555 I.C. Freq. generator

gopalyajur

Joined Jan 3, 2010
100
Actually, you would be using the high-current-capable MOSFET or BJT as a switch.



When fully switched on, the resistance across the MOSFET's high-power-capable Source and Drain terminals (often called Rdson) might be in the very low milliohms. And THAT means that even with many amps through it, it won't dissipate much power! So it won't get too warm! i.e. It will not be quickly detroyed! So if you hook one of its terminals directly to your power supply and the other one to your load, BAM! You get about the same waveform shape as your 555 output, but it's now controlling almost a "dead short" (switch) directly from your power supply!

One main performance (and reliability) issue, when using a MOSFET as a high-current switch, is that you "must" be able to make the MOSFET not spend too much time _between_ the "off state" and the very-low-resistance "on state", because between those two it has more resistance in the high-current path. And more resistance would mean (much) more "i-squared-times-r" power dissipation, which can get way too hot to touch, very quickly, and can very easily destroy the device.

So, in reality, you have to make sure that you can bang the MOSFET's gate very hard, and very FAST, when turning it on and off. So (and I am not sure whether or not it would always be necessary, when using a 555 to drive a MOSFET), you might even want an intermediate medium-power switching device in between, such as a smaller transistor, to be controlled by your 555 output to drive the MOSFET's gate.

And you would definitely also want to use a MOSFET with a very low gate capacitance, since any capacitance will form an RC "time constant" with the resistance in the circuit (and there always is resistance, even with no resistors), slowing down the turn-on and turn-off times, which is bad (for already-mentioned reasons).

With such high-speed edges (you hope), you also need to pay attention to trying not to allow the formation of high-frequency "ringing", i.e. damped oscillations, especially in your gate voltage, since the higher frequencies have more energy and can tend to heat up the MOSFET, and also might incite actual sustained HF oscillation, which is much worse and more-quickly destructive.



Cheers,

Tom

Thanks Tom for the detailed explanation. I still understand clearly the use of a small transistor to drive a MOSFET. Is it still not the same as driving the MOSFET directly. Please have a look at the attached pdf. Have I misunderstood you. Could you/someone else please let me know if I am thinking in a different way.

P.S. The output from 555 is fed through R1 to small NPN transistor.

Thanks in advance
 

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Kermit2

Joined Feb 5, 2010
4,162


Not a full tested schematic, just a sketch of how it should be arranged
The actual resistor values are subject to change, and notice that the PMOS device was changed to be NMOS.
 

gopalyajur

Joined Jan 3, 2010
100
Thanks Kermit. It was my mistake.

I still don't understand the use of small signal transistor?

In your revised schematic when the Mosfet is ON, its still the same without the transistor. No?
 

Kermit2

Joined Feb 5, 2010
4,162
They amplify the current sent into the base; allowing larger currents to flow from emitter to collector. A resistance in either emitter or collector lines will then develop a voltage drop across it proportional to the current flowing through it. By taking the output from that resistor, a small signal is 'amplified' into a larger signal.
 

gopalyajur

Joined Jan 3, 2010
100
Thanks once again Kermit.

I tried to use PWM based on the 555 timer chip to drive a modified CFL. The CFL did not bright enough. Hence I made a simple program on arduino (PWM signal) to drive a transformer (220 - 12 V; 600 mA) through a transistor. It lit the CFL pretty bright.

But, when I tried to connect a non polarised (0.47 μf) capacitor the cfl does not light at all. Also the capacitor does not charge when connected to the output. Why is it so? Is it because of very low the output current?

Also, I have no intention of using the setup to drive any electrical/electronic devices. I stared learning to use arduino and exploring what it can do.

I am also aware of the thread created by Bill Maderson on simple inverters based on 555 chip which is pretty interesting and useful.

I would also take the oppurtunity in wishing all the AAC members a VERY HAPPY AND A PROSPEROUS NEW YEAR!!!
 

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debjit625

Joined Apr 17, 2010
790
gopalyajur
But, when I tried to connect a non polarised (0.47 μf) capacitor the cfl does not light at all. Also the capacitor does not charge when connected to the output. Why is it so? Is it because of very low the output current?
I dont know where you connected the capacitor,its not in the circuit diagram you posted but one thing you should note that capacitors offers resistance to any AC signal ,its called reactance.So be sure that the caps value is proper for the frequency its working on.

It may be any other cause also.

Good Luck and a happy new year
 

gopalyajur

Joined Jan 3, 2010
100
Thanks debajit for the reply.

I did not show the capacitor in the circuit but i connected them across the output of the transformer after the diode. I will have to check the frequency with the oscilloscope. We have an oscilloscope in my office but I am waiting for the one I purchased from china .:)

Hello Ritesh

The site mentioned by Beratus is a good site to start with arduino
you can also have a look the projects using arduino here
http://hacknmod.com/hack/top-40-arduino-projects-of-the-web/

You buy one the boards here http://www.bhasha.co.cc/product.php?id_product=65
 
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