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  1. K

    Transistors heating up when sinking from motor

    You can "create" your own TIP102 equivalent by making Darlington pairs with your 2222s. Then you can get away with the 1k base resistors. Whatever PNP transistors you're using for your bridge can also be configured the same way. PN2222, 2N2907, 2N2907A, PN2907 are good to use.
  2. K

    How to chop straight DC?

    An IGBT could work if you can find one that can withstand a 1200V C-E voltage. But my thought is that a series resistor before the capacitor would also work. When the GDT fires, it essentially closes the circuit between the capacitor and the discharge transformer. The resistor would then drop...
  3. K

    LM7805CT Heating up

    You'll need a heatsink on the 7805. It's a linear regulator meaning it works like a resistor to reduce the voltage, and the extra energy is dissipated as heat. You'll need a decent sized heatsink to keep the 7805 from overheating. How much current does your device draw when operating? Let's...
  4. K

    Transistors heating up when sinking from motor

    A 330 ohm base resistor @ 5V gives you ~13 mA of base current. This is minor compared to the current draw of the motor. With a minimum gain of 40, I'd recommend a slightly smaller resistor, like 220 ohms, to ensure the transistor is fully saturated (on). If your transistor is controlled by an...
  5. K

    How to chop straight DC?

    Where are you getting 1200V from? It might be easier to "chop" a lower voltage and then step up through a transformer. Why wouldn't a low side switch work? The voltage only needs to drop across the tube for it to stop conducting, but it can still be 1200V relative to the low side. The high...
  6. K

    Decrease Current

    As long as the voltage is correct (12V) the fan will only draw the amount of current it needs. So the 0.5 amp adapter will run your fan no problem and it won't burn it out. Just make sure the "transformer" you're using puts out 12V DC and not AC. Most but not all "wall wart" adapters convert...
  7. K

    P-channel depletion mode Mosfet

    N-channel depletion-mode MOSFETS are turned off with a negative Vgs. Positive Vgs turns an N-channel FET on. One way to look at depletion vs. enhancement mode (for N-channel) is: the on-threshold is negative for depletion and positive for enhancement. P-channel depletion-mode MOSFETs are...
  8. K

    unknown IC

    It isn't a PIC, as the 14-pin PICs have Vdd/Gnd on pins 1 & 14. A quick look at the AVR micros show them not having Vdd(cc)/Gnd on pins 4/11 either. It probably is a microcontroller though. Are there any markings on the package?
  9. K

    Need help identifying a power regulator

    The key seems to lie in that blue part. Sounds to me like it is normally closed and it opens when there is a fault, and then the current passes through the LED and makes it light up. Maybe it's a polyfuse, also known as a PTC thermistor.
  10. K

    Need help identifying a power regulator

    I don't see a regulator at all, just a circuit with a 15-ohm resistor, LED and that blue thing that looks a bit like a varistor (surge protector). What is its application? And what voltage do you put in and what do you get out?
  11. K

    Transistors heating up when sinking from motor

    Another thing to consider is the maximum collector current of the 3904 is 200 mA, so you're pushing the transistor right to its limits. The motor may need more than 200 mA to start up, they always draw more until they're up to speed. I'd use a 2N2222 or similar transistor, which can handle...
  12. K

    P-channel depletion mode Mosfet

    Correct. With N-channel FETs the source is negative relative to the drain, and a positive Vgs turns the transistor on. For a depletion-mode N-channel FET Vgs would need to be negative to turn the transistor off.
  13. K

    Transmitt ASCII data

    My first guess would be the baud rate of the UART and the Bluetooth module don't match. What are you initializing the UART control registers to?
  14. K

    Help with Landline telephone

    If you think the switch is working ok (did you test it with a multimeter?) perhaps the switch controls some other circuit that handles the on/off hook. Have you traced where the switch wires go on the board? Normally the switch is in series with the phone line (after the ringer circuit), and...
  15. K

    World Clock Color Change

    One way could be to modify an alarm clock that has dual alarms. Have one alarm trigger the red LEDs and the other trigger the green ones. Otherwise, you'd need a clock that can provide a time output to something like a microcontroller, at which point you might as well just have the micro do...
  16. K

    Different type of potentiometer

    Maybe it's a pressure sensing strip? What does the other side look like? Can you press on it with your finger instead of the metal piece and have the voltage/resistance change?
  17. K

    inline ac polarity checker

    You say the supplies aren't grounded, I assume that means they have a 2 prong plug? Is the plug polarized (fits only one way in the socket)? If they are, and you can tell by looking at the board which is supposed to be "hot" and which is supposed to be "neutral", you can use a multimeter to...
  18. K

    Suggestions for proximity alarm circuit?

    How about an IR emitter on one side of the can and a receiver on the opposite side, facing each other. As long as the receiver sees the signal from the emitter, the alarm stays quiet. If something breaks the beam, have the alarm sound after a brief delay, a second or so, plenty of time for a...
  19. K

    Need help to understand circuit!!

    Good info there KrisBlueNZ. I'm mainly familiar with PICs and not the 8051-types. But looking at the circuit *something* needed to pull that line high, thus my assumption of either an internal or external pull-up.
  20. K

    Belkin power authority repair

    If I were to venture a guess about that ground (earth) wire, it looks like it was also connected to a varistor. You can still see part of a lead soldered to the wire. So there were probably two varistors, one between hot (black) and neutral (white), and another between hot (black) and ground...
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