# Picking transistor

#### Codyf1113

Jun 18, 2013
28
I've spun the whole week looking things up and looking though bunch of books from the library. I have herd how transistors work and bunch of other confusing things hundreds of times now, but I can't understand how you pick the right transistor for the project you would be doing. Could someone please help me end this headache I've got and tell me how you figure out this stuff? I'm trying turn 200mA from a 555 timer to 350mA for a led needing 3.0~3.2v 350mA

Side note looked at the "Got a question about driving LEDs?" and few other things on this web site but still having hard time understanding all of this.

#### eman12

Aug 24, 2013
31
Hi Codyf,

The common LED's do not need something more than 20-30mA, But if yours is one of those power LED's or you are trying to use several LED's in series or in parallel then you can use the below transistors which happily give you the current you need:
BD139
BD140

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
Firstly, you don't "turn 200mA from a 555 into 350ma"

What you do is that you use the 555's output (at well under it's maximum current) to control another circuit that will switch, or limit, or regulate (as required) the current to 350mA for a LED.

But, we need to know more.

What voltage are you running the 555?

What is your power supply? (do you have a different one for the LED?)

Is it just 1 LED you want to flash?

Without more information, I might suggest that you look at a circuit like the one in figure 3.5 in the tutorial

For this circuit you could use the following:

Q1 - N Channel Logic level mosfet (see below)
Q2 - BC548 (or some similar small signal NPN transistor)
R1 - 2 ohm 1/2W (use two 1 ohm resistors in series)
R2 - 470 ohms
R3 - 10k ohms

Rather than specify a particuar device for Q1, I would suggest you go to your supplier's web site and look for a logic level mosfet in a TO-220 package capable of at least double your supply voltage and at least 2A. There should be *many* to choose from. Pick the cheapest and get more than one just in case you damage it.

Edit: In some cases, the transistor suggested by eman12 above could replace the mosfet Q1 in the circuit I recommended. On it's own it would likely not work as you intend.

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#### eman12

Aug 24, 2013
31
Yes Agreed,
A mosfet is able to do the job nicely. Or if you do not know what exactly to do, then please try to see if a small "relay" can be helpful for your job or not.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
eman12, you seem to be ignoring the probability that the LED will need to be current limited.

In fact the certainty -- he specifies the device as a simple LED.

#### eman12

Aug 24, 2013
31
eman12, you seem to be ignoring the probability that the LED will need to be current limited.

In fact the certainty -- he specifies the device as a simple LED.

Yes I know Steve, But thanks for reminding. He can try to use a resistor to limit the current anyway.

#### eman12

Aug 24, 2013
31

Yet and apart from the heat and power loss subjects of a resistor I think he is able to use a current limmiting resistor. Yes, as I told before I am agreed that the best component to be used is a Mosfet, But the current limiting resistors work as well.It is in the case of LASER diodes which he would not be able to use a simple resistor as the current limiter or he surely will lose his laser diode

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#### Codyf1113

Jun 18, 2013
28
I'm wanting my power supply be 6volts (4x AA or D batteries) The 555 timer will be running at same voltage.

The leds are 1w blue and I got them off ebay. Looked around and most of the people selling them saying the voltage for them are 3.0~3.2 and 3.2~3.4. Going for the 3.2 be on the safe side. But no one is saying what the mA is but asked the seller of mine and he told me 350mAh

Im trying get one of these leds flashing before I try and make 6-8 of them flash

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
I'd recommend the circuit I suggested in post #3 on this thread then.

Be aware that AA batteries will go flat pretty quick with this sort of load, and even quicker if you're flashing more of them.

I would recommend that you run separate power and ground wires from the battery to the 555's (separate to the ones carrying the current to the LEDs) and have a diode/capacitor to isolate the 555's power.

I would also recommend that you make the "on" time for the LEDs much shorter than the "Off" time if possible as this will extend the life of your batteries as well.

#### Codyf1113

Jun 18, 2013
28
Sorry been sick past few days, I'm not to worried about the life of the battery this more of a starting project for me to practice with before I try and make bunch more of these leds flash.

Anyway I'm having hard time understanding how I can apply what you said to the schematics I have for making a single led flash.

Photo below is the schematic I used to make a led flash before. The photo beside it is the best I can think of how put a transistor in to the schematics. How off am I on this? also that diode you were talking about would go between the source (s of the transistor) and the ground?

#### Attachments

• 555 timere led flasher 1.gif
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• 555 timere led flasher 2.gif
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#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
That seems reasonable.

However you don't show that the mosfet is N or P channel.

It should be an N channel mosfet and there should be an arrow pointing to ward the gate from the substrate

#### Codyf1113

Jun 18, 2013
28
Rather than specify a particuar device for Q1, I would suggest you go to your supplier's web site and look for a logic level mosfet in a TO-220 package capable of at least double your supply voltage and at least 2A

It will be the N channel one but I still need know what is the name for the mosfet I need. Long as the transistor/mosfet is more than 350mA and same or higher voltage that the 555 timer would be using it should work?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
OK, let's go through a process of selecting a transistor.

(I note that we're not using a constant current driver despite my strong recommendation.)

1) Firstly go to digikey. (Yeah, even if you don't plan to buy from them.)

2) in the search box type "mosfet".

You get a list of broad categroies that might contain what you're searching for. Generally the one with the most hits is the one you want.

3) click on "FETs - single"

Now you're into the parametric search. We first need to start narrowing down by some obvious first choices:

4) Select from the "FET Type" box, "N Channel, metal oxide" and then click Apply filters.

5) Scroll to the right, and withing "Mounting type" select "Through hole" and click apply filters.

Your power supply is a low voltage, so you'll need mosfets which can be turned on with a low gate voltage.

6) In "FET feature", select "Logic Level gate" and click on apply filters.

There are still over 1500 devices, so lets narrow down on some ratings that may be useful.

7) in the "Drain to Source voltage (Vdss)" box, select all values between 20V and 60V. Then click Apply Filters. Whilst a Vdss outside this range would be fine, it's useful to have some margin at the low end and at the upper end the devices will be more expensive (all other things being equal).

The next important thing is the current. Although we want 350mA, I wouldn't pick anything capable of less than 1A. At the upper end, let's say 15A?

8) in the "Current - Continuous Drain (Id) @ 25° C" column, select all values from 1A to 15A and click Apply filters.

We don't want something fragile, so let's pick a device that can handle at least 2W of dissipation. and maybe up to 50W.

9) in the "Power - Max" column select all values between 2W and 50W and click on apply filters

OK, now I show a little over 40 devices, and that's small enough -- any fewer and I might open up my selections a little (and probably at the upper ends).

10) In the quantity box below the search boxes, enter "1" (or a greater number as appropriate) and submit it.

11) in the results table at the bottom, look in the price column and click on the small blue triangle at the left of the column to sort in order of lowest to highest price.

The first result is this. It's cheap and would do the job.

However, if you wanted to bolt the device to a heatsink, a few rows down is a TO-220 package here.

If you didn't want to have to worry about insulating the device from the heatsink, there's a full plastic version a row or so below. Here.

If you're not intending to purchase from, digikey, then perhaps you would take the part numbers from the first page of results and search for them on your preferred supplier's page.

In this case, we chose the specifications with plenty of excess capability, and we know that there are unlikely to be thermal issues, so we can probably go ahead.

In other applications (especially high current applications) there may be a number of other considerations.

#### Codyf1113

Jun 18, 2013
28
Thank you so much for all your help. My mind been going nonstop trying figure how you select a transistor.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
In practice, many people would already be familiar with the characteristics and uses of some devices and would tend to re-use them.

For example, once you build this device with some reasonably cheap mosfet, you might just use it again for another application without searching for other components.

You'll find a lot of circuits that use 2N2222's and BC548's.

2N2222 circuits tend to come from people in the US, and I'm not aware of the historical background (maybe someone else is). [As an update, apparently it came out as a metal can device early on and then repackaged in plastic]

BC548 and similar transistors are found in many European and Australian circuits because the BC108 was an early silicon NPN transistor. It was replaced with the TO-92 BC548, and people pretty much swapped over. I used them for years before I knew what the specs meant.

#### Codyf1113

Jun 18, 2013
28
I know i should be familiar with the things I'm using but I just like learn things as I go along and if I mess up just try again. Anyway I have one last question and thats how you calculate the resistor from pin 3 to the transistor, and will the resistor from the led to the transistor change or will it stay the same?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
The gate resistor to the mosfet should be calculated to limit the maximum current from the 555 (the gate looks like a small capacitor).

Since the 555 allows about 200mA max, then calculate the resistor on that basis. For a 9V power sullply, this means a gate resistor of at least 45 ohms (I would use 100 ohms).

For the LED resistor, assume the LED is operating from the full 9V and calculate it based on the forward voltage drop and the desired current. If it is a red LED and you want to operate it at 20ma, then...

R = (9 - 1.8) / 0.02 = 360

So use a 390 ohm resistor (the closest E12 value greater than the size calculated)

#### Codyf1113

Jun 18, 2013
28
Thanks and the led is blue at 3.2v and 350mA

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
Thanks and the led is blue at 3.2v and 350mA

In that case you should be using a constant current driver as I've discussed earlier. This will also result in a much higher value of gate resistor as you're operating the mosfet in its linear region rather than as a switch.

I believe I posted a circuit previously.

edit: No, I pointed you to the one in the FAQ and gave you part numbers and component values.

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