Which value I should use - Page 1
Forum section: electronics / beginners

author: Jasonbit
author_rank: Regular Contributor
timestamp: November 02, 2018, 05:29:21 pm
content: Hello folks,I am looking around on various datasheets and I was compare this two files, relating to BC547:1-http://www.mouser.com/ds/2/149/BC547-190204.pdf2-http://www.farnell.com/datasheets/59764.pdfOn two datasheets, from the same manufacturer, I have on table "Absolute Maximum Ratings" 500mA and 100mA to Ic - Collector Current. Which value I should look and how can I identify if this value is version A, B or C?Thanks

author: Benta
author_rank: Super Contributor
timestamp: November 02, 2018, 06:27:41 pm
content: BC547 was always a 100 mA transistor. A, B and C are the hFEsortings.

author: spec
author_rank: Frequent Contributor
timestamp: November 02, 2018, 06:54:32 pm
content: + JasonbitYes, datasheets can be a nightmare. The 'Mouser' spec is 2014, while the Farnell spec is 1997, so as far as specs are concerned go by the most modern you can find, but only use a datasheet from one of the top tier companies, like Fairchild for example.In terms of what to buy: a BC547 from either company is liable to be the same, they may even come from the same production batch.By the way, the fundamentally important parameter is the transistor's junction temperature and thermal resistance, rather than power rating which is a bit of a 'sales' figure. The maximum current rating is important, but you would rarely run a transistor at it's maximum current.As to the 500mA max IC, that just does not make any sense, as Benta implies.You ask how you know which version you will be ordering. Normally the BC547 will have the suffix printed after the base number. For example a BC547 would be marked BC547B and the ordering information should specify this too. Remember, if there is no mention of the suffix you will get a plain BC547 which has a very wide current gain (HFE). It's the minimum gain that counts and 110 is still respectable.Incidentally, if you are looking for a good small signal transistor for your spares box, the BC546 is the best bet: it has a 65V C/E rating. The PNP compliment is the BC556.If you did need a transistor with a bit more current capability, the 45V/800mA BC337/BC327 are a good choice.

author: Audioguru
author_rank: Super Contributor
timestamp: November 02, 2018, 11:02:09 pm
content: Why are you looking at wrongly copied spec's from a store instead of from the manufacturer?

author: Jasonbit
author_rank: Regular Contributor
timestamp: November 03, 2018, 03:15:36 pm
content: Quote from: Audioguru on November 02, 2018, 11:02:09 pmBut the Farnell is:QuoteThe Mouser is:Quoteat least On Semiconductor is:QuoteI can't know if the On Semiconductor it is a manufacturer and distributor or only a distributor.What do you mean to say is that I should look through websitehttps://www.mouser.com/ProductDetail/ON-Semiconductor-Fairchild/BC547?qs=UMEuL5FsraAqhvn1CRuL1g==,for example in this document:https://www.mouser.com/datasheet/2/308/BC547-1118654.pdfIs it this?Thanks

author: Jasonbit
author_rank: Regular Contributor
timestamp: November 03, 2018, 03:30:56 pm
content: Quote from: spec on November 02, 2018, 06:54:32 pmFairchild it is inside On Semiconductor. I was reading quickly history about Fairchild and Semiconductor and I think that one of them was acquired by the other (but I need read more about this subject).The Junction Temperature is 150ºC, but where I can find thermal resistance?If I have a maximum Collector Current around 100mA this means the factor Beta (gain) only change the way I get the value collector current . For example, If I have a 2mA in Base and use BC547A (with HFE 110) I will get 220mA in collector lead. But If I use a BC547C, with hfe equal 800 I will get 1.6A. Which means, by component limit, I only get 800mA. The versions with different values gains only change the speed with which I get the maximum current in the manifold. Right?Thanks

author: spec
author_rank: Frequent Contributor
timestamp: November 03, 2018, 07:01:44 pm
content: Quote from: Jasonbit on November 03, 2018, 03:30:56 pmYes, that is correct. Fairchild is now owned by ON, who also own Motorola. ON is one of the main tier companies who make high quality products.Quote from: Jasonbit on November 03, 2018, 03:30:56 pmYou noticedThe thermal resistance is not shown, but it should be! You can, perhaps, get the thermal resistance figure from another data sheet for the BC547 or, as the maximum power dissipation in free air at 25 deg C is given, you can work out what the thermal resistance is.Quote from: Jasonbit on November 03, 2018, 03:30:56 pmYes, quite right, but... you may get 220mA for a short time before the transistor failed, but most likely at 800mA the BC547 would fail immediately. It would not be possible to get 1.6A collector current, because of the internal resistance of the BC547. Don't concern yourself with the internal resistance of a transistor at this stage. In practice, it will not concern you when designing circuits with small signal transistors. This is because the signals (currents) will be small. A typical BC547, say the first gain stage of an audio power amplifier, would have a collector current of around 1mA. For a low noise preamplifier stage the collector current could be as low as 20uA.You should never exceed a component's maximum ratings. That is the golden rule for any design, electronic, mechanical, buildings, etc.The maximum collector current for a BC547 is 100mA. So when you design a circuit you must ensure that the circuit will never allow more than 100mA collector current to flow. In fact, you would be wise to limit the current to 75% of maximum, that is 75mA. Thus you provide a margin of safety.How to limit the current? There are two general ways: negative feedback, saturation. I can't give examples of these two approaches right now, because it would need a lot of explanation, but you can look it up on the internet or in text books. But, time allowing, I will see what can be done. Just to give give you a taster, if a transistor is saturated (no voltage between collector and emitter), you can push current into the base without the collector current increasing.If you do need more collector current for a design, you would need to use a transistor with a higher maximum allowable collector current. Generally speaking, there are families of transistors grouped by current and power rating.Here is a table showing some of the most popular types in the groups:SMALL SIGNALLOW POWERMEDIUM POWERHIGH POWERBC546/BC556BC337/BC327BD139/BD140TIP35C/TIP36CThese transistors are solid in their millions (jelly bean parts). Because of their low prices and good performance they are very popular with hobbyists. You may find it interesting to have a look at the datasheets for these devices.I get the impression that you are baffled by datasheets, and quite rightly so- I'm sure we have all been in the same position. But, take my word for it, once you get a bit of experience, the fundamentals of designing basic transistor circuits and choosing transistors suitable for the job is reasonably straight forward.

author: spec
author_rank: Frequent Contributor
timestamp: November 04, 2018, 02:50:14 pm
content: Here is the description of a design, to meet an invented requirement, where saturation limits the BC547's collector current (IC).REQUIREMENTYou would like to have a red LED turn on, in your workshop, when the wind is blowing. For adequate brightness the LED requires 5mA of forward current (If). At this If, the forward voltage (Vf) of the LED is 1V. However the LED will blow if If exceeds 9mA. You have a 12V power supply to power the LED.TURBINEYou make a little wind turbine using a model electric motor as the generator and blades made from bent aluminum sheet. You test the turbine with your multimeter and get the following results:Light Breeze: 5V DC output, but the generator will only deliver 250uA current.Gale: 50V DC and the generator able to deliver 250mA.DESIGN CONSIDERATIONSObviously, the turbine has more than enough current capacity to illuminate the LED in a gale, but not in a light breeze. So you decide to use a transistor to increase the current. You find from the datasheet that a BC547 will easily handle 5mA IC. The BC547 also has a high current gain (HFE) of 100 minimum and 500 maximum (round numbers).If the turbine fed its output current to the base of the BC547, with a light breeze the BC547 collector current (IC) would be between, 100 * 250uA= 25mA (min HFE) and 500 * 250uA= 125mA (max HFE). These currents would destroy the LED and obviously, at any higher wind speed the LED, would be obliterated not to mention the BC547. So there is a problem!THE DESIGNAttached is a design which solves the problem by feeding the LED with a constant 5mA for wind speeds from a light breeze to a gale.One thing to know is that when a silicon NPN transistor is conducting, its base voltage (VBE) is always 600 millivolts more positive than its emitter voltage. It does not matter if the collector current is 1mA or 100mA, the VBE is still 600mV (see note 1).Another thing to know is that the voltage between the BC547 collector and emitter (VCE) can never drop below 400mV (VCEsat) (see note 2). At VCEsat the transistor is said to be saturated.Choosing R1In a light breeze, the turbine produces 5V DC so the voltage across R1 is, 5V- 600mVBE = 4.4V.The BC547 IC is required to be 5mA to illuminate the LED, so the minimum required BC547 base current (Ib), assuming you have a BC547 at the lower end of the HFE range is, 5mA/100 = 50uA. The turbine can supply 50uA OK in a light breeze.Thus, the value for R1 is, 4.4V/50uA = 88k Ohms.Choosing R2When the LED's If is 5mA its, Vf is 1V and the BC547's VCEsat is 400mV, so R2 can only ever have a maximum voltage of (12V-1V)- 400mV= 10.6V across it. So R2= 10.6V/5mA = 2.12k Ohms.FINISHED DESIGNAnd that finishes the design: just one transistor and two resistors and you have a system where the LED is off in no wind and, in any wind, the LED has a constant 5mA forced through it.APPARENT ANOMOLYYou may be wondering about this circuit. After all, in a gale, the turbine output voltage is 50V. So the voltage across R1 is 50- 600mVCE= 49.4V. R1 is 88k, so the current flowing through R1 is, 49.4V/88k= 561uA which will be the IB of the BC547. And, If you had a BC547 with an HFE of 500, that would mean an IC of 500 * 561uA= 281mA which would not only blow the LED but also stress the BC547.But no, this is not the case. The maximum possible current that could ever flow through R1 and the LED is (12V - 1V)/2.12k. You can prove this by removing the BC547 from the circuit and connecting the lower terminal of R2 to the OV supply line (negative terminal of the battery).What happens is that when VCEsat is reached the BC547 is saturated and the current gain drops such that VCEsat is maintained.OPTIMIZATIONHaving arrived at the initial schematic you would then optimize the design. In this case only the value of R1 would be changed to ensure that the BC457 is fully in saturation at all times. From the datasheet the HFE of a BC457 in saturation drops to a minimum of 20. So the minimumBC457 Ib should be 5mA/20 = 200uA.From the previous calculations the minimum voltage across R1 is 4.4V, so the optimized value for R1 is 4.4V/200uA= 22k. This means that, with a gale and a BC457 at the high end of the HFE range, the maximum current through R1, and hence the IB for the BC457, will be 49.4V/22K = 2.25 mA. The BC547 will be quite happy with an IB of 2.25mA.COMPONENT RATIONALISATIONThe components on the schematic have the theoretically calculated values but for a practical design you would chose the nearest standard value.UBIQUITYThe rationalised and optimised circuit, and variations of it, are used universally for driving LEDs.HOW TO ARRIVE AT THE ARCHITECTUREThere are two ways to arrive at the architecture. For a standard architecture just look on the internet, or in text books. Very often the component datasheets have application circuits. For a non-standard architecture, for a unique application, you are on your own. But it is not quite that bad. Generally there are parts of other designs that you can combine and adapt. And, of course, experience helps.This is the end of the sermon.Note1 VBE does in fact increase a bit with collector current and base current. VBE is also inversely proportional to temperature (-2mV/degC), but just use a constant 600mV for general designs.Note2 VCEsat is more variable, but 400mV is a good value to use for a BC547 with 5mA collector current. Some high-power transistors may have a VCEsat of only 200mV with an IC of 1A.