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Offline GoldDigger1950
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« Reply #10 on: February 07, 2011, 01:24:56 pm »
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There's just so much I want to ask you that makes this discussion sort of pointless. Yours is a design effort in progress and you don't show your frequency source or what frequency you are using. Technically, the MOSFET circuit is working but drifts? What drifts? Any drift would be caused by the input to your amplifier, not the output which you seem to be using for control.

Unless I can see what your entire circuit is doing and why, in your own words, there's nothing more I can do to assist you.

This puts you up against the concept of exposing your idea to scrutiny here which is a double edged sword. You might get some really useful help or you might get some advice you won't like. Just keep that in mind.

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Offline levettpTopic starter
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« Reply #11 on: February 08, 2011, 10:08:31 am »
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Yesterday I started to rebuild my prototype board. It's almost completed and will begin testing soon. The MOSFET switch perfectly now.

Once I have test results I would love to have comment on my design. Whether it's what I want to heard or not, doesn't matter. I want a really good PI MD eventually so any criticism is welcome.

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Offline levettpTopic starter
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« Reply #12 on: March 13, 2011, 05:52:21 am »
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Got the PIC to switch MOSFET with same method as in the schematic I uploaded in a previous post.

Thanks for everyone's inputs

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Offline DOOLEY1972
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« Reply #13 on: May 30, 2011, 12:12:17 pm »
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hello levettp, regarding your voltage drop over the irf740 that you mentioned on your first post,
 
as long as the gate is driven with 5V or above , then mosfet will be fully on , 20v is max , i useualy drive with 12V , quite happy at that.

on resistance of the irf740 is i think around 0.65 to 0.75 ohm so simple ratio calculation at 12 v but only seeing 8v at the coil means that your coil resistance is 10.5 ohms , quite normal when driving something with such a low resistance.

less noticable when driving coils with a higher resistance.

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Offline DOOLEY1972
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« Reply #14 on: June 02, 2011, 01:45:13 pm »
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OOPPS !!! 10.5 ohm's that makes me look an absolute idiot , decimal places eh Huh? the rise and fall of a nation could hang in the ballance ment to say 1.05 ohm's.

the voltage drift on your op-amp by the way , when set for quite high gain , with long leads or in this case conected to a nice long wire (your coil) can pick up on fluresent lights , had this happen myself.

oh, and old oscilloscopes with crt screens , can give off interferance , had this happen , apparently someone said it was to do with the hv vertical hold circuit in the oscilloscope , old tv's the same inerferance.

and , they realy dont like it if you aproach them with one of those instant heat soldering guns , found that out when i thought i'd ad a resistor when circuit still on , will "flap" rail to rail , still all makes sense as those soldering guns are induction based.

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