Hello all, this is my first time to the forum and I have a quick question.

I am trying to make an electronic switch using a very simple transistor circuit. I need to do some testing to see what my microcontrollers voltages are for bitwise 1 and 0 (i will post that later), then I will amplify it and direct that into the gate of an N type FET transistor (or at least that is my current plan). I used EveryCircuit to design this and the problem is that I dont know what common transistor characteristics are. I learned about transistors at my college this last semester and I know how to read the data sheets( to an extent), but I dont know what transistor to use or what characteristics are fairly standard for application.

so I would like to start by asking what transistors you use or recommend. How do I know what transistor to use? and how do I find the correct transistor based on the other components in my circuit.

I guess trying to figure out the serial numbers and all of their characteristics is a little scary.

thank you in advance for any help you can give me.

 

For a lot of hobby work, a transistor is almost a generic device. One of the biggest reasons to choose one BJT over another, IMHO, is the amount of current you need it to handle, and the related power rating. For more advanced designs you may have special needs that require high voltages or high frequencies, and then you will need to choose more carefully. But for a low voltage, slow switch application, just about any will do. Switching a MOSFET at low frequency requires nearly zero current, just the gate capacitance and the pull-down resistor.

Oh, and by the way, as a rule of thumb a BJT needs 10% of the current being switched to be applied to its base, in order to be fully turned on (saturated).

 

depends how complex you want to go-what switching speed,how much current are you needing to supply to your load,how much current can your micro safely source etc....heres an example using transistors and a mosfet to switch a load....dependant on your load you may just need a single transistor ...

 

ive attached the basic concept of my circuit, the square wave source is acting as my micro controller, and will have a very small voltage that will probably need to be amplified.

also here is a transistor that I am looking at:

http://www.radioshack.com/product/i...ce=CAT&znt_medium=RSCOM&znt_content=CT2032230

the base/emitter voltage is .85 which is standard based on the labs that I have done. now does this mean that if I apply a voltage greater then this, it may blow the transistor? or is this the voltage loss over the base and emitter?

similar question for the collector emitter side.

I dont want to blow the transistor, but I need a high enough voltage to turn a motor like this one.

http://www.radioshack.com/product/i...ce=CAT&znt_medium=RSCOM&znt_content=CT2032230

and obviously .85 and .2 are not enough.

sorry for asking so many questions. I am an computer engineer in college and I want to familiarize myself with electronics, so I have been trying some projects.

 

As previously posted, a MOSFET requires essentially no current to be switched on. The important requirement is gate voltage, not current. If you choose a logic level mosfet, you can connect the output from your μC directly to the gate. You will need a pull-down resistor on the gate as well. With an N-channel MOSFET, you will be doing low-side switching to your motor. In other words, the +V will be connected continuously and the MOSFET will be making the connection to ground.

In answer to your first question, I use a lot of IRLZ44N MOSFETs even though they are overkill for most applications.

 

In answer to your first question, I use a lot of IRLZ44N MOSFETs even though they are overkill for most applications.

thank you for that, that helps a bit as a reference. I see what you mean by overkill. Its max ratings are very high.

As previously posted, a MOSFET requires essentially no current to be switched on. The important requirement is gate voltage, not current. If you choose a logic level mosfet, you can connect the output from your μC directly to the gate. You will need a pull-down resistor on the gate as well. With an N-channel MOSFET, you will be doing low-side switching to your motor. In other words, the +V will be connected continuously and the MOSFET will be making the connection to ground.

ok so based on your description It would look something like this? (see attachment). Am I correct in assuming that the drop-down resistor doesnt need to be that high. The voltage from the logic level that my microprocessor uses will be smaller, so I can use a little bit smaller resistor. Does this look correct? And no resistor between the drain and ground? or after the motor and before the source? We usually had some there for labs, but I guess the motor will have some natural resistance.

 

Heres a few ideas on how you can drive your motor,also you may use the ULN2003 which already has a complete transistor op stage (7 seperate stages) built in which will simplify things and cut down on the parts you need

 

MrChips has built an extremely useful list of generic electronic parts. Having a look definitely won't hurt: http://forum.allaboutcircuits.com/showthread.php?t=65137

 

Radio Shack carries a couple of assortments of transistors. Years ago, they were real "grab bags"; no telling what you might wind up with. Nowadays, with the NPN assortment you with five 2N2222, five 2N4401 and five 2N3904 transistors; good for about 500mA, 200mA and 100mA max collector current respectively - IF you can supply the Ic/10 req'd current to the base. Their PNP assortment normally has 5 2N2907, 5 4403 and 5 2N3906, complements of the NPN assortment.

However, you are wanting to drive a motor, which is an inductive load. You must provide a current path for the inductor to discharge for when the transistor is turned off, or the voltage across the inductor will build until the breakdown voltage of the transistor is exceeded, and the transistor is destroyed. A diode across the motor will provide a current path. Connect the cathode (end with band) towards the more positive polarity. Use the transistor's collector or mosfet's drain to switch the load's ground connection.

handy MOSFET to have around is the IRLD024; about $1/ea at Digikey.com and work great on breadboards.