Quick question about Transistor values - Page 1
Forum section: electronics / beginners

author: Szewczykm
author_rank: Regular Contributor
timestamp: March 12, 2014, 02:56:15 pm
content: For a 2N2222, there are three values that I don't completely understand.CBOcollector-base voltage open emitter2N2222 - 60 VCEOcollector-emitter voltage open base2N2222 - 30 VEBOemitter-base voltage open collector2N2222 - 5VCBO - Is this the voltage level that the transistor will "hold back" before it breaks down? IE: The transistor is "off" and if the voltage at the collector is more than 60v then the transistor will be damaged?CEO - Is this the voltage that the transistor can work with across the collector and emitter? IE: These specs would be goood for switching 24v, but the transistor will be damaged if you are switching 40v?EBO - Is this the voltage max that you can use for switching the transistor on and off? Is it 5v total, or 5v in reference to CEO? IE: Can I switch 24v by applying 24v to the base because it does not exceed 5v over CEO? Or is it 5v over ground?I realize that current is part of the equation, I'm just asking about the voltage specs right now.

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: March 12, 2014, 06:50:34 pm
content: Those all appear to be maximum values, so yes, if you exceed them, well, you are then experimenting, so yes, you may cause damage.

author: uwezi
author_rank: Unknown
timestamp: March 12, 2014, 09:44:39 pm
content: These three voltages mark the absolute outer edges of the safe operating range of the transistor.They tell you which voltage you can have at absolute maximum between pairs of two leads when the third lead is unconnected, hanging in the air, open.CBObreakdown voltage between collector and base, with emitter not connectedCEObreakdown voltage between collector and emitter, with base not connectedEBObreakdown voltage between emitter and base, with collector not connectedThis is nothing you would do in "real life" unless you use the base-emitter diode alone in a noise generator or as a pretty nice diode for precision rectifying.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 12, 2014, 11:18:51 pm
content: A good way to think of it (not necessarily correct, but I think it's representative in this case): there is leakage in both the C-B and B-E junctions.When testing Vceo, the base is open circuit, so any leakage current flowing from collector to base causes forward-bias in the base, which turns the transistor on slightly. That turning-on current flows out the emitter, which is fine, but the base kind of gets dragged along for the ride. The overall effect is, the transistor acts like a zener diode around 30V or so.If, instead of allowing the base to float, that leakage current is drawn from it directly, then there is no gain effect, and the breakdown voltage is as high as possible. This is Vcbo.And yes, one can effectively use a transistor as a variable zener diode, controlled by placing a resistor B-E to shunt away some of that leakage current. (I doubt the voltage is at all stable, let alonenoise free. So don't actually use this for something, just be aware that it exists.)The same effect should occur on the E-B breakdown, i.e., Vebo > Veco. Come to think of it, I haven't tried this, I should measure it.The practical effect of this is: if you're in a switching circuit... it depends! If your circuit actively shunts the base to ground with a seriously strong driver, then it will certainly be Vcbo*. If it's a much weaker drive, like a simple B-E resistor, it will probably be at Vceo for some time.*Except when it's not. It also takes some time to get there, too. If the transistor was just turned on hard, it will act like it's still slightly on for some time as excess charges dissipate. So a transistor might avalanche at Vceo while it's turning off, then gradually rise to Vcbo. This is important on high voltage switching transistors, like MJE13007, or horizontal output types. These often specify the RBSOA (reverse bias SOA), which have curves showing how much voltage you can expect to withstand after however much time.Practically, I would treat Vceo as equivalent to a MOSFET's Vdss. That is, a 40V transistor will be okay for switching no more than 25 or 30V -- allowing for supply variation, supply transients, and the normal switching transients that occur on every cycle.By the way... experimentally, it may be significantly higher than rated, too. I typically measure 2N3904s breaking down around 100V.Tim

author: uwezi
author_rank: Unknown
timestamp: March 13, 2014, 08:49:08 am
content: Quote from: T3sl4co1l on March 12, 2014, 11:18:51 pmSince manufacturers generally only state the lowest limiting voltage they want to guarantee for their products, this is perfectly normal...Let's have a look at the datasheet of the 2N3904 (Fairchild):You can see that only a minimum value of VCEO, VCBOand VEBOis filled in - a maximum value would not make sense, neither would a typical value, since you have to expect the worst case when designing your circuit.You can also see the manufacturers definition of the breakdown, or the test condition. These breakdowns are not expected to be infinitely sharp transitions from non-conducting to conducting. Rather it is a transition from leakage currents in the order of the stated cutoff currents to the value stated as test condition.So you would read the first row about VCEOas being the lowest voltage between collector and emitter in reverse direction (i.e. normal operation, for an NPN the collector is connected to the positive side of the test voltage) with the base not connected at all, where the collector current reaches/exceeds a value of 1 mA.

author: lewis
author_rank: Frequent Contributor
timestamp: March 13, 2014, 10:17:51 am
content: A transistor is two diodes (PN junctions) back-to-back with the intersection of the junctions at the transistor base.The Vcbo and Vebo values are the reverse breakdown voltages of those diodes. The 'o' on the end means with the other terminal open.Vcbo = 60V. This means the collector can be 60V max higher than the base for an NPN transistor. (For a PNP transistor the collector can be 60V maxlowerthan the base)Vebo = 5V. This means the emitter can only be 5V max higher than the base for an NPN. (For a PNP it's 5Vlower).We know the forward bias values for these diodes, Vbe and Vbc are about 0.6-0.7V. This means the base can never be higher than 0.6-0.7V above the emitter, or the collector. But Veb and Vcb are thereversebias breakdown values. Note the eb and cb are the other way around. It's like saying the Vakof a 1N4007 diode is 0.7V but the Vkais 1000V.Vceo = 30V. This means the maximum voltage between the collector and the emitter can be 30V.But wait! We said the maximum value between the collector and base is 60V and the maximum value between base and emitter is 5V so that means we should get 65V, right? Unfortunately not, because the transistor is notreallytwo diodes back to back. The Vceo is generally the most important parameter, I can't remember needing to rate a transistor according to the others.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 13, 2014, 07:43:44 pm
content: Quote from: uwezi on March 13, 2014, 08:49:08 amNot only that, but keep in mind the nature of the spec, too: JEDEC parts are datasheet-driven specs, not part-driven. If a given die meets or exceeds the spec for a given part, it can be marked and sold as one. As long as everything else (hFE, Vce(sat), fT, etc...) meets, a 2N3904 could be 60V or 400!Tim