Search results

Search Forums New Posts
  1. duke37

    Low battery shut off (and warning)

    About 10V should be enough to turn the MOSFET on fully, so just put a 10 or 12V zener between gate and ground to protect the gate. I will leave you to determine the input connections to the comparators. I do not know what device you are driving but if it is inductive then a "catching" diode...
  2. duke37

    [help plz] Measuring HIGH DC Pulse Voltage with DMM

    Electric fence energisers are normally rated by the energy in a pulse. 5J is the recommended maximum but I keep well away from one this powerful. The pulses occur about once per second so you have time or are forced to withdraw. I could not find what the energy in each pulse is or the repetition...
  3. duke37

    [help plz] Measuring HIGH DC Pulse Voltage with DMM

    Electric fence energisers are rated in energy/pulse, 5J is about the maximum and I keep well away from that. The pulses occur once/sec so you have time to withdraw. I assume that your device is also pulsed so measuring with a DVM would not be possible. Adding a rectifier and capacitor to give a...
  4. duke37

    Firing 3 LED's using two switches and 3 wires

    If you use an AC power supply, you can rectify to get a positive and negative output. Either or both can be switched to a single wire. Rectifiers (diodes) at the receving end will divert the signal to the LED required, With 3 wire (2 + screen?) cable one wire would be reference (ground) giving 2...
  5. duke37

    Building a pH meter

    Yes, screened cable will be made coaxially. Coax for radio frequency purposes is often stiff and a large diameter, this you do not want. Your probe cable should be connected to your circuit with a BNC connector and then to the op-amp with screened cable leaving no more than a millimeter or two...
  6. duke37

    'Soft starting' a DC motor

    The motor will have an effective resistance when under full load of about 1 ohm, If a resistance of about 0.5 ohm is placed in series, then the motor should still start. The resistance could be by-passed with a relay after a suitable time. Alternatively, A powerful MOSFET (IRF540?) placed in...
  7. duke37

    Building a pH meter

    With a very high input impedance, any external voltage variation can be coupled to the input with capacitance giving variable voltage readings. You will need to use screened input cable and keep away from devices which produce interference such as fluorescent lights.
  8. duke37

    [help plz] Measuring HIGH DC Pulse Voltage with DMM

    If your kV meter used electrostatic attraction to deflect the pointer, then a steady voltage is required, not a pulse.
  9. duke37

    [help plz] Measuring HIGH DC Pulse Voltage with DMM

    I have repaired electric fence energisers which give a high voltage pulse of about 6kV each second. I originally used a spark gap to estimate the voltage using a value of 3kV/mm. A 6mm metric or 0BA bolt has a pitch of 1mm. I have now been given a Gallagher digital meter for the same job and...
  10. duke37

    Low battery shut off (and warning)

    I have had a look at the data for the LM293 comparator. It does not act exactly as a standard op-amp. The output is an open collector transistor so can only pull the output low. The LED then should be connected between the output and V+ with a suitable resistor and the inputs connected the...
  11. duke37

    Building a pH meter

    You do not have a voltage folower, you have connected the probe to the - pin. To get a follower, connect the probe to the + pin and add a resistor from the output to the - pin and another from there to ground. A better op-amp will not help in this configuration which effectively shorts the probe...
  12. duke37

    12V circuit with 29 LED's

    What is the voltage necessary to turn on each type of LED? How many LEDs of the same type will be in each block? Draw a suggested block diagram with switches, I do not understand the three switches sentence, how about a logic diagram?
  13. duke37

    Blocking Diodes

    The diodes in the negative leads are not needed and in fact are the wrong way round and will stop the circuit working.
  14. duke37

    Low battery shut off (and warning)

    The current for the LED comes from the V+ supply through the amp. A pull-up resistor is not needed, the op-amp does that. I am more concerned about the transistor drive, you will not get sufficient current out of the amp to provide the base current and the voltage drop will be excesive...
  15. duke37

    Wiring center tapped transformer secondary for most power

    The power output is limite by the heating of the windings. The transformer will be designed so that total heat in the primary, secondary1 and secondary2 is minimised. If you use one secondary only, the heat will be doubled in that winding so the loading should be a little lighter. I do not...
  16. duke37

    Wiring center tapped transformer secondary for most power

    Option 2 is the way to go, the heat in he transformer is spread in both output windings and there is only one diode voltage drop. If you split the winings it may not be possible to place them in parallel unless they are closely matched. Two bridge rectifiers, each being supplied by a winding...
  17. duke37

    Complete Newbie - Please help with battery polarity & ribbon cable

    I thought that the striped wire went to pin 1 of the plugs. The polarity could be anything.
  18. duke37

    Yamaha YST-99CD fault

    Is there a battery to retain the memory, if so, is it dead?
  19. duke37

    Substitute transistors

    You may have two similar Zener diodes but you will not have two identical diodes, they will differ very slightly and one will take more current than the other, will heat up and take more current. It is difficult to advise without knowing your circuit but you might consider keeping one diode and...
  20. duke37

    Electromagnetic Microphone

    Hearing aids in UK have a T position to pick up signal from a telephone or audio loop. The signal is just the audio signal and the pick-up is just a coil, not a microphone. Most hearing aids have now gone digital and you may be able to find an old analogue hearing aid which would have both...
Top