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Forum Index : Electronics : Sensitive Mosfet in piclog circuit
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Cornelius Newbie  Joined: 26/12/2008 Location: NorwayPosts: 27 |
Just made the basic part of the piclog v1.2 (volt measurement and dumpload circuit), and it's working fine. Except that i can't get the hang of Mosfets, apparently... 
I'm using a P80NF55 from an old inverter, which are similar to the P40N10, except for the V/A. (55V, 80A) I've connected Source to v-, Drain to relay (and relay to V+), and Gate to Out2 on the Picaxe via a 100Ohm resistor. The problem is that the Mosfet are very sensitive on the Gate; just touching it with my finger turns on the Mosfet, and when Out2 goes low, the relay stays on... 
Is there something i miss here? I assume the mosfet are pictured with the front facing me in the drawing of the piclog, so the pins are G, D, S. |
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GWatPE Senior Member  Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
Hi cornelius, You should connect a pulldown resistor to the mosfet gate and ground, if you use any sleep functions on the picaxe. If you have omitted the protection diode across the relay, then this can potentially lead to failure of the mosfet, from the back emf when the relay is turned off. Gordon. become more energy aware |
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| Cornelius Newbie Joined: 26/12/2008 Location: NorwayPosts: 27 |
Yes, i do use a pause for 1 sec. in the code after sending data to the comport. (Without, Sertxd would send 10-15 times/sec., since i do not use any Count command yet.) I see in the circuit for the Picaxe based Windmill Controller/Charger, that a 100k pulldown resistor are used. Would this be suitable for the logger too? (I'll probably test before anyone answers though...  )
Cornelius |
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| Cornelius Newbie Joined: 26/12/2008 Location: NorwayPosts: 27 |
Now i've added a 100k pulldown resistor, and it's working excellent now!  (Also added the diode to the relay. I'm aware of the need for a diode over the relaycoil; just taking one step at a time...  )
Thanks, Gordon! 
But alas; i've no windmill to use this piclogger on - yet... But it's a work in progress. I wrote a Diary on it over at Otherpower...:
7 feet Windgen Excellent chips, these PicAxe's... |
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