Search results

  1. A

    Basic resistor question

    Dead right too - I must have been asleep. Any balance condition would do the trick.
  2. A

    Full wave rectification

    The two plates (anodes) are driven by the opposite ends of a centre-tapped transformer secondary winding. The AC potentials at the ends of such a winding with respect to the centre tap are in anti-phase to each other. That is to say, when one goes positive, the other goes negative.
  3. A

    Max current spikes in diodes

    In the situation you describe, with ceramic capacitors of probably quite low values, the inrush current will flow for only a brief period. For instance, with R = 0.1 ohm and C = 10 micro farads we get a time constant RC = 1 microsecond. This is much shorter than the 8.3ms often quoted in a...
  4. A

    Operation of a P channel MOSFET

    Conduction in the inverse direction along a MOSFET channel is generally possible. This is utilised in some types of synchronous rectification, and also in some analogue switches. What is not possible, at least for most (all?) discrete devices, is for the MOSFET to block a large reverse...
  5. A

    Basic resistor question

    If (and only if) all the resistors have equal value r, the bridge circuit simplifies very easily. Can you see what the total would be? Hint, imagine a voltage applied across the network: what then would be the voltage across the central resistor R3? Oops. Berkemann has already pointed...
  6. A

    Engineering Pmax in the given circuit.

    Of course, Vs will not be at its maximum value at all times, but it should not have dropped to anything near 24V unless C was very small, say 47uF. (IT =CV, assuming 8V drop in 10ms at 30mA gives 37.5uF.)
  7. A

    Engineering Pmax in the given circuit.

    You seem to have an incorrect value for Vs. What voltage would you expect across a capacitor fed by a bridge rectifier from 24V AC? Wouldn't you expect something closer to the peak value minus the bridge drop? The answer I get is actually a bit low even if we assume a 30V Zener, so I wonder...
  8. A

    Bridge Full Wave Rectifier Peak Voltages and Capacitcane Value

    A basic statement of Kirchhoff's Voltage Law is that around any closed loop, the sum of the emfs is equal to the sum of potential drops. You need to grasp that idea if you want to make sense of circuits. There is also a diode in the path leading to the bottom end of Rl. It too requires...
  9. A

    BC183L transistor crossover?

    BC183L has a different pin-out to many other types (1. Emitter 2. Collector 3. Base), 2N3904 is NOT the same, according to data sheets I've just looked up. You might be able to bend the leads to suit, but you need to be aware of this."...
  10. A

    Digital Ramp ADC Analysis difficulty

    As you know, the resistors are in the ratio R (MSB), 2R, 4R, 8R. To get actual values you will therefore need to decide the value of R. What are the constraints? The value of R must not be too small, or the counter outputs would be heavily loaded: this could do damage, or at least make the...
  11. A

    Engineering Transitor circuit help

    Have you been given information about the type of transistor used, or details of its characteristics, particularly its current gain and its base-emitter voltage? Do you have any values for the DC conditions required, such as the collector current, the collector voltage (Vc in the diagram)...
  12. A

    Windings question

    The wire used in some spiral-wound heating elements is not insulated. In this case, although adjacent turns of wire may look as if they touch, they are in fact held apart in grooves on an electrically insulating heat-resistant former. The design of wire-wound resistors also avoids the...
  13. A

    To all engineers/engineers to-be!

    If I had known how badly the engineering sector was going to decline in my country, I might have kept Electronics as an interesting hobby, and taken up something else as a career. Right now there seems to be only limited opportunities for engineers in the old developed world, particularly...
  14. A

    Photoelectric effect

    Electromagnetic radiation with a wavelength of 180mm would be described as microwave, not ultraviolet! Suspect a typo.
  15. A

    Exciting Current harmonics in Transformer

    The output transformer in an electron tube audio amplifier is rather a special case. Here the impedance driving the transformer primary will be relatively high, compared to that nomally seen by a voltage transformer of similar rating used for AC mains distribution. As a result, harmonic...
  16. A

    Question about dc power output

    The current available from a power supply of this kind will normally be limited by the ratings of certain key components. Exceeding these ratings is likely to result in failure, which can cause overheating, fire and electric shock hazards. Modifying mains powered equipment is not a job for...
  17. A

    Voltage vs Current curve isnt going through origin for Playdoh resistor, why?

    The voltage / current plots do seem quite straight, so these results could be characterised as fixed resistances (found from the slope of the line) in series with offset voltages (found from the intercept). I can think of a few things that could cause an apparent offset: 1) The ammeter may...
  18. A

    Is the earth has value 0V?

    Modern electronic voltmeters have very large input impedances. If you used this kind of instrument it probably would give a substantial reading, even if the only things "grounding" the secondary were the parasitic capacitances from the secondary coils to the core of the transformer. On the...
  19. A

    Buzzing incandescents

    I have read that sometimes when a filament fails, the current will continue flowing for a short time, due to an arc bridging the broken section. I'm not sure how convincing this is, but perhaps it has actually been witnessed in lamps with clear envelopes?
  20. A

    Having some problems with overshoot

    If your system can operate at 10MHz then it is not really "low frequency" (except perhaps as viewed by a microwave engineer). It is quite fast enough for parasitic capacitances and inductances to have serious effects. For instance, that 8" long connection to the rear monitor diode could be...
  21. A

    Energy of air in a room

    The volume of the room is constant, but unless we plan to suffocate the room cannot be perfectly sealed. Provided that there is some ventilation, the pressure P can remain the same as the room is heated. What then changes apart from T?
  22. A

    Foldback current limit circuit

    No, it is and crude and inefficient, but it does provide a foldback limiting characteristic. The current into a short-circuit will be lower than the initial limiting or "knee" current.
  23. A

    Foldback current limit circuit

    You should aim for a smaller short-circuit current (Isc) than 1A, since the maximum current (Ilim) will be larger. One way to begin the problem would be to decide that you want Ilim to be a certain number N times Isc. You gave two equations: Ilim = Vbe[(Ra+Rb)/RbRs] +Vout (Ra/RbRs)...
  24. A

    Foldback current limit circuit

    Yes, but those circuits used fixed resistors for the potential divider where you have the pot R4. It's OK to use a pot for adjustment, but you must never let the resistance Rb to ground become too small. That could easily happen if the pot were inadvertently turned too far - then there could...
  25. A

    Foldback current limit circuit

    Where did you get that circuit from? It does not look correct, are you sure that you have copied it correctly? If R3 represents the output load, then with the pot R4 wound down to ground, Q2 base-emitter would be in parallel with the output, and so would get the full output voltage across...
  26. A

    Foldback current limit circuit

    The series transistor does indeed drop voltage in current limit mode, but that's not the problem. The issues are that the transistor can't drop less than a few volts at somewhat lower currents, and that the dissipations in R4 and Q2 can be inconveniently big. A Darlington type transistor for...
  27. A

    Having some problems with overshoot

    Looking at this again, you seem to be using a rather large DC input bias (4V). Does this imply that you need about 8Vp-p input for full modulation? If so, are you sure that this will not exceed the amplifier input common-mode range? Also, what will be the minimum photo-diode bias voltage...
  28. A

    Having some problems with overshoot

    The first thing that occurs to me is that physical layout is very important for this kind of set-up. For instance, if you are using such things as banana plugs and crocodile clips for your input connections, you won't get such a good line termination at high frequencies. Generally the...
  29. A

    Inverter Question

    Vibrator-type power supplies were once used to obtain High Tension supplies for electron-tube (valve) equipment from lower voltage supplies, e.g. for vehicle radio. This sort of system was never very reliable and has been obsolete for decades
  30. A

    I am trying to do the circuit

    In the transmitter circuit, T1 base appears to fed with a relatively large audio signal without any added DC bias. I believe that this is likely to give a grossly distorted result - are you certain that this has been drawn correctly?