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Everything posted by autir

  1. Hello all. I am trying to design a low power guitar amplifier using only BJTs. Since the output impedance of a guitar is high, I was thinking of using a jfet instead of a bjt as the first stage. The problem is, I have no experience with fets and I do not know what to use. I have read in several fora that jfets as descrete parts are becoming obsolete and being replaced by mosfets. Why is that? The thing is, I am afraid of the mosfets' electrostatic sensitivity (and, being the first part to the amp, it could prove disastrous). Any thoughts? By the way, I would like someone to propose me a jfet model to use. It should be a classic, and easy to find in Europe. Something small signal, general purpose. Basically the BC548 equivalent of jfets :D What is your experience/opinion on the following: BF244 BF245 BF256 J120 Thank you in advance.
  2. Hi is the circuit of this guy http://casemods.pointofnoreturn.org/vregtut/tutorial-full.html a good example of star topology? any ideas on how to connect the LM317K (TO3 package), mounted on a huge heatsink? also I have comprehended the problem with the resistance between the 317's output pin and R1. But not the problem with the resistance between ground and R2 (the pot). Care to explain both a little bit more? Thanks.
  3. Hello all. I have built a simple circuit with a 555 as a monostable multivibrator on my breadboard. The circuit is the one documented in TI's 555 datasheet. The output goes to a 0-999 counter (three 7447s and three 7490s ICs). The 0-999 counter is a seperate circuit built on stripboard and works fine. I have a problem with the 555, though. When the "trigger" pin is unconnected and close to my hand the circuit starts to oscillate or counts double (produces twin pulses?). What happens? The final circuit I want to build is one where the counter will advance by one every time two metal wires touch each other. But leaving the trigger wire unconnected causes all sort of problems. I connected a 4069 IC to the trigger wire, so as to ensure that triggering will take place only with voltage and not by accidental grounding. But the problems persisted. I am thinking to build a 555 astable circuit with the junction of the metal wires as its power line. Or perhaps a transistor working as a switch... Any thoughts?
  4. Hello all. Recently I discovered that the TTL ICs praised in my books are in reality power hungry, ancient technologies - even the LS variant I have used to make some simple circuits. I would like you to propose an alternate logic family. I would like the following characteristics: DIP package or something suitable for stripboards. NO SMDs. wide array of functions. low price. low power consumption. wide array of voltages, suitable for battery operation. 9v could be nice, but 2.4 volts (two NiMH AAs) would be better. easy to find. After a search in the Internet, I am between 4000 and 74HC... any thoughts?
  5. Thank you for your reply :) One final question: If we have some rechargable cells and we are not certain if they are all of the same charge level, can't we just connect them in parallel? So the full ones will charge the empty ones and everyone will be ok. (I suppose that if the cells would have the same C it would be good, not to say needed ???)
  6. Are your two examples referring to devices that were always charging when not in use? 2500mAh batteries in a 100mA charger require 25 hours to charge fully. Taking the typical 66% efficiency in consideration, we have 25*1.5=37.5 hours. If this slight overcharging you have mentioned takes place for less than 20 hours, will it weaken the batteries - even slightly? Or there will be no damage at all? If I leave them for 40 or 48 hours, for example, what will happen in the long run? Also what will happen if I just leave the cells in the charger forever? (e.g. 1 week). Thank you.
  7. I have read a few articles regarding recharging methods for Ni-based rechargeable batteries (NiCd, NiMH). I have a few questions about the simple constant current method, that is without timers, dV/dt or dT/dt detectors, or anything else. 1) What do you think of the figures given here: http://www.powerstream.com/NiMH.htm Both in sections "Overnight Charging" and "Trickle Charging"? Do you like the C-based numbers? 2) Overcharging a NiMH battery can lead to the formation of small crystals of electrolyte on the plates, causing voltage depression. Is trickle charge harmful too? I'm asking this because of this: (text from http://en.wikipedia.org/wiki/NiMH) The bottomline is: I have got a GP PowerBank charger (http://www.gppowerbank.co.uk/powerBankRange.html), which claims to be able to be "always plugged in", and whose specs are 2.8V @ 100mA. I have also got some brand new 2500Ah NiMH AAs. Given that the charger's current is 0.04C, what should I pay attention to when charging the batteries in order to ensure long battery life? Thank you in advance.
  8. Thank you for your reply :D So: If I connect the anode of the segment to Vcc=+5V, I will need a voltage drop of 3V across the resistance (I assume that the voltage drop across the led will be 2V, like in a typical "lamp" led?). R=V/I =3/0.01 <=> R=300 Ohms, we will select 330 Ohms for safety? Are the above correct?
  9. Hi all. I have downloaded TI's datasheet about the 7446-7449 family of BCD to 7-segment display decoders. (http://focus.ti.com/docs/prod/folders/print/sn7447a.html) 1) The '46 and '47 decoders claim to "drive indicators directly", while the '48 decoder claims that "internal pull-ups eliminate need for external resistors". What is the difference between these two phrases? What will happen if I plug a led display directly to the output of a '47 and what in the case of a '48? 2) I am planning to use the 7447 along with the SA52-11HWA led display from Kingbright, (http://www.kingbright.com/) which, if I'm not mistaken, is rated at 10mA. In TI's datasheet certain values for maximum current are below 10mA. Which values are of importance to me? 3) Which ones are more common (are bought more often)? common-anode or common-cathode led displays? Why? Thank you very much.
  10. @Walid: I don't believe that there is anything wrong with my circuit apart from the wrong application of the Thevenin theorem (that is, regarding calculations. The choice of
  11. Alun thank you for translating the .doc file. Feel free to call me Autir ;D You are absolutely right. I have verified it by running simulations having used several values for R1 and R2. I use Thevenin in a wrong way. In the past you, among others, have advised me to stick with the BC546-550 family. Why were these issues not pointed out? BC548s are cheap and readily available where I live and appear a lot in scematics on the Web. I assumed that it was a fair choice. You are absolutely right. Thank you very much! :D
  12. Hello all. I have blown the fuses of my two "el cheapo" digital multimeters. What should I replace them with? Slow-blow or fast-blow?
  13. I have designed a class A preamp from scratch and written the procedure in a Word document. I would feel very obliged to all that can read it and point out any mistakes/ misconceptions etc. class_A_preamp_v.0.1.zip
  14. Because the electronics I have learnt is that of the computer scientist, that is very few and almost all of it digital. Plus my theory books were meant for physicists and electrical engineers. They are too complicated. Regarding electronics I am a hobbyist, not a professional. ;) @walid: Thank you for your reply. I have visited this link and found it to be horrible - the guy just throws in the Rb values and implies that Vce=0! No, thanks
  15. As always, many ;D The circuit without the emitter resistor is indeed a bad design, as it is dependent upon the hfe of each transistor and the shifts of the hfe value caused by temperature etc. change. I included it just to see how the formulas change with the transition from the simple (no Re) type to the other. Using Thevenin's theorem in your circuit dictates that the input impedance of the circuit will be Zin=R1//R2//(re+R4), which is roughly equal to R1//R2//R4. So Zin=976 Ohm and not 34900 Ohm. Or not? ??? How did you choose the Ib (2.4uA) to I(R1R2) (27uA) ratio? It is indeed a very stable circuit which, when being simulated in Multisim, provided exactly the same output with a variety of transistors and hfe's. When I added a bypass capacitor paralleled with R4 the above characteristic dissapeared. The Ib/Ic ratio in your circuit is proof of the hfe value of the transistor you used. Why, then, does it work the same with different hfe's? Does this mean that the Ib/Ic ratio can be decided by me without any consenquences regarding the circuit's function? The gain factor is calculated by the R3/R4 ratio, if I'm not mistaken? @Alun: Your link is invaluable. However, there are certain "shady" parts in the text. Abscence of any good reason? What happened with the Q point, isn't it good enough a reason?!?!?!? Isn't the Io/Ib ratio GIVEN AND DEFINED by the usage of Thevenin's theorem? "This takes time"... why? ??? ??? ??? ??? ??? Another question: Why do we use two distinct resistances in the Emmiter area? Could someone please explain the Re - Rg part a little better to me? Thanks.
  16. Electronic Principles, Albert P. Malvino The Art of Electronics, Paul Horowitz and Winfield Hill
  17. Hello all. For the past few months (ok, it was not my sole occupation
  18. Then the LM317 is unsuitable for you. Pay attention on the choice of your voltage regulator IC.
  19. Steveggz, what will be the maximum current flowing through the regulator?
  20. Anybody cares to explain how does the "150V input protection" part work? I am thinking of building a similar circuit for my computer. I don't care about output amplification and adjustment, I just want to increase the input impedance and protect my computer's soundcard from burning.
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