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  1. BobK

    Need opinions on regulation.

    A 7805 drawing 750mA from a 14V supply will dissipate (14 - 5) * 0.75 = 6.75W. Of course it will get hot. You need a major heat sink for that. The T0220 case can only dissipate about 1W without a heat sink. The alternative is a DC to DC converter, which will save power and not get hot. Bob
  2. BobK

    passive matrix mixer - need help

    This is why the author said you cannot connect it up without additional amplifiers. Any passive mixer will have this problem to a certain extent, but the values of the pots and mixing resistors you have used accentuates it. For less cross dependence you would want the pots to be lower in...
  3. BobK

    passive matrix mixer - need help

    Yes you are, and I was going to point this out to you. Each potentiometer in that circuit will affect all of the other inputs because they all form one big voltage divider. Bob
  4. BobK

    Test bench power supply

    Okay, that would be a single supply. Bob
  5. BobK

    passive matrix mixer - need help

    I don't see how you could get no sound out unless something is shorted somewhere. You should get something like 1/6 of the input at the output for any channel that is turned all the way up. Bob
  6. BobK

    Simple passive low pass filter

    10 Ohms, 47uF for a cutoff of 338 Hz. Of course, cutoff frequency is the frequency where the response will be down by 3dB, then 6dB per octave after that. Edit: or 1 Ohm 470uF or 100 Ohm 4.7uF etc Bob
  7. BobK

    LED's for model truck

    Dave: That is a boost circuit similar to the Joule Thief. Since all of the LEDs are in series, they are all getting the same current. Since some of them do not light at all, I suspect that current is < 1mA. Please read this on how to power LEDs...
  8. BobK

    Test bench power supply

    One what? Transformer or dual supply? Bob
  9. BobK

    Laser Diode Project. Help!

    I expect that 5mW is not the electrical power drawn, but the optical power output. The electrical power might be 5 to 10 times that high. Bob
  10. BobK

    The input given to Piezo Speaker is an analogue signal or Digital signal?

    And even if you expectation was true, it would still sound continuous. You have to get down to about 10Hz before we hear distinct clicks as opposed to a continuous sound. Bob
  11. BobK

    power inverter

    Since you did not specify a voltage, frequency and waveform, I will make my own assumptions. input 100V output 100V 1 Hz Square wave 10 A I would then make the circuit using an NE555 timer as an astable multivibrator driving DPDT relay rated for more than 100V and 10A. Bob
  12. BobK

    AC Coupled Common Emitter Amp Oscillation Problem

    I am surprised that circuit would oscillate since it is not high gain and it is low input impedance. It looks like it oscillates at about 700KHz. If you are not using frequencies that high, perhaps a low pass between the stages would help. What is the upper limit of frequency you want to...
  13. BobK

    Alternative IC for Constant Off Time PWM Current-Mode Controller for LED Applications

    Perhaps the reason you are not finding many alternatives is that you are looking for one that is explicity current-mode. Any voltage-mode controller can be used for current control simply by using a sense resistor instead of a voltage divider to the voltage sense input. Typically they need...
  14. BobK

    Adding Accent Lighting To My Raspberry Pi B+ Case

    Why not plug the LEDs into a GPIO connector on the Pi? Then you could also do some wicked awesome flashing patterns as well! Bob
  15. BobK

    Need opinions on regulation.

    A 7805 will not be much more efficient than a zener. In both cases, you need to drop 19V across either across a resistor or the regulator, which is 0.36W, which should not be a problem. A zener with a 1K resistor 1/2W resistor should work fine. Bob
  16. BobK

    Drilling pcbs

    That is not my experience. It works like a dream compared to hand drilling, which I did before I got the press. Bob
  17. BobK

    Drilling pcbs

    I use this: http://www.amazon.com/Dremel-220-01-Rotary-Tool-Station/dp/B00068P48O/ref=sr_1_1?ie=UTF8&qid=1425831268&sr=8-1&keywords=dremel+drill+press I have only broken bits when I accidentally hit them when removing the PCB, which is stuck to a piece of wood for backing with double-sided...
  18. BobK

    Test bench power supply

    Yes, I found cheap singles, but no duals for less than about $180. Bob
  19. BobK

    Choice of Op-Amps

    Yep. I looked a couple of controllers on Ebay and they used 13KHz and 15KHz. Not out of audio range but close. On my robot car I used 10KHz and did not hear anything, but then my ears fall off rapidly at 4KHz in one and 8KHz in the other according to my ear doctor. My wife also did not...
  20. BobK

    Test bench power supply

    Actually I looked, and dual supplies do not come that cheap. I think your best bet would be to get a toroid core of the appropriate size and wind your own transformer. Bob
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