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  1. duke37

    Pull-down resistors for MOSFETs

    You should come to Derbyshire, they tell me it is balmy here, I think they meant the weather. A pack of silica gell to absorb the moisture may help but the box needs to be sealed for this to work. A bjt transistor will have a gain of up to 1000. A fet will have a gain of millions even with a...
  2. duke37

    Pull-down resistors for MOSFETs

    There will be drive current only when the fet is turned on which presumably will be switching a much higher current. Also, 10M seems a high value. It may well work in Alice Springs but not in Darwin with its higher humidity. You will need high quality printed circuit board.
  3. duke37

    12 V power supply.... Is this normal?

    The transformer outputs AC so there is no such thing as a positive output. The measurement of a pulsed waveform with a DC and AC component will depend on the way the meter works. An oscilloscope would be a better way of measuring this.
  4. duke37

    Pull-down resistors for MOSFETs

    In the second diagram, when one input is high, the resistor will not pull down any of the other inputs. What do the diodes do? With 10M across 50 fets, the total resistance (all inputs high) will be 10M/50 = 200k With 12V input, I = 12/200 mA = 60uA. Are my calculations right?
  5. duke37

    Audio Preamplifier produces glitches (cracks)

    I think Harald is on to something, I have had electrolytic capacitors which short over internally. This gives a crack and then they then take some time to recharge. Is there a very short time with lack of sound after the crack? You may be able to see a voltage change with a fast analog or...
  6. duke37

    Protecting Driver Transistor from Transformer/Coil Back EMF

    Perhaps you could give us details of the NTE261, it is not in my transistor book. The transistor rapidly turns off the primary current and the inductance forces this current to continue for a short time. As a result, a high voltage is generated which can force current through the turned off...
  7. duke37

    another motor question

    My guess is that if it has an iron core, it should be placed in series with the motor. Either lead should be OK. It could be to smooth the current to ease the stress on the commutator. If it is air cored or ferrite cored it should be in the input leads.
  8. duke37

    Troubles coming up with capacitor without guessing or trial and error fudging

    The drop out voltage will be much less than the pull in voltage, I measured the voltages on a 240V AC relay running on DC. Pull in 60V Drop out 7V To hold the relay for 1sec, the time constant will be about 1sec. so C = 1/280 = 3600uF If you use a transistor to drive the relay, the...
  9. duke37

    Electret Microphone Replacement

    The electret microphone will be fed with a resistor. If this is replaced by a potentiometer, then the sensitivity can be varied. A low pass filter can be built but will need some room and a small amount of power. Have you tried some PVC sticky tape over the microphone which will drop the...
  10. duke37

    7500EXL GENERAC GENERATOR NO AC OUT

    First determine the type of generator. What is the specification. If it is AC out, then there is likely to be a good connection from output to the stator windings. Check the stator windings for continuity. Check any fuses between the stator and sockets, as recommended previously. Look at...
  11. duke37

    another motor question

    The Bitish standard horse, given sufficient oats, can produce 746W. Other horses, ch, PS or CV differ. Input 90V 14.5A = 1309W Output = 1.5HP = 1119W Efficiency = 85% These measurements will be made at full power and should not be exceeded except for very short transients such as at...
  12. duke37

    Old computer power supply has low voltage.

    Schottky diodes have a lower voltage drop than standard silicon diodes.
  13. duke37

    Home made electronic ignition

    I made a transistorised ignition some 30 years ago and also bought a Sparkright unit. These were based on self oscillating invertors using the new 2N3055 power transistors. The inverter charged a capacitor which was discharged throuh the coil with a SCR. This put a short on the inverter output...
  14. duke37

    Universal motor bi-direction speed control

    Your Variac was a bargain. Make sure you do not exceed its current rating.
  15. duke37

    please help

    Is your power supply big enough? The motor will take about 2A so you need a power supply capable of 3A or so.
  16. duke37

    Temperature sensors?

    The first thing to do is to chose a sensor. A thermocouple will give linear output with temperature change but has a very small output so will need a good amplifier. Also, the thermocouple needs a cold junction which must be stable. If very thin wire is used, then it will react quickly. A...
  17. duke37

    Quick question about equal input voltages for op amp?

    I think you are right. I have never had to balance an op-amp in the hobby jobs I do, they have been adequate without balancing.
  18. duke37

    Anti-thyristor?

    It seems that you wish to switch a low power signal. A transistor will do this. Show us a diagram of the parts you wish to switch and your working relay solution so that suggestions can be made on how to do it. It may be preferable to inhibit one circuit rather than disconnect it.
  19. duke37

    Voltage or Amplitude?

    R5 will contol the gain of the seond circuit.
  20. duke37

    Universal motor bi-direction speed control

    You have not said the voltage of the motor, at 840W I assume that it is mains, 110 or 220V. To generate a 220V PWM source would be something special. A Cds is perhaps a cadmium sulphide (CdS) light sensitive cell which replaces the potentiometer to control the triac. I do not know if these...
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