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  1. Hero999

    Capacitor charging using low voltage supply

    The zener diode only wastes power when the capacitor has charged to 50V. You may be able to make a precise 50V zener diode from the TL431. The problem is it's only rated to 36V but a common base amplifier can be added to boost it to 50V. The extra gain may cause oscillation though. I haven't...
  2. Hero999

    Capacitor charging using low voltage supply

    Sorry to hear about your mum. A transformer is a good idea but bear in mind the reading on the meter is the RMS voltage, not the peak, so if you rectify 50VAC you'll get 50*sqrt(2) = 70.7V minus the losses in the bridge rectifier. Use a 230V:24V transformer or a 120V:12V transformer.
  3. Hero999

    comparator for DC voltage

    It isn't a comparator. A common ground is needed because all voltages in a circuit are relative to one another. Measuring potential difference is like measuring distance: you need to start from somewhere.
  4. Hero999

    comparator for DC voltage

    No, I don't know what you're talking about. The circuit doesn't have a common 0V reference. Yes, the current through Q is zero because the two batteries are only connected together via one wire and there needs to be two wires to complete the circuit.
  5. Hero999

    comparator for DC voltage

    How on earth is that a comparator? It's a potential divider. A comparator compares two inputs and turns on when one is greater than the other. It has a digital output and two analogue inputs.
  6. Hero999

    the finer points of electronics - I

    It's difficult to analyse circuit without a 0V node because all voltages are relative. Ignoring that. The negative side of the capacitor gains electrons and the positive side loses electrons.
  7. Hero999

    comparator for DC voltage

    That's not a comparator circuit. This is a fairly simple comparator circuit.
  8. Hero999

    a circuit that cuts off current when voltage drops from a given level.

    A comparator. Look up low battery cut off circuit.
  9. Hero999

    Capacitor charging using low voltage supply

    Your circuit is working, at least in simulation. Increasing the values of the capacitors in the voltage multiplier circuit will speed things up. Try replacing the 10uF capacitors with 100uF capacitors. The maximum voltage on the capacitor will always be lower than expected, due to the diode...
  10. Hero999

    scr gate calculation

    It's a trick question. An SCR might not even switch on and stay on at 20mA and it will certainly drop less than 2V, at that low current. SCRs don't work like BJTs. The gate current doesn't need to be on continuously. You need a small gate current, for a short length of time to trigger the SCR...
  11. Hero999

    a circuit that closes when power is cutoff

    A normally closed relay. https://www.google.co.uk/search?q=normally+closed+relay&ie=utf-8&oe=utf-8&gws_rd=cr&ei=uHWlVKf_E4jjaoSwgaAN
  12. Hero999

    Capacitor charging using low voltage supply

    What do you think? A boost converter increases the voltage of a DC supply. The voltage multiplier circuit I linked to converts AC to DC, as well as boosting it, unless you're talking about the CMOS charge pump.
  13. Hero999

    Capacitor charging using low voltage supply

    A fairly good description of a voltage multiplier can be found on Wikipedia: http://en.wikipedia.org/wiki/Voltage_multiplier The charge rate of a capacitor can be calculated by rearranging the following formula: Q = CV Q = It
  14. Hero999

    Capacitor charging using low voltage supply

    That may be true but capacitors don't store much energy, compared to batteries and will only charge as quickly as the energy is supplied to them. If your 5V generator only gives 1mA once it's been boosted to 50V, then it will take 500s to charge up, assuming the current is constant.
  15. Hero999

    Capacitor charging using low voltage supply

    What do you intend to power with the capacitor? How about gearing it up to increase the speed?
  16. Hero999

    Running a 555 based timer with a SMPS power supply

    What voltage does the solenoid run off? Unless it's mains then it won't have mains rated insulation so isn't safe to use a capacitive power supply. How much current does it draw? Capacitive power supplies are only suitable for low currents. Above 20mA or so, a small transformer becomes more...
  17. Hero999

    Capacitor charging using low voltage supply

    More information is needed. Where are you getting the 5V 1.5kHz signal from? What's the wave from? Is it 5V peak, peak to peak or RMS? A capacitive voltage multiplier or a transformer with a rectifier on the output would work.
  18. Hero999

    Running a 555 based timer with a SMPS power supply

    What are you switching with the 555 timer? How is it triggered? Capacitive power supplies are only suitable for low currents and are not isolated from the mains so the whole circuit, including everything connected to it, needs to be treated as mains and heavily insulated. The correct solution...
  19. Hero999

    What does 12V - 4.5Ah means?

    Obviously a 4.5Ah battery won't hold as much charge as a 10Ah battery. The other issue could be maximum discharge current and charge current, if it has a built-in charger.
  20. Hero999

    AM - FM Radio

    There are plenty of AM/FM radio circuits about. What parts can you not get hold of? Here's a schematic for a high quality receiver which you should be able to find the components for: http://quazar31.home.comcast.net/~quazar31/index.html Another option is to buy a kit...
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