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  1. duke37

    Capacitor Question

    But you cannot use all the 12V so the capacitor will need to be bigger. The energy required is 4mA * 10sec * 12V = 0.48J The energy in a capacitor = C*V*V/2, there will be 0.23J in the 3.3mF capacitor. It all depends on how tolerant your circuit is to a falling voltage, I would try 10mF or more.
  2. duke37

    Theremin Questions

    Q1 is a variable frequency oscillator, Q2 is a fixed oscillator. these are mixed in Q3 to produce an audio signal. Q6 is used as a variable resistance to adjust the volume and Q7 acts as an amplifier with a gain of one, giving a low output impedance. Q4 is another variable frequency oscillator...
  3. duke37

    Is it possible to detect mosfet failure?

    Redundancy is good but if you have two sensors that do not agree, what do you do? In some cases, three inputs are used and a vote is taken on which of the two of the three match best. Then, how reliable is the voting system? In your case, perhaps the fets failed due to over current caused by...
  4. duke37

    Is it possible to detect mosfet failure?

    There is a whole science dealing with reliability and risk. Do you have two or four engines on an aircraft? There has been a shift from eight to four to three to two. Adding a change over circuit will add unreliabiliy and, if the first circuit failed, would the second circuit survive. It...
  5. duke37

    Sequence 10 leds on power up

    Hooray! That's great.
  6. duke37

    tachometer.

    That is it exactly.
  7. duke37

    perrya

    I need a reviver and also lightening. I have mended several electric fence energisers which are subject to lightning strikes. Sometimes some components are blown apart but I have never seen any long term reliability problems.
  8. duke37

    tachometer.

    LM3914 The output from P7 has two functions, a stabilised voltage and a current sense to set the led current. The led current is about 10 times the current supplied by P7. So with 1.25V, a 1k resistor to ground will give a led current of about 12mA. P8 should be connected to ground. The 3914...
  9. duke37

    Sequence 10 leds on power up

    According to the National data, the led current is about ten times the current through R2, so the higher the resistance the lower the led current. What are you getting? Any led on at all? Have you got the leds in the right way round? Try connecting a 10k resistor from ground to one of the leds...
  10. duke37

    tachometer.

    The 8 pin version is the one which I had information on. The circuit of the 8 pin version needs to be modified. The op-amp is inside the LM2917. You do not need an extra one. The + pin of the op-amp is internally conected to the F/V stage and to pin 3. See the National data sheet. To modify...
  11. duke37

    tachometer.

    Your link shows a 14 pin chip. the National data sheet gives details on an 8 pin chip. The pins do nor correspond 100%. The 8 pin chip has the following connections. 1. Input 2. Capacitor 3. F/V output 4. Output 5 +Vcc 6. Zener (7,56V) 7. Op-amp- 8. Gnd The three diagrams I give are to...
  12. duke37

    tachometer.

    My disturbed brain cell would not let me sleep last night so here we go. I coudn't get your link working properly but got details of the LM2917 from the National website. Set the F/V convertor to give 4000rpm/V 7000rpm gives 1.75V subtract 1.75V Set op-amp gain to 8 i.e. 500rpm/V then...
  13. duke37

    tachometer.

    I was thinking of an extra op-amp to give the required voltage but I have had a look at the National data sheet and see that there is an internal op-amp which can be got at. The gain (rpm/volt) can be set with one resistor and a offset can be connected to the - op-amp input. With the lower...
  14. duke37

    Coil amplifier

    I do not know what a coil amplifier is but your circuit seems to have some strange details. 1. There is a 1k resistor across the power supply, this just takes power to no pupose. 2. The first op-amp provides a voltage reference for the other two amps. This feeds a capacitor and a 1k to ground...
  15. duke37

    tachometer.

    The LM2917 circuit you have shown gives 1V/1000rpm, the list of outputs you require is not linear at the bottom end so there will be a problem. At the top end, you want 1V/250rpm so an amplifier with a gain of 1000/250 = 4. Giving an offset to get 0V at 7000rpm, you get 0V 7000rpm 1V 7250rpm...
  16. duke37

    Optocoupler as a relay.

    The phone will have a battery much higher than 2V. You should be able to drive a transistor through a resistor from the 2V source to drive a relay at the battery voltage. Do not forget a freewheel diode across the relay coil.
  17. duke37

    not so cordless drill

    The battery is DC so your supply must be DC and connected the right way round. The 18V battery may not give 18V under load, so a lower voltage may be adequate. I suggest using a 12V simple battery charger of adequate current rating feeding the fattest capacitor you can find. You will get more...
  18. duke37

    Optocoupler as a relay.

    Tell us more about the application. Do you want to switch a mosquito treadmill or the local power station? AC or DC, volts and amps? Optocouplers pass the signal without a direct wired connection and you normally get out less current than you put in. Optocouplers use a led to generate light and...
  19. duke37

    LED dimming and ac fan wierd problem

    You can get transient suppressors which look like a ceramic disc capacitor which are placed directly across the mains. These have a high resistance until a certain voltage is reached when they conduct. You need to chose one suitable for your mains voltage. Computers have mains inlet filters...
  20. duke37

    VA rating?

    The 6VA will be the total input, so 1A output split into two secondarys i.e.500mA each. Nos da.
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