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  1. duke37

    Laptop DC Adapter - Burnt Out

    Ash, I think you have some die back ! The transformer has almost certainly failed and the unit is scrap. These small transformers often have a thermal fuse embedded in the windings which pops off if the transformer is too hot. I have tried replacing the fuses but have never had success...
  2. duke37

    Power source for heating a .025 wire

    With low resistances, the four contact method is the way to go because it eliminates contact resistance troubles which may be bigger than the resistance to be measured. Set up a current through the wire with the battery charger and then measure the voltage across say 6" of the hot wire. R =...
  3. duke37

    Power source for heating a .025 wire

    I supplied my doctor with the necessary parts to cut poystyrene glider wings. The data I generated has gone but various wires were used including suture wire. You will need stainless or resistance wire to take the heat., I do not know what a guitar string is but nylon will not do !! The wire...
  4. duke37

    Wow, Im on an electronics forum!! So cool!

    The connection should be from the remote terminal to either one or the other adjacent terminals which I think you have. If the terminals are burnt, the contacts inside will probably also be burnt which could give a significant resistance. The switch should be replaced with new clips to keep...
  5. duke37

    Can capacitor be made with 2 pennies + paper?

    Wikipedia says capacitance is C=8.854E-12 * e * A / d where e = dielectric constant (air = 1) measurements in meters so yes, you need a large area and small separation. An interesting exercise is to unwind a plastic insulated capacitor. There will be a very large tape with a very thin...
  6. duke37

    Wow, Im on an electronics forum!! So cool!

    The switch should conduct from the contant one end to either one or the other contact at the other end. 0.8 ohm seems to be too high but perhaps it is the meter leads. Any contact betweent the two adjacent terminals means the switch is dead. Microswitches are very common. They come in...
  7. duke37

    My first circuit

    To get positive at the top,which is the norm, the battery has to be inverted. The leds will then conduct. The arrows should point down to conduct in pnp or npn transistors or leds.
  8. duke37

    Microwave Transformer Isolation

    The fuse ahould have a current rating above the normal load current but not too much. If charging big capacitors there may be a problem on switch-on and a soft start circuit would help. Most fuses are 250V, these would not be safe above this voltage. Many years ago I operated some equipment...
  9. duke37

    Blowing fuses on 1980's Fisher CA-880

    Adam, Which of your threads do you wish me to comment on?
  10. duke37

    SMPS design

    Mosfets are used because they have a very low resistance if turned on fully. A diode will have drop a considerable forward voltage, perhaps 1V. When rectifying 100V a 1V drop is not too serious (1%). but if rectifying 3V it is a dramatic loss. Schottky diodes are better at low voltages but...
  11. duke37

    LEDs (and diodes?) have CONSTANT voltage?

    You were close in your original explanation. As Bob says, the voltage across the led will change little with a change of current so cannot be considered a resistance. With 12V supply and 3V or so across the led, the resistor has to drop about 9V.
  12. duke37

    Parallel resonance on oscilloscope

    Please forgive me, I am grumpy today. You should post things correctly, you were out by a factor of ten for your capacitor and a factor of one thousand million for your signal generator. There is a lot of difference between 50 mHz and 50 MHz. Note that I have used a capital H for Herr Hertz...
  13. duke37

    Blowing fuses on 1980's Fisher CA-880

    I thought I had made a contribution here before. Do not ever replace a fuse without finding the fault, they do not fail for the fun of it. Do you have a circuit diagram (schematic) ? Unplug the speaker and put a resistor of several ohms (say 100) across one of your blown fuses. Put this...
  14. duke37

    Diodes for rectification

    Schottky diodes are normally low voltage. If you have a high voltage, then a moderate voltage drop is not too important. Schottky diodes are fast and so can be used in switch mode power supplies.
  15. duke37

    Blowing fuses on 1980's Fisher CA-880

    The fuse is more likely to be 4A 250V. Do not replace a fuse without finding the fault, they do not fail for the fun of it. I assume this is a transistorised amplifier. Have you got a circuit diagram? You can solder a resistor across one of the blown fuses to put some power into the amp...
  16. duke37

    Help with Small Project Please

    As Mrs Beaton would say "First catch your bison" Thermocouples are used in gas boilers to detect the pilot flame, these are quite cheap. You could make your own from copper/constantan(resistance wire) but it would be a hassle to make the junction and provide the insulation. PTFE will go to...
  17. duke37

    Two-coil latching relay arrangement

    Hi Kris, Would the circuit work without the bridge rectifier (D1 to D4) and a diode between D6 and C2. This would keep the 'ground' line at neutral and so safer. C2 may need to be a bit bigger but relays are inductive and can stand some ripple. Duke
  18. duke37

    I don't "get" capacitor usage if it blocks current

    In the water analogy when the diaphragm bursts, it will not work again and needs to be replaced. In the capacitor case, when it fails it may just go short circuit or, if using high voltage and power, may explode. I always use safety glasses when using over 100V. Do not use a capacitor in...
  19. duke37

    Parallel resonance on oscilloscope

    There seems to be some errors in you post. The generator is likely to be 0 to 50 MHz rather than 0 to 50 mHz. The reactance of 1H at 40.1 Hz is about 250 ohms. In order to get resonance at 40.1 Hz, the reactance of the capacitor will also be needed to be 252. This means the capacitor will...
  20. duke37

    Samsung 2223NW overheating

    It is obviously very close to working or not working. It can be heated with a hair dryer if working or cooled with a blast of air from a small fan to cool it if not working. The outlet from a vacuum cleaner is warm and perhaps could be utilised. Go gentle on the heating.
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