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  1. Hero999

    2N3906

    This is the problem with using no name parts, using branded parts solves the problem. Do you have a multimeter meter with a  transistor test function? If so the gain will be higher when the collector and emitter are connected to it the right way round. Another option is to build the test...
  2. Hero999

    JFET Mic Pre Advise Please

    Your circuit is fine for driving a 10k load - leave it as it is.
  3. Hero999

    JFET Mic Pre Advise Please

    Originally the circuit had such a high bias current that Q4 was overheating, now the bias current is lower so Q4 will no longer overheat. To drive a lower impedance load a class AB stage should be added. It's a silly idea to use a class A amplifier like this to drive a low impedance load.
  4. Hero999

    Design new 0 to 30V power supply.

    R20 and R23 are in series so  for the sake of this discussion I mean R23 as well as R20 when I mention R20. R20, C5 and U1C form an integrator or low pass filter. The cut-off frequency is equal to 1/(2πRC), in this case R20 and C5 are R and C respectively. The interesting thing about an...
  5. Hero999

    Is ultrasonic transmitter and receiver has polarity

    No, they're AC so it doesn't matter as long as you're aware that one of the pins is connected to the case.
  6. Hero999

    current regulation help

    You can't regulate both the voltage and current simultaneously. If you limit the current then the voltage will drop, see Ohm's law.
  7. Hero999

    washing machine motor controller circuit

    See if you can work it out for yourself. Google for DPDT, if you've never seen one before. All you need to do is reverse the polarity of power going to the motor, this should be easy to figure out how. Post your schematic here and we'll verify it for you.
  8. Hero999

    LCD MONITOR

    LCD monitors are actually very complicated pieces of equipment with nearly 100 possibly failure modes. What do you mean it's not working? Are they completely dead? Check the fuse before anything else.
  9. Hero999

    washing machine motor controller circuit

    RL2 needs to be replaced with a DPDT relay and the wiring needs to be changed to work with a 12V permanent magnet motor. That circuit wouldn't work with some of the washing machine motors I've seen which are universal, not induction.
  10. Hero999

    Design new 0 to 30V power supply.

    R20 is not supposed to alter the current limit, that's R21's job. R20 controls the delay before the current limit kicks in so when it's set to the highest resistance setting there will be more of a surge before the current is limited. There will be a surge anyway due to C7 rapidly discharging...
  11. Hero999

    Design new 0 to 30V power supply.

    I'm not meaning to be picky but don't put MA when you mean mA or MV when you mean mV. The upper case M means mega which is a million, m means a thousandth. I obviously know what you mean but not everyone will. In theory you should be able to set the current all the way down to zero but I accept...
  12. Hero999

    Looking for Knowledge/Assistance

    That's slightly more complicated, although not much more complicated. You need to use a timer, to make sure the minimum pulse length is at least a second or so. Here's a solution but it will require some learning before you'll be able to build it and even more if you want to understand it well...
  13. Hero999

    Design new 0 to 30V power supply.

    Good, I'm glad it's performing as expected. You probably can use a 0.1R resistor, in fact may be the circuit should be modified so 0.1R can be used? The trouble is the resistor will need to be rated to 12.5W, even higher if the current is slightly over 5A, I'd probably suggest 15W. For 5A I...
  14. Hero999

    Design new 0 to 30V power supply.

    Good, that seems to be much better. The current limiting still isn't behaving as it should. As I said before, changing the value of the pot won't alter the actual current limit, it'll just alter the length of time for the currrent limit to kick in. Changing R5 to 0.1 will lower the current...
  15. Hero999

    opamp roll-off

    I'll post what I did before but deleted. Here's how the roll-off limits the high frequency gain and how a reduction in closed loop gain and increased negative feedback, increases the bandwidth. Going back to the graph posted by audioguru...
  16. Hero999

    Looking for Knowledge/Assistance

    It depends on where you live. In the UK Maplin is a high street shop which sell some pretty loud piezo buzzers, in the US there's RadioShack which, although similar are a different company. Many online companies are much better: in the UK, Farnel, RS, Components and Rapid Electronics are...
  17. Hero999

    Looking for Knowledge/Assistance

    There is no real circuit diagram. The two pieces of metal can form a switch, when the pieces of metal come into contact with each other, the circuit is made and the beeper goes beep. You can buy piezo buzzers which make a beep noise rather than a buzz when powered up.
  18. Hero999

    Pass signal at 0V input?

    Please post a schematic. Is this what you want? >0 output high = 0 output low How near zero does the voltage need to be? There has to be a switching threshold, it could be 0V but in order for the output to be off, the input would need to be below 0V i.e. a negative voltage. So if you want...
  19. Hero999

    inverter and transformer rating....

    You should use MOSFETs instead of BJTs as they have a lower voltage drop. Here's a schematic I found using Google. You don't have to use all four MOSFETs, two will do as it's only lower power. I'd recommend using a 9V transformer because it's easier to get hold of and will give you closer to...
  20. Hero999

    Design new 0 to 30V power supply.

    Did you remove U5? So you remove R8 and R14 without removing U5? If the current from the supply isn't limited it will destroy U5. The tap change occurs when the voltage across R17  goes above the voltage across R13. If Vref is changed, then the switch over point will change because the...
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