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  1. duke37

    Replacement Motor - Sewing machine (Stepper or Servo?) More torque needed

    I have a speed control for a coil winder using a foot pedal which is just a variable resistance. This works well down to very low speeds. To get more torque and less speed, use a smaller pulley on the motor or a bigger pulley on the sewing machine.
  2. duke37

    UPS

    A transformer takes a considerable inductive current which could be the cause of the trouble. A better configuration would be to transform the mains down to 110V and then use a 110V in/out UPS.
  3. duke37

    Transistor-triggered photogate

    You would simplify things if you do not complicate matters by asking for advice on things you do not want If you have an inductance, just connect it to the charged capacitor and short the other end to common with a SCR. You may be able to find suitable circuits if you look up electric fence...
  4. duke37

    Slotted optical switch

    The 2N7000 is a diddy transistor in a to92 case and may not be strong enough for the job. Most fets need a considerable voltage source to gate to turn on. With a 9V supply you may not get more than 5V out to the motor. A Darlington may be better.
  5. duke37

    Low Hum Coming From SONY ICF-C218 Clock Radio Speaker? Can It Simply Be Fixed?

    A lot of low powered devices do not switch off the internal power supply when parts are turned off. This is likely to be in the case of a clock where you do not want to lose the time. The hum may not be coming from the speaker but from the mains transformer. These always vibrate to some extent...
  6. duke37

    Replacing light bulbs with LED's for illuminating a radio's scale

    You do not need a difuser, what you need is the light going into the edge of the glass. The light stays inside the glass due to total internal reflection When it encounters the print, the light can escape. Light bulbs must be placed close to the edge but leds will be better since the light...
  7. duke37

    Help, trying to replace a transistor!

    Measure the voltages on the transistors. Silicon transistors will have about 0.6V between base and emitter. The collector will be more positive than either with an npn transistor. Measure voltages relative to 'chassis' which with npn transistors will be connected to the battery negative. The...
  8. duke37

    audio output transformer specs meaning?

    The 1000Ω and the 8Ω is the impedances that the transformer will match. If connected to an 8Ω speaker, the input to the transformer will appear like 1kΩ. If you connect to a 16Ω load, the input will appear like 2kΩ which is three times the optimum but may be all right. Remember that the 16Ω...
  9. duke37

    Help me design a water heater circuit that automatically turns-off when the heater reaches a boiling

    All electric Kettles I have seen have a thermal switch above the water. This does not switch off untjl it is heated by steam thus a temperature setting of about 95C is good enough. The switch is an overcentre type and needs to be reset. This is to allow a fast contact break. The one I use now...
  10. duke37

    Now I am going to ask 3 stupid questions, please dont laugh. but i cant seem to find info

    I made a box for my neigbour to control a hot wire cutter. This used a triac light dimmer feeding a transformer. I put a bulb in series with the primary to indicate the current and to protect against possible transformer saturation. He has made some very good model aircraft using polystyrene...
  11. duke37

    tip120 driving led array

    1. You will be able to put two LEDs in series and perhaps three. This will reduce the total current by a factor of two or three. Each leg should have its own resistor so that all legs have the same current independent of the temperature. An overall resistor is not necessary. 2. Put the LEDs in...
  12. duke37

    Now I am going to ask 3 stupid questions, please dont laugh. but i cant seem to find info

    A Variac is an autotransformer with one single layer winding around a magnetic torroid. This winding is machined flat and plated with noble metals and the output current is taken by a carbon brush placed on this winding at the appropriate voltage. Excess current will distort the winding and the...
  13. duke37

    tip120 driving led array

    If you are taking the output from the emitter, then the transistor will not be fully on unless you have a subsidiary higher voltage supply to drive the base or gate. Ground the emitter/source and use the collector/drain to feed the LEDs. Then a voltage is available above the ground.to drive the...
  14. duke37

    Now I am going to ask 3 stupid questions, please dont laugh. but i cant seem to find info

    I have seen it stated that using a Variac on a SMPS is not a good idea since it may not like a low voltage. Running a valve radio gently is also not optimum, at low voltage the rectifier does not work then a little more voltage does not limit the current. I use a lamp limiter for running up old...
  15. duke37

    Now I am going to ask 3 stupid questions, please dont laugh. but i cant seem to find info

    A 10A Variac (other autotransformers are available) will be a heavy lump, do not drop it on your toe. Assume that you set have the input at 120V and a current into a resistance of 10A, then the resistance is 12Ω. Dropping the output voltage to 12V will provide 1A into the resistance. 12W. 12W...
  16. duke37

    Inverter power, the voltage becomes low

    Give more details. Make and model number. Voltage input, AC or DC Voltage output DC or AC and what frequency and waveform Power rating. What load is required to drop the voltage?
  17. duke37

    Electronics AutoVolt Common problem. HELP

    You need much more details on the faulty equipment. If it is a switch mode supply, at a high input voltage the input capacitor has to stand a high voltage. At the lower voltage, the input capacitor has to stand a much higher ripple current. Is it up to the task?
  18. duke37

    What is the difference or similarity between these 2 pot circuits?

    A shunt resistor could be connected across the whole circuit that is to the two connections on the board. I cannot understand what you are doing. Is the large resistance 500kΩ or 200kΩ? Edit: If this is a regulator, the pots are possibly the voltage sense feedback and excess resistance could...
  19. duke37

    Help finding issue with this circuit.

    The old system of control was three resistors switched in series with the motor to set the speed. Considerable heat was developed in the resistors. This transistor does not look as if it has been bolted to a heat sink. The transistor therefore has to be either fully on or fully off. To get...
  20. duke37

    Help finding issue with this circuit.

    What was the trouble? Is this the type of car which likes to go on fire? If so, get it repaired free of charge on the recall. Make sure the MOSFET is properly fixed to the heatsink.
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