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  1. KrisBlueNZ

    Quick question regarding op amps

    If you're generating your derived ground rail using a two-resistor voltage divider (usually with a capacitor from the rail to the negative supply), the values of the resistor are a tradeoff between avoiding wasted supply current (this pushes for higher value resistors, so less current flows...
  2. KrisBlueNZ

    Capacitor and Resistor ordering

    I can't tell what question you're trying to ask. If you mean during construction of a circuit, it's usual to install IC sockets, connectors, link wires, resistors, capacitors and diodes before you install other semiconductors, then install socketed ICs last. Within those groups, order is not...
  3. KrisBlueNZ

    Problems building first circuit - capacitor ESR meter

    Replace the circuitry to the right of the device under test with a half-wave or full-wave precision rectifier. See http://sound.westhost.com/appnotes/an001.htm (which is linked from the Wikipedia article on precision rectifiers) for a survey of possible circuits with advantages and disadvantages...
  4. KrisBlueNZ

    LED Current Calculation

    You can't really do what you're trying to do. The zener voltage and the LED's forward voltage are close together, and will vary depending on various factors, so how much of the current flows through each of them will be poorly defined. You haven't explained the purpose of your circuit. It looks...
  5. KrisBlueNZ

    arcade machine issues

    Could it be a missing power supply rail, or some other signal provided by another board or module in the cabinet, that isn't needed by the board that works fine, but is needed by the other board?
  6. KrisBlueNZ

    LED Current Calculation

    ... Why would you like to do that? Is the zener diode already present in an existing circuit? If so, no, you shouldn't add the LED in parallel with it. Doing so will most likely affect the circuit. It would help a lot if you would post your design as it stands, and explain what you want to do.
  7. KrisBlueNZ

    Problems building first circuit - capacitor ESR meter

    Typical ESR values for aluminium electrolytics range from less than 0.1 ohms to more than 1 ohm. An electrolytic may fail and go out of spec without having a hugely high ESR, so to be useful, an ESR meter needs to be able to measure ESR down to less than 0.1 ohms. A typical silicon diode such as...
  8. KrisBlueNZ

    Help wanted

    Be aware, anyone who gets involved in this project, that the intended application here is to cause current to flow between two electrodes attached to a person's head. Look up TCDS (trans-cranial direct stimulation) for more information. Equipment for direct connection to human beings may have to...
  9. KrisBlueNZ

    Problems building first circuit - capacitor ESR meter

    I doubt the circuit will work using a diode rectifier (D1). The AC voltage present at the capacitor under test will be very small, unless its ESR is very high; not enough to cause a diode with a 0.15V forward voltage to conduct. I would use an active rectifier based on an op-amp to detect the AC...
  10. KrisBlueNZ

    Circuit problem

    Just load the image into an image editor, flip it and rotate it 180 degrees, and you got it right first time!
  11. KrisBlueNZ

    Help with voltage regulator circuit, please

    Also, if your battery is also used to supply an electric starter motor and/or an ignition system, you may need some clamping and/or filtering on the 12V supply going into the regulator/charger, to suppress spikes and surges due to load dump and ignition noise.
  12. KrisBlueNZ

    Circuit problem

    What type of battery? What is the battery's rated voltage, exactly? Is it actually 12V? Some "12V" batteries are actually specified as 13.8V. Donkey's suggestion is good (although it's drawn upside down, with negative at the top, and you need to confirm that the charger output connector is...
  13. KrisBlueNZ

    Comparator inputs

    Steve is right. That's why I didn't suggest a single comparator, although you only need one. The LM393 (dual) is fairly widely available, and is the same physical size as a through-hole single comparator; the LM339 (quad) is even more widely available. Also, they're not all the same...
  14. KrisBlueNZ

    Problem with Heathkit IO-102 oscilloscope - vertical deflection doesn't change

    Those were helpful tests. It looks like Q1 may be OK. Those voltages are kind of compatible with a working device. Can you do some more testing. Q1 operates as a voltage follower, so its source voltage should follow its gate voltage, which in turn comes from the input. Can you check with both...
  15. KrisBlueNZ

    Problem with Heathkit IO-102 oscilloscope - vertical deflection doesn't change

    I don't think that's a sign of damage. The little bit of silicon inside the package is far more vulnerable to damage than the wires are. Probably they're just made of a different metal, or are untinned vs. tinned on the other semiconductors, or something like that. To the chassis, or the ground...
  16. KrisBlueNZ

    Does this Circuit make sense?

    No. The LEDs are in series, so the SAME current flows through each of them. Not just the same AMOUNT of current; actually the same electrons! So you don't add the currents together; there is only one current flow. You do, however, add the VOLTAGES (forward voltages) together, since the LEDs are...
  17. KrisBlueNZ

    Discretes in cheap designs (EDN mag)

    Discretes in cheap designs - EDN magazine Martin Tomasz "As part of my job at Touchstone Semiconductor, I’ve been doing some teardowns of consumer products recently—and from time to time I’m still finding lots of discretes in the analog portion of those designs. A wireless door chime...
  18. KrisBlueNZ

    Does this Circuit make sense?

    Well, the forward voltage specification at a particular current is fairly accurate but will vary somewhat with temperature, from batch to batch, and from component to component within a batch. It's not a tightly controlled parameter; LEDs are current-driven, and the voltage across the LED at a...
  19. KrisBlueNZ

    Does this Circuit make sense?

    As Steve as already said, 270 ohms is a preferred value. I suggested two 560R in parallel because 560 ohms is a preferred value and the resulting resistance is 280 ohms. It doesn't need to be exactly 280 ohms though. The forward voltages are just approximate anyway. Yes, LEDs are operated...
  20. KrisBlueNZ

    Op-Amp Problems, constant output voltage

    Yes, the TLV247x looks very nice and ideal for your application. I'm really impressed with TI's op-amp range, and they're not expensive.
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