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  1. duke37

    To SCR or not SCR?

    This is the type of circuit that is used in electric fence energisers, they work well. In order for the SCR to turn off, the current in the SCR must drop to zero and there must be no trigger voltage. If you are driving an inductance, it can be arranged that the inductance can ring with a small...
  2. duke37

    Help 15vdc too much

    Radio Shack have a bridge rectifier, 25A 50V #276-1185 $3.49, which should do. Remember that this will dissipate quite a bit of heat so will need boltng to a heat sink if used at full current. While you are at the shop, get a capacitor to replace the dodgy one as others have said. Have...
  3. duke37

    Help 15vdc too much

    You will need a 10A diode in series with a wire from the power supply to the charger. The diode will dissipate 10W at full current so will need a heat sink. You may be better off getting a 10A or 25A bridge rectifier which can be bolted directly on a heat sink. You can use one diode to get a...
  4. duke37

    The best way to test cap voltage with adc?

    I was wrong! It should be 1M and 1.8k.?
  5. duke37

    The best way to test cap voltage with adc?

    Thats right. !M and 18k would give a division about the same as you have with 1/17 the drain.
  6. duke37

    Inverter idea - would it work?

    This seems to be a complicated way of going about it, in particular, you will have to make an alternator. The speed may not be constant with varying supply voltage. A 4047 CMOS chip can produce signals to drive two mosfets in antiphase. This is simple and cheap. Mosfets will be more efficient...
  7. duke37

    Simple speaker switch

    The amplifier should be connected to the two centre connectors and the speaker to the connectors at one end or the other. It would be easier to use a switch for left and another for right although one switch could be used with care in arranging the connections. Wires can be connected using...
  8. duke37

    Help 15vdc too much

    Welcome to the forum. I doubt if 0.15V is significant but a silicon diode in series will drop about 1V. You will need a diode that can take the current that the charger demands and may need a heat sink.
  9. duke37

    Simple speaker switch

    Welcome to the forum. You need a double pole, double throw switch (DPDT). Boxes are available from the switch vendors or you can make one from aluminium sheet or plywood. You do not say where you are or what shops you have in the area. Maplin or Radio Shack are two, I buy from Robert Potts...
  10. duke37

    Add two voltages without the use of a OPAMP

    The best option is probably to use a twin op-amp designed to operate from a single supply (LM358N). Use a non-inverting circuit with a gain of 2 so you are not involved with negative voltages. This will give very little load on the sources, if you are worried, you could even use a series...
  11. duke37

    Add two voltages without the use of a OPAMP

    If your signal source can source and sink current, you can connect a resistor to each output and use the voltage at a junction of the two. This will give an average of the two voltages. To get the sum, use an op-amp with a gain of +2. You could buffer the sources with a couple of op-amps with...
  12. duke37

    Help with AC/DC conversion/inversion

    The diddy power supplies are of no use to you, you will need thick wires and large terminals to handle your current. The last one you show, may do but I would not go for it as it gives no details. I looked on e-bay "12V power supply 30A". Look for one which has a stabilised output and a low...
  13. duke37

    Help with AC/DC conversion/inversion

    An invertor is used to convert DC to AC. You want to convert AC to DC. Radio amateurs use 12V transmitters which need a powerful 12V power supply (actually 13.8V). These are available up to 30A. Try e-bay? 100W from the radio should be enough to rattle the windows, try this first before...
  14. duke37

    Transistor as a switch

    I am not a great fan of relays but think that they will be the best option in this case. The circuitry will be simpler than an opto-isolator. The response of a small relay can be very rapid. Use a supply voltage at the top of the recommended range to get a rapid pull-in. Use a zener diode in...
  15. duke37

    Transistor as a switch

    A silicon transistor needs above 0.7V to turn on as previously said. The TIP122 is a Darlington pair with one transistor feeding another, thus needs at least 1.7V to turn on. The Darlington configuraton means that the saturation voltage is larger than a simple transistor but this is unlikely...
  16. duke37

    Why wont this circuit work?

    Even if you get the phase right, you will not get quite identical outputs so there will still be an argument. A 1 ohm resistance in each output may alleviate this problem. Why not use a more powerful amp instead. Antiphase connection is used to increase the output power without increasing...
  17. duke37

    I need help with my first flyback converter

    At the very least, the switch on pulse must be shorter than the power pulse otherwise the SCR is kept conducting. I doubt if it is anything to do with rate of rise of voltage. The transformer can reverse voltage if it rings, this will turn off the SCR when the current drops to a low enough...
  18. duke37

    Variable power supply

    R2 the lower resistor can be made up of a fixed resistor to limit the lower voltage in series with a variable resistor to control the output voltage. The potentiometer should have one end and slider connected to ground and the other end connected to R1 and the reference input. With R1 = 240...
  19. duke37

    Why wont this circuit work?

    The op-amps have their outputs connected together, they have the inputs connected to produce opposite phase outputs so they will be fighting each other. U1 has a negative feedback to set the operating condition, U2 does not and will likely be driven hard in one direction.
  20. duke37

    Variable power supply

    The LM317T needs a potential divider to set the output voltage as you have determined. There are two resistors in this divider, R1 and R2. The LM317T controls the passes current to get 1.25V between the output and the sense terminal. The easiest way of detrmining the resistors is to decide on...
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