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  1. BobK

    Hypothetical Question on Potential and Resistance

    Yep, that is correct, and for higher power LEDs, a constant current supply is the best way to go. Bob
  2. BobK

    Hypothetical Question on Potential and Resistance

    The power is split between the resistor and the LED. In your example of 120V and a 6K resistor, the current will be: (120 - 3.4) / 6000 = 19.4mA. The total power is 120 * 0.0194 = 2.33W The power in the LED is 3.4 * 0.0194 = 0.07W The power in the RESISTOR is (120 - 3.4) * 0.194 = 2.26W So...
  3. BobK

    LM2903N differential op-amp

    Not sure, but at least the current draw would not exceed the specs of the sensor. Bob
  4. BobK

    Which type of large decoupling capacitor for amplifier audio output?

    1000uF and 8 Ohms will be down 3dB at 19.80Hz. Unless you are using a huge subwoofer, the speaker will cut off way above that. A typical 5 or 6 in speaker is not going below 40Hz or so. The speaker and enclosure are much more important to bass response, as Kris said. Bob
  5. BobK

    Voltage Level Switch

    And, once again, is the 12V regulated? If not you need a voltage reference. Bob
  6. BobK

    Which function generator to buy? $300 to $400 budget

    Adam, did you change the sort to log to high? There were some reasonable ones under $300 there. Bob
  7. BobK

    LM2903N differential op-amp

    Except that it went only to 0.8V, which is way higher than need be, and after making the resistors larger it started working, not quite to zero but close enough. Bob
  8. BobK

    LM2903N differential op-amp

    This is where you went wrong. The resistors should be LARGER than one that would draw 2.5mA over the full voltage range, otherwise the sensor will not be able to provide the current needed through the resistor to make the voltage difference between the two sensors. R = V / I R = 5 / 0.0025 =...
  9. BobK

    Advice on SMD Soldering

    Halve the amount of solder once more and you will be there! Bob
  10. BobK

    transistor and speaker help

    I am aware of that. But since you can only get about 1/10 W out of it anyway, I don't think any speaker is going to be damaged by the small constant current. I also experimented with using an 8 Ohm emitter resistor and a capacitor to the 8 Ohm speaker and got similar voltage to the speaker...
  11. BobK

    Resistance range to output voltage range

    The OP said it should output 0.1V at 10 Ohms for the empty indication. I can see the gas gauge having E marked somewhere above the 0 point so that you can distinguish between empty and meter off. I believe your method will have something like 0.3V output for 10 Ohms, which means it will never...
  12. BobK

    Resistor help

    Next time, when you say "measured", measure it first! ;)
  13. BobK

    Using Vref as the input to comparator module

    The internal reference can be connected to the comparator inputs internally without any I/O pins involved. Just set the mode bits in CMCOM to 010. Also, see figure 10.1 which shows all the configurations. Bob
  14. BobK

    High Speed PCB Design

    I am not an expert, but: Make it as small as possible. Place the components to minimize the distance of all connections. Use a ground plane. I have seen old amateur radio circuits that were build on a ground plane on a single sided pcb, with through hole components mounted like surface mount...
  15. BobK

    transistor and speaker help

    I did some toying around with that circuit. You will get the most output by simply using the speaker as the emitter resistor and then running about a 1K resistor from v+ to the base to bias the transistor partially on. I was able to get 2Vpp swings that way, though quite distorted. There will...
  16. BobK

    Maintaining DC voltage across capacitor with signal?

    If that is all you want to do, why does it need to be done with lethal voltages? Nothing you have described here required 2000V. Bob
  17. BobK

    Advice on buying a mini solar panel for a solar powered usb charger

    To charge a cell phone, say 2000mAH, it takes 5V at 1A for two hours if 100% efficient. That is 5W. So to get that kind of performance you need a 5W solar panel minimum, and realistically a 10W panel. A 1m2 panel puts out about 100W, so you would need about 1/10 of that or about a 31 cm (or 1...
  18. BobK

    Resistance range to output voltage range

    The 10 Ohms at empty is what bothers me. Couldn't we subtract the voltage at 10 Ohms with the op amp to get a 0.1V at empty and adjust the gain to get the full scale value of 4.5V? I think this would improve the circuit. Here is a modification of the circuitry around the op amp in Kris's...
  19. BobK

    Capacitor voltage as input to transformer?

    If you put a transformer primary across a capacitor, you have and LC filter. Bob
  20. BobK

    Resistor help

    Okay, I think this is explained by the fact that your "9V" wall wart is one of those that puts out 9V at a specific current, relying on it's internal resistance to drop from a higher voltage. What current is it rated for? I would suggest you use a voltage regulator after the wall wart, or get...
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