Jump to content
Electronics-Lab.com Community

autir

Members
  • Posts

    196
  • Joined

  • Last visited

    Never

Posts 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 Autir,
    I also built my LM317 variable supply on stripboard. I had the load connected to the heatsink tab, with a terminal screwed on with the same bolt that attaches it to the heatsink, then connected the current-setting resistor to the output pin without any load current in it. Then the output "star" connection is inside the IC.

    I connected the supply's ground, the load's ground and the pot's ground all together in one spot, called a ground "star"connection.

    It works perfectly. ;D


    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


    Hi Autir,
    If you have it wired so that load current travels through a wire or pcb trace to which R2  (the pot) is connected to ground and the resistance of the wire or trace develops a voltage across it, then the regulation is much poorer.
    National Semi explanes a similar problem that is caused when load current travels in a wire or pcb trace between the regulator's output and the current setting resistor R1:


    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:


    When a multi-cell battery of cells in series is over-discharged, the weakest cell gets completely discharged first, then the remaining cells continue discharging through it forcing the current in it to be backwards, causing crystals that short the cell.


    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. Hi Autir,
    I have a Ni-Cad powered cordless drill that has been "simple-charging" (about 0.08C) for many years without problems. I worked with wireless boardroom microphones that used series'd Ni-Cads that were always "simple-charging" and when their battery failed I asked the customer, "What happened?" and their answer was always, "We forgot to turn it off".


    Are your two examples referring to devices that were always charging when not in use?

    Your charger takes "forever" (about 32 hours) for your cells to reach full charge, then slightly over-charges them for a trickle-charge. At least it doesn't grossly over-charge cells that weren't completely discharged. ;D


    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)

    Some equipment manufacturers consider that NiMH can be safely charged in simple fixed (low) current chargers with or without timers, and that permanent over-charging is permissible with currents up to C/10 h. In fact, this is what happens in cheap cordless phone base stations and the cheapest battery chargers. Although this may be safe, it may not be good for the health of the battery. According to the Panasonic NiMH charging Manual (link below), permanent trickle charging (small current overcharging) can cause battery deterioration and the trickle charge rate should be limited to between 0.033×C per hour and 0.05×C per hour for a maximum of 20 hours to avoid damaging the batteries.


    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. Alun thank you for translating the .doc file.


    Hi Walid,
    I wish your schematic wasn't a negative pic.


    Feel free to call me Autir  ;D


    1) Your R1 and R2 divider doesn't have enough current. R1 can't supply the base current without even having R2. Your error was caused because you selected a Thevenin parallel combination of R1 and R2 to supply a current equal to the base voltage, but the divider's current must be much more than the base current. I use a ratio of 10 times for Idiv/Ib.


    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.


    2) Your choice of an old BC548C transistor limits your purchase since few manufacturers still make it and few other transistors have such a high curremt gain. I used BC548C transistors with Philips about 40 years ago. They invented it but no longer make it.


    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.



    3) You choice of 15mA for the emitter and collector currents is very high and cause a calculation for R3 and R4 values that are not common. I would have used 2k for R3 and 100 ohms for R4 or 20k for R3 and 1k for R4 if the load impedance is high enough.
    Also, The MCC datasheet has few spec's at 15 mA but has full spec's at 2mA.

    4) Your Vbe value must be accurate since the emitter voltage is so low. My MCC datasheet shows typically 0.74V with a collector current of 15mA. The difference of 80mV from your 0.66V causes the emitter and collector currents and to err by 5.6mA which is more than 37%. The collector voltage will also err by more than 37%.
    Also, the Vbe spec has a range of Vbe that isn't spec'd at the high collector current of 15mA. The range is from 0.55V to 0.7V at a collector current of 2mA.
    Therefore a bypassed emitter resistor must be added to raise the emitter (and base) voltage so that the range of Vbe causing a change of the emitter and collector currents is insignificant. ;D


    You are absolutely right.
    Thank you very much!  :D
  11. I wonder why Autir doesn't learn this basic stuff in school. ???

    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
  12. Any questions?


    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.

    In the absence of any good reason for making some other choice we might just as well assume that the available voltage should be shared equally between Re, Rc, and the transistor. We therefore want about 5 volts across Re, 5 volts across Rc, and 5 volts between the collector and emitter. This means that the amplifier should have Vc=10 V,  Ve=5V, and  Vce=5V.


    Abscence of any good reason? What happened with the Q point, isn't it good enough a reason?!?!?!?

    In practice the simplest convenient choice is to pick something like Io=25*Ib so I suggest you choose that. Note, however, that you could choose almost anything from Io=Ib up to Io=100*Ib and it would still probably be possible to make the amplifier work despite having chosen very different currents and resistor values!


    Isn't the Io/Ib ratio GIVEN AND DEFINED by the usage of Thevenin's theorem?

    To change the voltage across Ce we have to move charge in or out of the capacitor. This takes time. So if we keep changing our mind and waggling the input voltage up and down quickly we don’t give this a chance to happen.


    "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.
  13. 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.

×
  • Create New...