Step 5: How a Does a Transistor Work

A transistor works by changing its resistance between the pins collector (power going in), and emitter (power going out), depending on how much current flows through the base to the emitter.

Unfortunately, all transistor's base collector and emitter pins , are in different places varying from transistor to transistor, which is why you'll never find any transistor pin assignment diagrams, that apply to all transistors, and that is why you should never listen to any which aren't exclusively for your transistor.

Transistors unlike relays, can open up by specific amounts, which are directly proportional to the current going through the base.
This proportion is the gain.For example, if a transistor had a gain of 100, then for every 1ma flowing through the base, 100ma could flow through the collector to the emitter, which technically is considered to be an amplification effect. However when you do this, a transistor tends to get rather hot, transistors operate best either when they are fully ON or fully OFF.

All transistors have a maximum input before the input starts to have no effect on the current gain, and eventually, if it gets too high, the current stops all together, which happens only when the voltage on the base is too close or the same to the voltage on the collector.

When we talk about using transistors just as on/off switches, we generally operate at currents that would saturate, or fully switch on, the transistor which is what i will focus on in this guide.

Here is an animation to show you how a transistor works. In the animation, the arrows represent the flow of water, and show that the smaller source is enabling the larger source to flow. This of course is meant to represent the flow of electricity as well, but its easier to just think of it as being water.

From this example its easy to understand why the base must always be less that the collector.
If the flow from the base was the same as or greater than the voltage at the collector, the hypothetical base water flow, would take up the entire pipe, which on its own would block the collector current as there is no room for it.
A situation like this though often results in a combusting transistor.
<p>Finally i can actually different relays and transistiors. Although, what does npn and pnp mean?</p>
<p>This site may help you: </p><p>http://www.learningaboutelectronics.com/Articles/Difference-between-a-NPN-and-a-PNP-transistor</p>
<p>Is the description of N/C and N/O backwards in his description? : &quot;N/C: Short for normally closed, this terminal is not connected to COM when there is no signal, but when there is a signal, the pin inside the relay is pulled down until it touches N/C, which would connect it to COM&quot;</p><p>I thought N/C should mean that without input(relay off), it's closed in a circuit, i.e. connected to COM. ??, </p>
<p>Exactly my thoughts. In the animated .gif it is also different from the description (the gif is the correct one).</p>
<p>A great wealth of information! Thank you for explaining the difference between the components in an easy to understand way. It's just what I was looking for. I look forward to reading more of your instructables! </p>
<p>Thank you!<br>Finally i catch up with transistor's mechanics! ^_^</p>
<p>Great work.Would like to get more knowledge about relay. A brief history of relays and their applications in the electrical.</p>
<p>Nicely done with excellent examples... I appreciate all the time you put into this...!</p>
<p>Great explanation! Thank you! <br><br>Does that mean that if 2.5mA was applied to the Base only 250mA would be allowed through the Collector creating a total of 252.5mA at the Emitter?<br><br>Also why do people say that transistors amplify? - is it just based on the fact they combine both the Collector and the Base?</p>
<p>Thanks for this tutorial man, this exactly what i was looking for. But I still need to ask you this. Can i use a regular transistor to replace a burnt out mosfet in a pioneer 760w car amplifier? Or would it cause it to get worse? The transistor i want to use is a ML7808A and the mosfet i want to replace is a P55NE06FP.</p><p>If not possible, can you suggest what other kind of transistors I could use to replace it? Just like you i took apart old electronic devices that i had in my garage and wouldn't like to drive an hour to the closest electronic store for such a small component. Thanks in advance.</p>
<p>To answer your question simply: no, you can not replace the MOSFET with the transistor. Now here comes the disclaimer/explanation: the circuit in that amp was designed to drive that MOSFET not a transistor. A transistor uses current at it's base to &quot;control&quot; the flow of current through the collector and emitter pins. </p><p>Think about a transistor as a garden hose with your thumb over the end (pretty sure we've all used that trick to squirt someone). The flow varies with the way you move your thumb, just like a transistor. The MOSFET, however, is like the super-soaker. As soon as you pull the trigger (apply voltage), the gun is shooting water. There is no half way off or half way on, you either are blasting someone with as much water as possible or you're not (and usually running cause you're out of water).</p><p>The most important thing to anything electronic is the datasheet. The datasheet will tell you EVERYTHING about the device you are working with... even the maximum limits... which... well... you can test if you want. Hence the name, Fusekiller. </p><p>Use websites like: datasheetcatalog.com or findchips.com to look up who sells the parts. Even the search engines can lead you to the datasheet. Most of the time they are PDFs. </p><p>Putting P55NE06FP into findships.com came up with a distributor offering it for $8.0880. Now, with the datasheet, you can find a cheaper, comparable MOSFET from Newark.com, digikey.com, mouser.com, etc. by using the specs in the datasheet!</p><p>Hope this helps,</p><p>Fusekiller</p>
<p>I've guessed how relays work since I've opened a few of my car but never understood the transistor switch. Helped me a great deal. Thank you very much.</p>
<p>Thanks very much for this. I understood relays and I thought I understood transistors but could never get them to work on my project board but I didn't know the base current had to be lower than the collector and how you explained that was perfect. Nice one!</p>
<p>Hey Can I use any transistor to Flash LED If yes then how........</p><p>If no then Explain............</p>
Thanks man, that post really helps understand the relay.The one I'm wiring is in a motor control panel and has 8 &quot;pins&quot; though. I'm familiar with NO and NC, COM is a new term to me. I'm not sure how to see what's what with my meter while I'm putting it together inside the panel with no power on. here's a picture that points to it
<p>Great Instructable!</p>
<p>Thank you for taking the time to write this, it's explained very nicely.</p>
Hi. Thanks for the write up, much appreciated and massively useful. I've been told I can use a power MOSFET insted place of a relay on an automotive circuit. The circuit is designed to switch in excess of 14 volts to the headlight bulbs. In the relay circuit, the existing feed to the bulbs is used to trigger the relay and heavier gauge wiring is used to supply the switch side with full power (via a fuse) to the bulbs. <br><br>At maximum, ie. for main beam, having both bulb filaments on (H4 bulb), the combined wattage would be 120 watts. A fully charged auto battery with a charging system in good order, supplies in the region of 14.4 volts. Therefore the current requirement for the relay is 8.33 amps.<br><br>Most auto relays are easily capable of this and are very resilient devices. For my circuit, would a power transistor suffice or do I need a MOSFET as I've been advised??<br><br>Many thanks :-)
<p>great introduction for someone who know nothing about the concept. thank you</p>
<p>Thank you very much for this!</p>
<p>Thank you so much. Will certainly use some of the tips. Cheers.</p>
<p>Fantastic overview. Thanks for all your work. It was well written and the animations were very helpful!</p>
<p>Good effort on how to explain about transistors. Keep it up!!</p>
<p>This document is nice.. </p>
<p>hi,</p><p>what is the difference between current controlled switch and voltage controlled switch?</p><p>how it differs from each other?</p>
<p>a current controlled switch is controlled by current, a voltage controlled switch requires a set amount albeit tiny amount of current, but the resistance is determined by the voltage. voltage controlled transistors are FETs like MOSFETs</p>
<p>the gain proportion mentioned here is the transconductance (Gm) ??</p>
<p>i dont actually know. Ive only ever known it as &quot;gain&quot;, look up a datasheet and it should hold them term if its not the common aplification gain.</p>
<p>Would I be able to use a FET transistor to turn on/off the signal to an ESC signal wire to control a brushless motor, or would it distort the signal? I want to be able to control whether the signal comes from an Arduino or a RC receiver by switching on or off a FET (one FET for each signal source). I would be controlling the &quot;gate&quot; voltage with the Arduino.</p>
<p>it wouldnt distort the signal, or at least, not enough to do anything if it did, the PWM signal varies in the milliseconds, so you could probably even do it with a relay.<br>however, a fet isnt really suited to the role, youde just want a regular transistor, Mosfets are sensitive to static and cost more than the same power rated transistor. since your just controlling a pwm signal, which i beleive is around 5v at a few mA, and most any transistor could turn on and off a thousand times between each cycle of the signal, a transistor is much more suitable.</p><p>in short, you dont need a mosfet, a high gain transistor will be fine.<br><br>mosfets are only really suited for fast repeated switching of extremely high currents. or super accuracy/precision waveforms.</p>
Oh ok. I was talking to my electronics teacher, and he said that Field Effect Transistors would work better. Then again I don't think he understood my question.
<p>well yes, it would work better, but its totally unnecesary, like in the same way a Ferrari is better than a station wagon, but that fact is irrelavent when you just need to drive down the road to the supermarket</p>
<p>Hi i would like to ask some help after reading your tutorial about transistors ive understande the use of transistors a bit more. but still not much but i would like to ask some help. can i use a transistor as a switch to control charging/discharging of a battery. what transistor should i use? thanks in advance :) </p>
well this gets a bit tricky. no, you wont want a transistor any more than you would use a push button switch, its possible yes but its wasteful, inneficient and difficult.<br><br>for batteries like SLA nimh and nicad, all you need is to output 1.8-2.4v in a bistable circuit that turns off when you reach a certain voltage, that function though is done with transistors. for adjusting the voltage youll want a 5v variable voltage regulator, it uses 3 pins, in, adj/gnd and out, and the output voltage is determined by a resistor connected between two of the 3 pins. for lithium based batteries you need a special kind of variable charger specific to that kind of battery, or just a slow charger which takes days at a safe minimum
<p>i just wanted to make a circuit to measure the capacity of a rechargeable li-ion battery with the use of a arduino. so i cant you use any transistor? how about a relay?</p>
<p>Im not sure exactly of the function you want to perform, if you for whatever reason just wanted to do a low voltage cutoff with the transistor then thats fine, for that ide recomend to use a relay though moreover, since its going to be always on, and also, for 3.7v lithium cells, try to find a 5v-12v transistor since they have lesser resistance.</p>
<p>Great tutorial. Thank you</p>
<p>what transistor(s) would you recommend for doing the setup you have described above? I'm trying to make a strobe light, but the LED I have is a high power Luxeon LED, wich obviously can't run off my 555 timer.</p>
<p>well, this depends on the required current, if its an amp or more, just do a general search for transistors which are classified as &quot;power&quot; transistors. In general power transistors are never used for signal deliveries, but instead, switching heavy loads. <br>one in particular i know and use is called the sd882 NPN (sb772 is the pnp version), it can handle at least an amp but i think can go as high as 2, with a peak of 3 amps. Its a medium-low power transistor and is very cheap. <br>On the other hand, keep in mind that you can use several transistors in parallel if individually they arent up to the task, though its not recommended since the failure of one could damage the rest.</p>
Thanks for the quick answer!
<p>This was a great tutorial; very informative. I am just getting into the world of building circuits, and this write-up was very clear and easy to follow. With regard to switches, is it correct to say that all they do is enable a secondary power source (switch it on)? So if, for example, I wanted to turn a lamp on and off with signals from a microcontroller, it would still need to be plugged into the wall, but with an open circuit, which the switch would then close? Thanks!</p>
yes, but keep in mind, its not so much the fact its a switch, as it is, its a low energy high speed switch which can offer variable currents between saturation and being completely off, plus it CAN be controlled by a microcontroller, wheras a relay or mechanical switch cannot. <br>A transistor for example, unlike a relay might be used at a super high frequency with PWM, to dim a lamp which which wont turn off between the ON-OFF-ON phase. SOmething like a incandescent bulb would do this. <br>But in essence yes, a transistor just enables a secondary power source, though, since the signal is encorperated into the current, its primarily thought of an an amplifier<br>
Say I had a circuit that used a manual button to do something. How would I connect a transistor to the existing circuit, without removing the button, so that the transistor can activate the circuit in the same way the button can?
well, the simplest way would be to use an NPN transistor. since it operates best when your working with positive voltages, or rather its easier to understand.<br><br>Now, find the directionality of current, find which terminal on the button is connected to positive voltage, and which is negative in comparison. Power needs to flow INTO your collector and OUT of your emitter, so find which button terminal is the IN and which is the OUT. if it leads directly to ground or negative then that terminal is your OUT.<br>When triggering the transistor make sure you use a lesser voltage than your transistor is driving, and ensure that if you are using a seperate power supply to trigger the transistor as what you are driving with the transistor, it wont work unless you connect your negatives together, because the base input needs to circulate through ground/- and back through your power supply to complete a circuit, you know how it goes. <br><br>Does this answer your question? <br>To reiterate, Collector takes current in and emitter emits current out.<br>
I got the gist of it, and that's certainly a more thorough explanation than I've th gotten from anywhere else. If I understand this right, I want to put the transistor circuit in parallel with the switch. That makes sense. Thank you!
actually depending on . how old yr set up might be. it could be the electronic board in side could be screwed up.
thank u really this notes useful for me

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