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  1. duke37

    Help! Need advice choosing proper resistor value.

    You will need a more complicated circuit than this. 1. The frequency is determined by the transistors running out of steam. This can work but puts them under stress. The limit will be when the current provided by the transistor does not drive the inductance which may have saturated. 2.The bases...
  2. duke37

    Transmitter circuit operation explanation

    There are many types of oscillator, given different names. This one would be a classic Colpitts without C3. It has a whiff of Clapp about it. The best exposition of oscillators that I have seen is "Electronic Designers' Handbook" by Landee, Davis and Albrecht.
  3. duke37

    Ballast fault

    Interchanging L and N should not affect the operation. Please state what you did wrong. Show a picture.
  4. duke37

    Transmitter circuit operation explanation

    There is a parallel circuit with a several capacitors across the coil. The preset capacitor is to set the frequency to that required. It is FM modulated by the two variable capacitance diodes driven by the audio amplifier. The fet is a source follower to drive the circuit and the gate is driven...
  5. duke37

    relay sticking

    You could leave the old relay in and connect the SSR with wires tacked onto the coil. Check the current drawn.
  6. duke37

    relay sticking

    I used to look after an old electric fence energiser.. This had a relay to produce the pulse. The relay would stick in and needed its brass screw adjusting so that the pole pieces did not touch. It would wear down as the relay worked once a second. You may find an anti-stick screw in your relay...
  7. duke37

    relay sticking

    You must find out if the relay contacts are welded together or if the supply (12V DC?) is still there. A 12V relay may well stick in with less than 4V. A solid state relay will solve the problem of welded contacts but will not help if the problem is due to a control transistor leaking.The...
  8. duke37

    Cooling Fan blowing the fuse

    I was asked to look at a combine harvester which was blowing fuses. The fault was hidden, worn through wire insulation, probably helped by a mouse. Shake, rattle and roll does not help. Put a headlight bulb across the fuse connections to limit the current and give an indication of current. I...
  9. duke37

    Variable Frequency Op-Amp Astable

    It all depends on what you mean by close to zero. Any variable oscillator can not get down to zero. The way round this is to have two oscillators, running at slightly different frequencies and then mixing the two and running through a low pass filter to select the difference frequency. This is...
  10. duke37

    Cheap Motion sensors , chinese I think

    If two resistors are connected in parallel, the resulting resistance MUST be less than the lower one. 1/33k + 1/2k = 1/1.89k
  11. duke37

    Cheap Motion sensors , chinese I think

    1/30k +1/2k2 = 1/2k05 so more than a few hundred.
  12. duke37

    power station question

    I believe that in the early days, a bulb was connected between the two supplies. When this went out, the contactor was energised. Three phase supplies used three bulbs.
  13. duke37

    Mosfets in parallel

    What is the pupose of the 470μF across the FET? I would think that this will slow down switching and increase fet dissipation.
  14. duke37

    Mosfets in parallel

    Yes. The fets will tend to share unlke junction transistors where the one with most gain will hogg most of the current.
  15. duke37

    Haake Chiller/Circulator

    You need to measure the voltage getting to the compressor. This will tell you if the board is faulty. What sort of motor drives the compressor? If it is an inductance motor with a start capacitor, perhaps this or its circuit is faulty. Starting a compressor against a retained pressure is...
  16. duke37

    Can someone help me with explaining a part of this circuit?

    Nothing happens. A resistor between IC1 and D2 to limit the current would be advantageous.
  17. duke37

    Use AC Tap From DC Power Supply

    I do not see why you should not get a good sine wave from the transformer but you will not have isolation so must not use the DC output when using AC.
  18. duke37

    How do you use a 12v 10a capacitor as a momentary backup battery for bridge

    dV/dt = I/C so C = dt/dV * I C = 0.5 * 10 / 1 if a 1V drop is allowed. C = 5 F = 5000000μF This is getting a bit big:)
  19. duke37

    Can I get a relay that reverses polarity for the load when not powered

    The Amazon relay liooks OK but is limited to 5A load. What is the load?
  20. duke37

    Can I get a relay that reverses polarity for the load when not powered

    Show a circuit diagram of what you want. I think all you need is a change over relay connected the right way round. You can get relays with 230V AC coils with various sizes of contacts to suit the load. You should use the 12V supply to energise the relay, then you do not have the danger of high...
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