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  1. duke37

    Need help modifying ac ammeter (shunt)

    Assume you have a transformer with a turns ratio of 1000. One amp through the primary will give 1/1000 A (1mA) through the secondary. If this current goes through a 100Ω resistor, then a voltage of 1/1000 * 100 (100mV) is developed, this can easilty be measured. There is a reflected voltage in...
  2. duke37

    Need help modifying ac ammeter (shunt)

    The different resistance should not affect the current. This will be the input current divided by the turns ratio of the transformer. The higher the burden resistance, the higher the voltage drop across the input. Do not get voltages and currents mixed.
  3. duke37

    Boost converter

    Are you running at 20kHz or 200kHz? Are you turning on Q1 fully.
  4. duke37

    Converting a PWM wave into pure DC

    You can look up Wikipedia "Precision rectifier". The article is taken from Horowitz and Hill "The art of electronics". I still do not understand what you want. The PWM signal is used to vary the average output voltage. The signal should be sufficient to turn on the switching transistors , say...
  5. duke37

    Need help modifying ac ammeter (shunt)

    In other words, you want to halve the sensitivity. In order to do this, the burden resistor will need to be halved, not doubled. Have I got things wrong, yet again?
  6. duke37

    Need help modifying ac ammeter (shunt)

    Doesn't he want to do the opposite? Adding a resistor of equal value in parallel will drop the sensitivity by a factor of two. The wires will need to be able to pass the increased current without overheating.
  7. duke37

    Converting a PWM wave into pure DC

    The diode and capacitor will give a value of the peak minus the diode voltage drop. To get better than this, you will need an ideal diode which can be simulated with an op-amp but will need a higher voltage power supply to drive it. A low pass filter will give the average voltage which is of...
  8. duke37

    Satellite and Radio signal

    Radio signals in the medium wave and short wave bands encounter vatious ionised layers in the atmosphere, these can absorb the signals, reflect the signals or focus the signals. The ionisation is affected by the sun. Modern TV signals, mobile phone signals and satellite signals are of much...
  9. duke37

    find resistance r1 and r2 if only vin and out is given?

    12V is reduced to 9V so one third of the voltage is across R1 and two thirds across R2. Chose a resistor (any value) for R1 and then R2 will be double this. [mod edit: solution removed]
  10. duke37

    Is there a name for this type of motor ?

    Older windscreen wiper two speed motors have brushes at different angles, A universal or DC motor runs at slow speed when the stator is at high voltage and will run at higher speed when the field is weakened. The field is presumably taken from the brushes which will have different voltages at...
  11. duke37

    Will this purchased adaptor be okay?

    I assume that the blue light is to indicate that there is output voltage. It will stay on until the reservoir capacitor discharges.
  12. duke37

    B&O Beogram 1900 bad motor?

    I have never dealt with a motor lke this. 330Ω seems reasonable. Is the motor supposed to reverse? The two windings are in series with 55V across each, 117v total. I think the resistor and capacitor are there to suppress interference when switching. Connect the two windings in series and to...
  13. duke37

    No sound

    What is a boombox? What is the power supply and how big are the speakers? Have you been running it at silly volume? If the volume goes up and down, it is unlikely to be the volume control. Small amplifiers can have poor connections at the headphone sockets which should turn the speakers back on...
  14. duke37

    Regulated output voltage of 12v 100ma.

    I have not checked in detail but what I have seen is good. 15V AC is 21V at the peak of the waveform, this is held by the capacitor, then the other half of the waveform goes to -21V so the diodes have to withstand 42V. There are two diodes in series so it should be possible to use 25V diodes...
  15. duke37

    Acceptable Zener regulator tolerances

    If you want good regulation, why not use a band gap reference e.g. LM317T. Set it to the voltage you want.
  16. duke37

    DIY Electric Shocking Mouse/Rat Trap

    The local rado club held a construction contest and one entry was a metal plate with a smaller plate mounted on insulating pillars. The two plates were connected to the mains (240V) via a bulb limiter and food was placed on the top plate. It was said to work as a rodent killer but I have...
  17. duke37

    1980 Soviet Radio Cassette amplifier problem.

    When you get the new transistors, measure them to check the meter. Do you realise that V9 is npn and V10 is pnp and may well need a different connection to the meter? A gain of 345 is possible. I believe that you set R18 to get half the supply voltage at the emitters of V9 and V10 but I think...
  18. duke37

    Beginner regulated power supply circuit help

    As shown, dV/dT = I/C Rearranging, dV = dT*I/C Taking dT=10ms, I=100mA, C=1000μF dV = 0.01*0.1/0.001 = 1 This assumes the pulse charges the capacitor in a very short time and that the discharge current is constant. A 7812 regulator chip will be simpler and better than a Zener diode and will...
  19. duke37

    Beginner regulated power supply circuit help

    I presume that you are using the mains for the power. The time between charging pulses will depend on whether you have full wave or half wave rectification and the frequency of the supply. In the UK we have 50Hz so the time for one cycle is 1/50sec or 20ms. With full wave rectification the time...
  20. duke37

    1980 Soviet Radio Cassette amplifier problem.

    The amplifier runs as a feedback loop. Putting a signal in part way is not going to tell you much. The signal is compared with an attenuated version by VT2 and VT3. The attenuation is R31/R32 so 15k/58 = 260 This gain is very high and a little bit of hiss in an input transistor will be...
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