Power Supply for MK111

Discussion in 'The Projects Forum' started by Lightningbolt, Feb 4, 2013.

  1. Lightningbolt

    Thread Starter New Member

    Feb 4, 2013
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    I have a 16 VAC circuit which when closed operates an electro-mechanical solenoid causing its plunger shaft to hit a bell/chime. Since the switch on this circuit is an automatically controlled relay that may cause the circuit to stay closed from 1 - 5+ minutes, this causes the solenoid to stay energized for too long (bad for its power supply as well as for the solenoid). Furthermore, I only get 1 "ding" sound no matter how long the 16 VAC circuit is closed.

    I would like to get multiply "dings" from the solenoid at a low duty cycle (i.e., a "ding" every 60 seconds or so) and have it user adjustable. I have found that the MK111 Adjustable Interval 555 Timer module would probably satisfy my needs for controlling the timing of the circuit. See following url: http://www.vellemanusa.com/products/view/?id=350686

    My problem is that the MK111 requires a 12-15 VDC power supply for its circuitry. In order to provide this, I am thinking about tapping into the 16 VAC circuit (parallel connection) and adding a full wave bridge rectifier. The bridge rectifier specs state it has a 1.1 V drop across each leg. Therefore, I presume the resulting voltage would be 14.9 VDC, but it will not be very smooth. Would the uneven VDC harm the MK-111 or cause it to malfunction?

    How smooth/regulated must the power supply be to the MK-111?

    Thank you for your thoughts on this matter.
     
  2. #12

    Expert

    Nov 30, 2010
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    No. When you rectify 16 VAC RMS you get 22.6 peak volts. Then you subtract the 2.2 volts for the rectifier loss and capture 20.4 volts in a capacitor. Then regulate it down to what you want. An LM7815 regulator might be what you need if the current is in the right range. That chip has a nice, smooth output.

    It all hinges on how much current is needed. Can you tell what that is?
     
  3. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    The MK111 is just a 555 operating a relay. The recommended PS is 12 V at 100 mA, so an LM7815 or LM7812 would work well. It probably wouldn't even need a heat sink.
     
    #12 likes this.
  4. Lightningbolt

    Thread Starter New Member

    Feb 4, 2013
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    The MK111 manual states it requires a 12-15 VDC / 100 mA power supply
     
  5. Lightningbolt

    Thread Starter New Member

    Feb 4, 2013
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    So, when I use my multimeter to test the voltage on the 16 VAC circuit, the result is a RMS value, not a peak?

    Why subtract 2.2 V? When the bridge rectifier spec's state a voltage drop of 1.1 V / "leg", what does "leg" mean? Each of the 4 diodes making up the bridge configuration? Each pair of diodes engaged when the current flows in one direction?
     
  6. #12

    Expert

    Nov 30, 2010
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    yes, RMS. I think it's 1.1 volts, not 2.2 volts like I said earlier. Anyway, doesn't matter. The 7815 chip can stand up to 40 volts so the difference between 20.4 volts and 21.5 volts doesn't matter to the chip.

    You still need a filter capacitor. Can you do the math?
    Rock/paper/scissors for who does the schematic?
     
  7. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    It's past my bedtime. :)
     
  8. #12

    Expert

    Nov 30, 2010
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    10 characters
     
  9. Lightningbolt

    Thread Starter New Member

    Feb 4, 2013
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    Why would I still need a filter capacitor with the LM78xx regulator?

    If I chose to use a LM7812 regulator, would it make any difference if I used a filter capacitor of 100 uf - 1000 uf?

    I could do the math, but do not know the formulas. Can you provide?

    Can you also provide a schematic?

    Much thanks for your help!
     
  10. #12

    Expert

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    Must have been in a time overlap.
     
  11. Lightningbolt

    Thread Starter New Member

    Feb 4, 2013
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    Where did the 17.5 V min to chip value come from? Would this be the voltage of the LM7815 [15 VDC] + the voltage drop of the LM7815 [2.5 V]? [15+2.5=17.5 VDC] Is this correct?

    I assume the 21.52 [V] peak @ cap was derived by: (√2)*16 VAC [input RMS voltage] - 1.1 V [rectifier drop voltage] = 21.52 VDC. Is this correct?

    Vrip [is this the max voltage ripple? (Vpeak - Vmin) ] = Er in formula provided?

    F [is this the frequency of voltage flucuation? 2 [for the 2 half waves] * 60Hz=120 cycle/sec]

    So, if my interpretation of the above is correct, then the value of the capacitor is inversely proportional to the voltage ripple, with a minimum based on the LM78xx regulator I use. Is this correct?

    If I choose a Vrip = 7 V [max for a LM7812: a 28 % variance in V] then C=84uf [minimum for a LM7812]
    If I choose a Vrip = 4 V [max for a LM7815: a 19 % variance in V] then C=147uf [minimum for a LM7815]
    However,
    if I choose a Vrip = 0.6 V [a 2.8 % variance in V] then C=980uf

    So, for my particular project, if I use a LM17812 regulator, I should be able to use a filter capacitor of 100-1000 uf correct?

    What pros/cons would there be for using greater than minimum size filter capacitor (say 1000uf vs 200 uf)?

    Though the manufacturers of the LM178xx regulator indicate for the use of device as a "fixed output regulator" to add a 0.22-0.33 uf capacitor on the input side and a 0.1 uf capacitor on the output side (as shown on your schematic), what is the purpose of these capacitors? Are they really needed in my situation? What would be the effect if I did not include them?
     
  12. #12

    Expert

    Nov 30, 2010
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    You are about the sharpest beginner I've ever seen. You caught every bit of math correctly. Now for some more bits...

    Mfg. studies show that using a filter capacitor which is rated at "up to double" the voltage actually required will increase the survival time of the capacitor. The small capacitors listed on the datasheet are definitely necessary to keep the regulator from oscillating. The best reason to use a filter capacitor that is a step larger than the minimum is because you can't trust electrolytic caps to be full labeled value, especially after a few years.

    You can put 1000uf where 100 will do, but it costs unnecessary money to do that...unless you have a heating problem from huge currents...which you don't have today.

    and a final pointer: The grounds on the schematic are drawn funny to draw attention to the idea that the small caps need to be physically close to the regulator, like, within a few inches. You can typically do a lot better than that. In a stand alone circuit like this, you can have the small caps snuggle right against the regulator.

    Edit: You can use a 12V regulator, but why inflict the extra 1/4 watt of energy on the regulator when the Velleman kit can use it?
     
    Last edited: Feb 5, 2013
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