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  1. audioguru2

    OVER-/UNDER-VOLTAGE PROTECTION CIRCUIT ANALYSIS

    Hi Indulis, The old LM358 low power dual opamp and the old low power LM393 dual comparator were made at about the same time, have the same amount of voltage gain and have very similar circuits. The opamp slews 2.5V in 10us. The comparator slews 2.5V in 0.38us positively and even quicker...
  2. audioguru2

    a very simple question.....

    A capacitor should show that it is charging from an ohm-meter. It is a very crude and accuracy-less test. It is easy to test a capacitor: 1) Measure it on a capacitor tester. It should charge then stop drawing current. 2) Charge the capacitor from a DC voltage then time how long it discharges...
  3. audioguru2

    white led

    Half the white LEDs I bought that were made in India didn't work. Cheap junk. The store (Sayal, owned by people from India) replaced them all with excellent ones but I don't know where they are made. I won't buy anything from India anymore but the store has excellent American surplus products...
  4. audioguru2

    transformer getting too hot.

    If you short circuit a voltage supply without something to limit the current then it gets too hot. Hopefully it is a fuse that gets too hot and it blows to prevent a fire. Keep away from electrical things until you learn the basics. The mains frequency in my country is 60Hz. It is 50Hz in some...
  5. audioguru2

    500W sine wave inverter design

    It is complicated to make a 500W sine-wave inverter that doesn't burn up. Most inverters that are sold make a simple square-wave and they don't get too hot. We have one in our Projects section: http://www.electronics-lab.com/projects/power/033/index.html It uses a Cmos oscillator IC, two quad...
  6. audioguru2

    opamps

    Just think of an opamp asd having infinite voltage gain. The two resistors for the negative feedback make the voltages at both inputs the same. Then simply use Ohms Law to determine the voltage at the output and the amount of closed loop gain.
  7. audioguru2

    OVER-/UNDER-VOLTAGE PROTECTION CIRCUIT ANALYSIS

    The 741, 1458 (dual 741), TL07x and many more ordinary opamps have outputs that go down to about +1V above their negative supply which could be ground. It is a little more than 1V if the current is higher than about 5 mA.
  8. audioguru2

    Circuit Help

    A new 9V alkaline battery will power it all day.
  9. audioguru2

    IC replacment for transistor tester

    I typed CD4047 by mistake before. The CD4027 is the IC that is used in the transistor tester project, a CD4047 and a CD4020 are completely different. Digikey has thousands of CD4027 ICs in stock for only $.55US.
  10. audioguru2

    Circuit Help

    Your schematic is a fuzzy JPG file type. It is too bad you didn't save it and attach it as a clear GIF or PNG file type so we can see the parts values. What are you going to use it for? It just makes a continuous beep and quickly kills the 9V battery. Do you have an AM radio that operates at...
  11. audioguru2

    IC replacment for transistor tester

    An ordinary CD4047 IC is available nearly everywhere (except RadioShack and a few weird countries). It costs only $..55US at Digikey. They have thousands of them in stock.
  12. audioguru2

    OVER-/UNDER-VOLTAGE PROTECTION CIRCUIT ANALYSIS

    Hi Walid, The transistor is not straining so hard without the LED. With the LED, its load was 60mA for the relay and about 25mA for the LED if it had a current-limiting resistor. The total collector current would have been 85mA so the 8mA base current is fine. Without the LED then you can...
  13. audioguru2

    OVER-/UNDER-VOLTAGE PROTECTION CIRCUIT ANALYSIS

    You need R4 to turn on T1 as hard as it can. The LED needs to have a current limiting resistor in series with it then the circuit will work fine. Don't change R4. A graph in the datasheet for the BC547 transistor shows that the collector saturation voltage is typically 0.2V when the collector...
  14. audioguru2

    OVER-/UNDER-VOLTAGE PROTECTION CIRCUIT ANALYSIS

    The red LED is about 1.8V so the lowest voltage for the transistor's collector is +12V - 1.8v= +10.2V before one of them burns out. With a base current of 8mA then the transistor is turned on as hard as it can with an extremely high current and it will be extremely hot. Look at the datasheet...
  15. audioguru2

    OVER-/UNDER-VOLTAGE PROTECTION CIRCUIT ANALYSIS

    Hi Walid, The LED doesn't have a series current-limiting resistor. The transistor has a base current of 8mA and has a current gain of at least 80 when it has a collector current of 100mA. So the current through the LED and transistor will be much more than 100mA. The max allowed current for the...
  16. audioguru2

    white led

    Hi Pier, Yes, your schematic is correct. If the supply is 12V then two LEDs can be connected in series and in series with one current-limiting resistor then half as many resistors could be used. We cannot determine the value of the resistor until we know details: 1) Is the 6V regulated? Or will...
  17. audioguru2

    OVER-/UNDER-VOLTAGE PROTECTION CIRCUIT ANALYSIS

    The circuit is not designed properly: 1) The LED will blow up! It doesn't have a current-limiting resistor. Maybe the transistor will also blow up when it tries to saturate. 2) An LM324 was used in the circuit but it is a quad opamp. An LM358 dual opamp is exactly the same opamp but is a dual...
  18. audioguru2

    ringing detection circuit

    The protection diode conducts when its voltage becomes higher than only 0.6V. Then when the AC ringing voltage is reversed across the protection diode then the LED conducts and limits the voltage to only about 1.5V. So the 1N4148 has a max voltage during ringing of only 1.5V.
  19. audioguru2

    white led

    LEDs will draw as much current as the power source will supply until they burn out. You must use a series current-limiting resistor or a constant current source to reduce the current to less than 30mA for each ordinary LED.
  20. audioguru2

    ringing detection circuit

    Nearly every LED has a max reverse voltage rating of only 5V. The ringing signal has a peak voltage of 127V so the diode conducts and protects the LED.
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