100 watt light bulb driver

nero

I want to drive two light bulbs,,,alternating,,,using a 555 timer.with· either a relay or· a mosfet..How do I calculate the gate resistor value of the mosfet? Probably· a relay will be easier?· Any help

Leon

Why do you need a gate resistor? A MOSFET has a very high gate impedance.

Leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

nero

So a 555 timer will drive the mosfet directly?

Philldapill

It should do fine. However, the mosfets might get a little hot due to the 555 not being able to source enough current to turn them on quickly. If the frequency is low enough, it won't matter.

Leon

You might also need to run the 555 from something like 12V, to ensure that the MOSFET is fully turned on. It depends on the MOSFET. You could always use a MOSFET driver, like the Micrel MIC4426, of course. That will source plenty of current.

Leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Post Edited (Leon) : 1/12/2009 6:12:10 AM GMT

Beau Schwabe

nero,
·
If there is any doubt, and you are not trying to control the brightness of the lights, just the on and off state rather, go with a Solid State Relay (SSR).· For 100 Watts You will need at least 3 Amp... (doubling the safety rule of thumb).
·
If it is brightness that you want to control, then neither a simple 555 or MOSFET solution will work, you need something more sophisticated that's going to operate around the zero crossing of the AC.· BTW - many commercial SSR's have a narrow window near the zero crossing where they will turn "ON".· This ensures that they run cool by switching before the voltage peaks, but at the same time it makes them a bad candidate if you want to control brightness.
·
I would suggest taking a look at this document regardless of if you just want to turn the light on or off or even adjust the brightness.· It shows how to safely construct an Opto-isolated SSR without an upper turn on limit and also how to construct an Opto-isolated Zero-Cross detector to work with the SSR to control the brightness.
·
http://www.parallax.com/Portals/0/Downloads/docs/cols/nv/vol8/col/nv146.pdf

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Mike2545

I agree with Beau, a simple opto-isolator and triac is what you need here. the stamp can drive the led in the opto-isolator and let the triac handle the 120v. Simple and safe.
For the opto-isolator i would recommend the moc3011
and for the triac use 2N6073a.
For a few dollars you can easily do this.

nero

Wow,,,you guys are the greatest thanks I read jon williams ,,my project is for on off only,,,and I want two lamps to alternate...some simple..logic should work here, say a 1k resistor to ground and the 555 driving that,,,can I pick off my two logic states at that point?
Also how does the lamps dim on jons circuit,,do you need to change the program,,to dim the lamps?

Beau Schwabe

nero,

The article I provided a link for earlier tells you what you need to know. Using two Opto-isolators, one to monitor the Zero-crossing of the AC voltage, and another to control a triac driving the light. The program involved controls the timing to the triac telling it when to turn ON upon detecting each Zero-Cross. See page 3 of the link below.

http://www.parallax.com/Portals/0/Downloads/docs/cols/nv/vol8/col/nv146.pdf

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.