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  1. duke37

    5.1 Surround System fault

    7805 seems like a 5V regulator. They normally have over temperature shut down so it is working correctly. The problem is likely to be in the circuit which the regulator supplies or poor thermal contact to the circuit board.
  2. duke37

    High Current DC Switching

    A solenoid will need a fair voltage to turn on, then a lower voltage to hold on. Thus it will start an engine even if the voltage drops to less than 10V. A mosfet does not have this hysteresis so if the voltage drops significantly, then the mosfet is not turned on fully and will dissipate a lot...
  3. duke37

    High Current DC Switching

    I would think that a FET solution would be cheaper than a solenoid but the car makers use a solenoid. There must be a reason for this. A dismantled solenoid will show how severe the duty is.
  4. duke37

    Led On Indicator

    Get the polarity correct.
  5. duke37

    Questions conservation electronic

    What is wrong with silica gel? Store them in a sealed plastic box with a lid clipped shut. I store my rolled oats (for porridge) in such a box. Put some well-dried grain in to absorb water from any leaking air, in other words use this as a substitute for silica gel. The grain (wheat, oats...
  6. duke37

    Tank circuit oscillator design

    Why does the frequency matter? If a transmitter torroid and similar receiver torroid are at the same temperature then they will track each other.
  7. duke37

    Simple Resistance Division

    The transistors are biased for distortion as already said. Dropping the voltage will elevate the distortion maybe sufficiently to stop the amplifier working at all. R3,R4 are critical as are R6,R7 to get the correct transistor current.
  8. duke37

    110V/220V switch in an old power supply

    Some form of limiter will enable some voltages to be applied without blowing fuses or other components. I use it for tube radios and other devices such as mains motors. You can use a bulb with less than 50W since you will not be taking power out till you have found the fault. The quiescent power...
  9. duke37

    AM modulation

    Tell us where you found the first equation. The second equation is just efficiency = output/input and applies to any energy using device such as energy from sun to heat into a room via a tree. Oil energy to delivered electrical energy. Efficiency is always less than one. The lost energy goes...
  10. duke37

    Current Regulator

    You have shown a voltage regulator, I thought you needed a current regulator.
  11. duke37

    Simple Resistance Division

    Please post a schematic otherwise all is guesswork. I do not think anything fires. Dropping the voltage from 9V to 4.5V may have a profound effect. Post a schematic. Do not use nunbers to over 30 places, this is absurd. You will never measure to better than 1% accuracy. You need to specify the...
  12. duke37

    Reed switches with LED strip lights

    20mA will be OK if the load is not inductive. The best way to ensure this is to use a transistor with a suitable base resistance. I would only use a relay if isolation is required and make sure is has a protection diode. I have had reed relays which have been unreliable but on the other hand, I...
  13. duke37

    110V/220V switch in an old power supply

    S5 should be inserted for 220V OR S6 for 110V. DO NOT INSERT BOTH. C904 is connected across the mains and is not necessary to get the set to work. If suspicious of it just disconnect it. It is there to reduce interference. Take out all other fuses and bodge links and measure the voltage across...
  14. duke37

    Simple Resistance Division

    You need to define what is input and output and where you are measuring. In the third circuit the centre resistor is shorted and plays no part. There are two seperate resistors connected to ground. R1 and R2 are not in parallel since the top ends are not connected together. The top right...
  15. duke37

    Reed switches with LED strip lights

    Reed relays are not very good power switches. I would use them to switch a transistor to either switch the load directly or switch a relay.
  16. duke37

    Reverse Photoresistor????

    I do not think that there is a reverse photoresistor but a single transistor can reverse the action.
  17. duke37

    2 questions about speakers

    A loudspeaker has a radial field so a bar magnet wll not do. The clearance must be very small in order to get sensitivity. I can never repair a speaker because I can not get the clearance right.
  18. duke37

    DELL Laptop Charger Repair

    A lamp limiter is a filament bulb placed in series with the mains supply. The maximum current which can be drawn is that of the lamp so things start up gently. If the lamp stays bright with no load on the power supply, then something is wrong but you are unlikely to blow fuses. I use a lamp...
  19. duke37

    DELL Laptop Charger Repair

    I do not think that 100μF will be much worse than 120μF. They are not made to high cacacitance accuracy. Of more concern is that the picture shows a 420V capacitor, not 400V. You do not say what your mains voltage is. If you try the new capacitor, start it up through a lamp limiter. Get a...
  20. duke37

    Power circuit question

    I think you will need motor type (universal?) and voltage to get current.
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