil). Noise in background! Can't detect anything and irf is very hot! I don't own oscilloscope. Is background noise normal? Because i think that something blew up!
With coil of 21 turn and radius 15cm and wire lenght 0.9mm. It detects big things but max to 2cm. And irf dosen't heat up.

[Qiaozhi]

I tryed with 18 turns and same thing happends again(with normal coil). Noise in background! Can't detect anything and irf is very hot! I don't own oscilloscope. Is background noise normal? Because i think that something blew up!
With coil of 21 turn and radius 15cm and wire lenght 0.9mm. It detects big things but max to 2cm. And irf dosen't heat up.

If the MOSFET is getting very hot, then the TX pulse width is probably too long. Check you've fitted the correct value components for R2, R3 and C1.

[lazarjovancic99]

If the MOSFET is getting very hot, then the TX pulse width is probably too long. Check you've fitted the correct value components for R2, R3 and C1.

It gets hot only when i connect 0.6mm coil. With 0.9mm coil (you know, i said before) dosen't gets hot! And i checked 2 times components and they are ok but delay pot of 150k i replaced with 250k. I use 12v for vsup

[Qiaozhi]

It gets hot only when i connect 0.6mm coil. With 0.9mm coil (you know, i said before) dosen't gets hot! And i checked 2 times components and they are ok but delay pot of 150k i replaced with 250k. I use 12v for vsup

The TX off-time is defined by: t1 = 0.685 (RA + RB) C
and the TX on-time is defined by: t2 = 0.685 (RB) C

where:
RA = 100k
RB = 3k3
C = 22nF

Hence, the TX on-time should be 49.7us

The TX pulse rate is defined by: f = 1.46 / [(RA + 2RB) C], which gives f = 622.5pps

A TX on-time of 50us with a 300uH coil should not be long enough to make the MOSFET get so hot that you cannot touch it.
You probably need to get access to an oscilloscope to understand what's going wrong with your circuit.

[lazarjovancic99]

The TX off-time is defined by: t1 = 0.685 (RA + RB) C
and the TX on-time is defined by: t2 = 0.685 (RB) C

where:
RA = 100k
RB = 3k3
C = 22nF

Hence, the TX on-time should be 49.7us

The TX pulse rate is defined by: f = 1.46 / [(RA + 2RB) C], which gives f = 622.5pps

A TX on-time of 50us with a 300uH coil should not be long enough to make the MOSFET get so hot that you cannot touch it.
You probably need to get access to an oscilloscope to understand what's going wrong with your circuit.

Thanks a lot! Now i will find problem easier! My friend has digital oscilloscope and i will ask him to use it to find a problem. Thank you for your time. I appreciate it.

[Qiaozhi]

Thanks a lot! Now i will find problem easier! My friend has digital oscilloscope and i will ask him to use it to find a problem. Thank you for your time. I appreciate it.

Good luck. Hope you find the problem.

[crni1]

У колико би желео да повуче новчић, када имате проблем кондензатора.
Утисци после поправке.

[Qiaozhi]

У колико би желео да повуче новчић, када имате проблем кондензатора.
Утисци после поправке.

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