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Registered Member #3781
Joined: Sat Mar 26 2011, 02:25AM
Location:
Posts: 701
ok I fixed it? after many blown 555's and some melted plastic, I figured out it was either a problem with the power supply, or mosfet, or both, im not sure... the powersupply says it's 12v dc 500mA. Although I checked it on my multimeter and it said 20v, so that might be the problem. The fet is I'm using my computer power supply in the meantime Is there any way I can check if my mosfet still works or not without risking the circuit?
Registered Member #3766
Joined: Sun Mar 20 2011, 05:39AM
Location:
Posts: 624
20V is probably blowing your 555's, are you using a wall wart? I've had those go quite a few volts over before. Melted plastic makes me thing that theres a short somewhere, pin 7 is notorious for an unseen short, it essentially switches directly to ground.
You should be able to make a simple setup on a breadboard pretty quickly with a FET, LED, pushbutton switch (if you have none you need to scrap more junk), and a few resistors (don't blow the LED or FET). Ohms law is your friend.
Registered Member #3610
Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506
That mosfet is way too low voltage to be reliable driving a flyback transformer. You will want one rated at least 500V, but 800V would be better. 15-25A is more than enough. The mosfet you selected is intended for things like high current low voltage motor controllers. It has a current rating of ~10 times and a voltage rating nearly 1/10th what would be ideal.
Also you can't drive the gate directly from the 555 and expect it to work well, especially not with a mosfet that large. You have to use a driver, this can be as simple as a 2N3904/2N3906 totem pole and you want a gate resistor, say 5-10 Ohms. With a properly designed circuit, you won't blow the 555 if a mosfet fails.
As for the power supply, the voltage listed on it is at full rated load. A common unregulated iron transformer wall wart will deliver substantially higher voltage into light loads.
magnet18, what the heck kinda scope do you have that's limited to 20V? Are you sure that isn't 20V/div? Do you have a 10x probe? That will increase the voltage range by 10x.
Registered Member #3781
Joined: Sat Mar 26 2011, 02:25AM
Location:
Posts: 701
ah maybe, it's meant for a led rechargeable light... The melted plastic was a little over exaggerated haha I will go set it up the circuit right now, I have a small box filled with switches haha. @James That seems mighty high, I had a 250 volt fet that made 4" arcs and it worked until I ran it to long on 20vdc
Registered Member #3766
Joined: Sun Mar 20 2011, 05:39AM
Location:
Posts: 624
@james, parallax USB oscilloscope, It's meant as an educational tool, not an analytical scope. I'm in the process of building a divider (as in thinking about it).
@camp Badger, yes, a lower voltage will work... untill a nice spike comes flying out of nowhere. Like the one time the coin lands on it's edge, except less improbable. And for continued usage, usually you use parts rated for well above what there limit is. It's a trade-off, endurance for performance. Like for a 18mA circuit that's going to be on all the time, you should use a 25mA LED.
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