Any help will be greatly appreciated. This may be a very basic question. I have search for the answer and cannot find a solution to my problem. I am currently working on a research project from school. The idea is to make and Arduino flip a 12v relay, just to understand basic EE concepts. I have been able to acquire a circuit diagram from Jeremy Blums Video series on youtube. The problem is I'm not sure how to purchase two parts from the circuit. Specifically the diode and the transistor . Below is the circuit. enter image description here

Don't get me wrong I have found plenty of tutorials explaining what these part do. I just don't understand how to calculate the specs given the verbiage from the back of the package of these two parts from radio shack. Below is a screen shot from the transistor. enter image description here

And the Diode is:enter image description here

I have been unsuccessful trying to find what Vceo, Vcbo, IC, Pd, fr, Hfe, Vce(sat),and Ic(max) stand for and their role into the calculation of the transistor. As well as voltage(vz), current(iz) for the diode.

Again any explanation or simply a link to a site in which I can read about these naming conventions would be great. Thanks you for your help in advance, and I apologize for my ignorance.


3 Answers 3


The 2N4401 will do fine in your circuit. However, I would ditch the capacitor accross the relay. Just about any regular diode will do, but the one you have is inappropriate. That is a zener diode meant to act as a voltage reference. You want a ordinary silicon or possibly Schottky diode.

To calculate the minimum specs required, you first have to identify the relay you want to use. The relay datasheet will tell you the current it requires at 12 V. That tells you directly the current the transistor must be able to handle. Most small 12 V relays require 15-50 mA, which is well within the capability of a 2N4401.

You also have to think about how much collector current the transistor can support for the base current you are giving it. Your sketch shows a 1 kΩ base resistor. Figure the B-E drop as 700 mV, which leave 4.3 V accross the resistor when the relay is supposed to be on. That means there will be 4.3 mA into the base. Figure a 2N4001 can be relied on to have a gain of at last 50. Your specs say 100-300 for Hfe, which is another way to say gain, but what you post is a dumbed down snapshot of the datasheet. In any case, 4.3 mA times 50 is 215 mA, which is lots more than any reasonable "small" 12 V relay is going to require, so all is fine there.

The diode has to be able to take up to the relay current in forward mode, and block at least the power voltage in reverse mode. A 1N4148 is a common small signal diode that can do this. Those usually top out at around 50-75 mA (there are a lot of variants out there), but again, that is still more than a reasonable relay will require.


Olin has pretty much summed it up. Pretty much any NPN will work in this application. The 2n4401 is not a very common transistor, it also has the "european pinout", with the base in the centre, the 2N3904 is much more common (in the US), while the 2N2222 is popular, and the part number is a easy to remember. Just bear in mind that some of the plastic pack transistors, and almost all of the larger and surface mount transistors will have the collector in the middle.

The 1N4148 is the obvious choice here, (and if you were switching a much larger relay/solenoid then a 1N5819 is the obvious choice, but obviously you would need a bigger transistor/MOSFET)

The capacitor would not normally go where you have indicated, possibly you meant to attach the lower end to ground? and you should indicate it's value unambiguously, you appear to have "1mF" this is 1000uF? (quite a large capacitor , not normally written as mF, except in circuit simulators) or did you mean 1uF or maybe 1nF? (In the old days of valve radios, capacitors were sometimes marked "MFD" meaning microfarad or uF)


Oops, Your question was two pronged , OK, I'll explain the verbiage to you: Semiconductor specs usually include as a minimum, two types of data: Absolute Maximum and operating values. The absolute maximums are the "red lines" of operation, somewhere above this the part will self destruct. In many applications you may not be able to reach some of the maximum limits as you will have exceeded another. Note that all the specs that look like "Vabc" are the voltage between the two terminals a and b when the other terminal is at condition c.

Vceo = Voltage collector to emitter with base open Vcbo = Voltage collector to base with emitter open , easy to measure with a zener diode tester, but no real circuit uses a transistor this way. Note that if you short the base to gnd, and the emitter is grounded too, then the maximum voltage between collector and emitter is numerically the Vcbo value. So any real circuit like yours, with some resistance in the base leg will have an effective Vce somewhere between the 40v and 60v value for a 2N4401. The significance of Vceo is that the collector-base junction behaves as a zener diode, and the leakage current will sharply increase when the voltage exceeds a certain value. This leakage current increases with temperature, so what the manufacturer is telling you is that, provided the supply voltage is < 40v then there will never be enough leakage current flowing into the base to accidentally turn on the transistor.

When the supply voltage exceeds 60v, the leakage current across the collector- emitter junction will be sufficient to heat up the junction, even if you short the base to ground.

Ic = collector current , this heats up the transistor, and can cause localised hot spots. Ic is the maximum continuous current, be aware the test conditions are nothing like real life, so always ensure your value is lower than half this.

Pd = power dissipation, this is the power level that will cause the junction to exceed 175C with specified cooling applied to the transistor, your circuit may not have the same conditions, may be crowded, may be in high ambient, so plan on only using half of this. Note that Pd is determined by the package, and not what is inside, so all devices in a TO92 package will have the same Pd, even if if its physically impossible to get this power level.

Ft is the transition frequency, this is the frequency at which the common emitter current gain drops to unity. Above this frequency, a piece of wire has more (common emitter current) gain. So for example you wouldn't use this transistor for circuits operating faster than 25MHz, Your relay operates in 5mS so no problem.

Hfe This is one of the "H" parameters , assumimg the transistor is a black box with a pair of wires going in and another going out. So its "H parameter" "forward" "common emitter" so put 1mA into the base, ground the emitter and 100-300mA appears at the collector. Note that Hfe changes with the operating point, so it's never a fixed number. Vce(sat) is the voltage across the transistor when its "hard on" , probably at the 600mA mentioned, this gves 600mA x 0.4V = 240mW, so they have arrived at Ic by working backwards from the Pd. Vce(sat) also varies with operating point.

ZENER: Vz: This is the voltage at which 1mA flows through the zener, this changes slightly with temperature. Iz: The nominal operating current (this drops as the zener voltage goes up ) Pd: is 1W , all parts with this package have a 1w rating, this assumes certain lead lengths, PCB areas etc, zeners will happily operate close to this value, but will get very,very hot, and may damage surrounding parts. Best to go for half this value. Note that multiplying 20mA x 12v is only 240mW , but you will find that zeners 10- 20v will be 20mA, and 20v to 40v will be 10mA, manufactures just bunch them up in tables. Zeners always fail short circuit, so only operate them close to the limits if a short circuit does not cause a cascade failure.

Thermal impedance: Not mentioned by you, but is seen in data sheets. Basically it takes a certain time for the junction to heat up, so a short pulse that exceeds the maximum continous rating can be safely applied in certain circumstances. So for example the transistor in your circuit will pull several amps to charge the capacitor briefly, not a problem for a 1nF capacitor, but "briefly" becomes too long for a 1000uF capacitor. Which is why your schematic raised the eyebrows of your readers.



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