Search results

Search Forums New Posts
  1. duke37

    DIACs gone the way of dinosaur?

    Hi Twister, I have not investigated the characteristics of diacs but I have used them to switch a SCR about once a second. A capacitor was charged with a very low current and when the diac fired, a large current switched the scr on. If I used a zener in this circuit, I woud only get the input...
  2. duke37

    DIACs gone the way of dinosaur?

    A diac is not the same as two zener diodes back to back. When the diac turns on, the voltage across it will drop unlike the zener diode case, thus it can discharge a capacitor not just hold it at a certain voltage. See Wikipedia.
  3. duke37

    Car wiring project - backup battery

    Your description is long and detailed so I will not deal with it all. 1. A battery needs to be charged more than it is discharged. The voltage of 14V or so you measure is about right but these measurements must be made at the battery posts. 2. The currents used in a car are huge so any...
  4. duke37

    Help in understanding the math behind the schematic.

    The photocell conducts when it is illuminated, providing current to the transistor as you have found. The resistor and photocell need to be interchanged to get the opposite effect. Some change of resistor value may be necessary.
  5. duke37

    AC Power Adapter Use

    But, does the LED sign have its own mains to 18V supply?
  6. duke37

    seemingly impossible class project

    BobK The current will not be proportional to the voltage if the resistance changes, it does not violate Ohms law.
  7. duke37

    LM393 comparator does not work...

    I do not want to spend time looking for data sheets. Most comparators need a pull-up resistor at their output, they only pull down as you have found. The fet is connected wrong, interchange the ground and 9V (assuming it is +9V)
  8. duke37

    AC Power Adapter Use

    Tip - put a zero in front of the decimal point, then it will not be lost. The adapter must have the right voltage and if DC, the right polarity. Get these things right and it should be OK.
  9. duke37

    Heat Generation

    The perforated metal tube was stuffed into the pile of grain and the perforations allowed the damp air in the grain to diffuse into the tube so equilibrating with the hair moisture content. The variation of the length of the hair relative to the tube length pulls a pointer round. You could...
  10. duke37

    seemingly impossible class project

    Is a shocking dog collar legal in your country? You could look up details of electric fence energisers. These work by generating a very high voltage and feeding through a high resistance. Please tell me of a circuit that does not obey Ohms law.
  11. duke37

    problem with digital multimeter

    The current is less than expected since the meter has some resistance as you say but also because the battery has an internal resistance so its terminal voltage will drop when a current is drawn and will sink over time. You could try measuring the battery terminal voltage without and with the...
  12. duke37

    help

    The 0V is the reference to which everthing else is measured. Car chassis (ground) probably. The 15V zener diode is there to protect the base of the fet from spikes and will not normally conduct so a quarter watt diode should do. The diode across the relay only conducts for a short period...
  13. duke37

    Heat Generation

    An old style of grain moisture meter consisted of a perforated metal tube with a horsehair filament up the centre which pulled a pointer round, depending on the moisture content of the air. It took a few minutes to stabilise. You could perhaps use a similar system to move a potentiometer or...
  14. duke37

    help

    An IRF540A has an on resistance of under 0.1 ohm and can take 5 or even 10A. I show a circuit with some components to protect the FET from spikes.
  15. duke37

    help

    The relay needs 12V and 120mA to work properly. The signal source is 10V but you have given no indication of the current capability. You give no information on the current demand of the test light. If the source has a very low current capability, at 10V it can still be used to switch a power...
  16. duke37

    help

    You need to supply more information. What is the signal source? What is the relay resistance? Try the 12V relay on 10V, it may well work. Could you use a 6V relay with series resistance?
  17. duke37

    Transformer clicking.

    Capacitors will also click if they get an internal short, sometimes they explode! Look for any distortion of the capacitor's cases. The current in the reservoir capacitors will depend on the circuit loading.
  18. duke37

    DC volts regulator

    I have a Watson supply which is several years old, it uses mains power. I e-bayed 'power supply ham radio' and got several hits. They are all mains input. There is a choiice of linear stabilisation (clean but heavy) or switch mode (light but maybe dirty). Newer switch mode supplies can be...
  19. duke37

    Can I use 2 x transformers?

    You could leave both transformers in circuit and move the output from the ends to the centre of the transformers. Two pole change over switch required.
  20. duke37

    Need help with strobe circuit

    If you are starting from scratch, consider a 4093 oscillator (one resistor and one capacitor) driving a 4017 decade counter, using two outputs for your double flash. This may do what you want.
Top