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Picking transistor

Codyf1113

Jun 18, 2013
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sorry about me being a pain but you talking about the
Q1 - N Channel Logic level mosfet (see below)
Q2 - BC548 (or some similar small signal NPN transistor)

or the TO-220 tansistor off digkey site
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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sorry about me being a pain but you talking about...

I'm talking about what I wrote in post #3 in this thread.

The circuit is in the LED faq.

The TO-220 mosfet is the same as what we have selected for recently. However, depending on the voltage you're running at, you may need a heatsink (I'd recommend one for anything above 6V)
 

Codyf1113

Jun 18, 2013
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The TO-220 mosfet is the one I like to use but you also wrote about the BC548 that went along with the figure 3.5 in the tutorial. Just making sure that had nothing do with this.

Im going be using 6 volts and when I get it all up and running I'm going be using 12 volts to power multiple of these leds. The heat sink for the transistor actually cost less than what they transistor did. Also Im going need this go along with it? http://www.digikey.com/product-search/en?x=9&y=16&lang=en&site=us&KeyWords=HS417-ND
 

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That's not a heatsink. That's a kit ogf parts to connect a TO-220 device to a heatsink.

If I recall correctly, I did recommend a "plastic" TO-220 device as that removes the need for most of that hardware and you can just get a small nut and bolt from a hardware store.

In terms of the heatsink, let's assume you have 3.5V across a LED at 350mA and everything is being powered from a 12V source. The amount of heat dissipated in the transistor will be (12 - 3.5) * 0.35 = 2.975W (let's say 3W)

If your max ambient temperature is 30C and you want the device to be no more than 60C, then you need a heatsink rated at (60-30)/3 = 10 degrees C per watt or LOWER.

Go here. (I searched for heatsink and chose the obvious one)

Then scroll to the right and choose all ratings of 10 deg C and lower in the "Thermal resistance natural" column. Then apply the filter

Set a quantity of 1 and sort by price.

Here is the top hit for me.

Note that it's slightly better than you need, and it has appropriate mounting holes for TO-220.

That unit is designed to be bolted to the device and onto a board. Others have small legs so you can mount them standing up on the board and solder them in place.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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The TO-220 mosfet is the one I like to use but you also wrote about the BC548 that went along with the figure 3.5 in the tutorial. Just making sure that had nothing do with this.

Yeah, you also need the BC548 and the various resistors that I specified.
 

Codyf1113

Jun 18, 2013
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How would I apply the bc548 in to that schematic I showed in my other post
 

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You wire it up just like it's shown in figure 3.5 of the LED tutorial..

The input to it comes from pin 3 of the 555. The power supply connections go to your power supply as you have done in the circuit you've shown above.
 

Codyf1113

Jun 18, 2013
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I know but I'm having very hard time understanding how I add that BC548 along beside the other one.

I know you just told me but honestly Im having still having hard time adding that to the photo I drew. I never had deal with the collector base emitter pins before and looking at that photo from the "Got a question about driving leds" to me it all just running around circles looking at that photo of the two transistors. Know its not much of a excuse but I'm bit dyslexic its not out much.
 

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Here's a circuit diagram for you:

attachment.php


It seems like you want to draw the circuit diagram with things which look like the components. Don't do that. It doesn't help your understanding of the circuit.

Once you have a design, the next stage is to think about how you're going to build it.
 

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Codyf1113

Jun 18, 2013
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I understand that better now, thank you so much. And reason I draw the schematics like I do just helps me understand after I found out how the circuit works so I don't have figure it out again later when Im putting it together.

Side question. The C2 0.1uF is a non-polarized capacitor nothing special. Looking at digkey web site for the BC548 all seem be the same but the DC Current Gain section.Will any of them work?
 
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I understand that better now, thank you so much. And reason I draw the schematics like I do just helps me understand after I found out how the circuit works so I don't have figure it out again later when Im putting it together.

I have added pin numbers onto the 555 to make that easier.

The next step is to get the transistors and note which pins are collector, base, emitter, or drain, gate, and source. To do this, you may have to look a the datasheet for the component you purchased because there is generally no standardisation.

The C2 0.1uF is a non-polarized capacitor nothing special.

Nothing special. Pick something small and cheap. Often they will have a 50V rating and that is fine in this application.

Looking at digkey web site for the BC548 all seem be the same but the DC Current Gain section.Will any of them work?

Yep, there is always a spread of values for gain in transistors. I picked this as a general purpose transistor and you could replace it with almost any NPN transistor with almost no change in performance.

If you already had something else (perhaps a 2N2222) then you could use that too.

The BC548 should be cheap and easy to get. I prefer them to 2N2222's because they have a higher gain and because I grew up on them (probably mostly the latter)
 

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¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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The mosfet I recommended? (This one) <-- that's not a 2N2222

A 2N2222 is, in this circuit, a potential replacement for the BC548.

It's probably worth grabbing 2 of the mosfets and at least 2 of the BC548s as it will save you time and effort (and possibly money) if you destroy one.

When you get them, the mosfet is static sensitive, so don't remove it from its packaging until you know what precautions to take.
 

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Yep, that's perfect.

While you're waiting for them to arrive, you need to go back to the links and download the datasheets for all components other than the resistors and capacitors.

From these you will get the pinouts (you already know what they are for the 555).

Then you can start thinking about how you will lay out the circuit on a breadboard or later on veroboard, matrix board, or your own PCB.
 

Codyf1113

Jun 18, 2013
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Ok, If this all works out ill post video/photo of it. Doesn't Ill just keep on trying till I get it going. Thank you for all the help you gave me :)
 

Codyf1113

Jun 18, 2013
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Ran in to small problem with the C2 0.01uf capacitor. I can't find one at any store in my town. They only have them in something called ceramic and mylar film capacitors. Is it one of them two?
 
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