Search results

Search Forums New Posts
  1. duke37

    Charging a tuned L/C circuit

    If you look at the crystal set circuit, the aerial and diode are tapped into the circuit at different places. The idea is to match the aerial to the tuned circuit so that maximum energy is transferred and the diode load tap is chosen to get a compromise between selectivity and sensitivity...
  2. duke37

    Charging a tuned L/C circuit

    A pendulum swings from side to side. At one extreme, the bob is a little higher and has potential energy. When it is swinging in the centre, it has kinetic energy. This interchange of energy continues until friction damps the oscillation. In the case of a tuned circuit, energy is interchanged...
  3. duke37

    Was, Were, Wasn't and Weren't

    Voice activation will need persons to speek ploply. No-one seems to distiguish between their and there, cote and coat, weather and whether. Books are now kept in lybwies, in it?
  4. duke37

    Was, Were, Wasn't and Weren't

    Gee man That is why Harald is so good at english
  5. duke37

    Toggle Switch - Momentary Pulse Circuit (problem!)

    That bank of relays seems to have transistors to feed the relay coil so should be much more sensitive than the ones you have but you will have to observe polarity.
  6. duke37

    Transformer Help!!

    150W out at 12V means about 15A in is not trivial. I have made transformers to step up voltage using a TV line output transformer core wound with 1 turn/volt. I used a pair of fets in push/pull driven at about 10kHz. I have no experience of generating voltages above 500 but insulation will be...
  7. duke37

    Toggle Switch - Momentary Pulse Circuit (problem!)

  8. duke37

    Toggle Switch - Momentary Pulse Circuit (problem!)

    Perhaps the energy given by the 330μF capacitor is sufficient to light the led but insufficient to switch the relay. You could use a 555 monostable to give a fairly hefty pulse.
  9. duke37

    Solar cell charger

    A 7805 needs about 2V to operate so needs 7V input. There are low drop out regulators which need less than 2V.
  10. duke37

    repairing an amplifier

    There has been a lot of heat around the diodes as the board is cooked. The diodes should be replaced with high current diodes or a high current bridge. I mentioned a light limiter, you could make one by connecing some twin flex across a blown fuse and then connectiong this flex to a...
  11. duke37

    220V to 12V 2Amp power supply modification

    Keep a 7.6Ω resistor well away from this circuit. You could pass excessive current or generate much too high a voltage. Put everything back to what it was and test that you have not damaged anything then put a 8.2kΩ across the 2.2kΩ resistor. If you do not have one, then use higher value say...
  12. duke37

    Home Security system takeover

    Shield the cell phone with aluminium foil?
  13. duke37

    repairing an amplifier

    It is the mains input fuse so the fault could be anywhere after this. I would use a lamp limiter (40W) to run the amp without a fuse blowing all the time. There are 1.25A fuses to the amp supply. Remove these to isolate the fault within the power supply. If the capacitors have rounded ends from...
  14. duke37

    Op-amp task

    R5 and R6 are connected in series with a positive voltage at on end and a negative voltage at the other end. You can find the current from the total voltage and the total resistance. Then calculate the voltage across each resistor and find U+ relative to one end and then shift the reference to...
  15. duke37

    220V to 12V 2Amp power supply modification

    There will be 5V across the 2.2k resistor so the current will be 2.27mA 2.27mA through the 3.3k resistor will be 7.5V so the total voltage will be12.5V If the voltage is to be increased to 14V then the voltage across the 3.3k will be 9V and the current will be 9/3.3k = 2.79mA There is therefore...
  16. duke37

    Problem with new microphone preamp.

    The plot shows a gradual change in the middle which indicates something to do with a capacitor. Is the input capacitor the wrong way round? The base of the transistor will be a little positive and a dynamic microphone will be connected to earth.
  17. duke37

    LED Signal Dummy load

    110V 50Hz 0.3A C = 0.3/(2*pi*50*110) = 8.7μF
  18. duke37

    LED Signal Dummy load

    If the supply is AC, you could use a capacitor to pass the current without any heating. What is the voltage and frequency of the supply.
  19. duke37

    Low voltage cutout help

    I have thought about this in the past and cannot get my brain cell around it. If you take half the turns off, you get double the current with the same voltage and so the same ampere-turns and the same magnetic force. Am I wrong?
  20. duke37

    Motor run capacitor theroy with added PFC fun

    I do not understand why you are so concerned about power factor. Are you taking your supply from a small alternator? C1 as shown is a run/start capacitor as shown, not a start capacitor. A power factor correction capacitor is placed across the mains input to cancel the lagging inductive current...
Top