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can someone check the math on this one?

billabott

I had a real cool electronics "aha moment" this afternoon.  A package came from Amazon.com with a 16V 4A laptop power supply.        When I ordered it I had the impression that the output was AC.        Boy, was I surprised.  I need it to power a 315 MHz wireless home       security system which I had previously opened the enclosure and       inspected because it did not power on.  Turns out the wall       transformer was DOA.  That xfmr was hard wired to the main board       and there is an  fuse on the board.  I noticed that the 16V AC was       going straight into the board and that four fat rectification       diodes were sitting there.  So, I said: okay, that is a bridge       rectifier for converting it to DC and I observed some nice big       capacitors too - obviously the power supply.
So ,today, when the laptop power supply came and I looked hard at       the label, it was telling me 16V 4 amps DC with the center       positive on the barrel connector.  I frowned and my brain started       percolating like one of those old coffee pots.  I tested the       output voltage with a DVM and confirmed it was functional before doing the       Frankenstein routine on it.  It was fine.  So I proceeded to wire       the 16V DC output into the 16 V AC input and ... drum roll please       ...  it works great!   I even used wire nuts and shrink tubing.
So my question to you is "In your expert opinion, is this setup       stable for long term operation?" I have a  feeling that a 14 volt       4 amp DC laptop       supply would work just as well.  Care to clue me in on the math?

  • If this solution works now should work also long term. However, schematics would be interesting to check if power supply is adequate because 16V AC after rectification and filtering can reach about 20V DC (depends on load). 16V DC is slighty lower but I think this security system has some regulator because most security systems needs 12V DC.

  • in reply to RWM

    Good Point.  I agree.  Especially since I know the backup battery is rated 12V 1.2 Ah.

     

    I am upgrading the battery to 12V 8Ah and am concerned about the potential damage to the bridge rectifiers if they have to dissapate 2.8 Watts for a longer period of time while charging the battery from a zero Ah state.  I am looking at the PTC Thermistors at Newark (Element 14 US sales rep).

     

    So, I am thinking: what can I put in series with the battery to limit the current inflow when charging and not limit its ability to perform the power backup function at the designed 12 volts and current demands of the system?   Clearly not simply a resistor or a diode.  Based on my internet  research I most likely will need a PTC Thermistor ( a "Positor" ).  PTC  is Positive Temp. Coefficient as opposed to NTC or Negative Temp. Coeff.

    Here is my candidate device:  http://www.newark.com/murata/ptgl13ar0r8h2b71b0/posistor-ptc-circuit-protection/dp/74M3909?Ntt=posistor 

    These positors range in value from 0.8 to 10 ohms.  So the only unknown  is whether or not the voltage drop across the positor is too much at the  estimated less than 0.5 amp draw (actually closer to 0.2 amp) when the system is operating from the  backup battery?  It may need a diode to bypass the positor so the  voltage drop is 0.7 volts or less.

    But then again the charging circuit itself must already have current limiting features to insure that the functioning of the alarm system continues while the battery charges AND also to not boil the liquid in the battery and cause it to "explode".  Additionally the sealed battery itself should exhibit a limit on the amount of current it can process into stored electrons.

     

    Comment?  Please share your thoughts.

  • in reply to billabott

    My percolater has been working again.  Maybe all I need to do is MEASURE the charging current when I first hook up the battery and then make a judgement about if anything needs to be done about limiting the charging current.

  • in reply to billabott

    Maximum charging current for this battery (8Ah) is 2.4A. In security systems battery acts as buffer (constant voltage charging) and power supply should have current limiter. If there was previously used 1.2Ah battery, circuit should limit current to about 0.4A. I think all should work until next battery change after 3-5 years image

  • in reply to RWM

    The 8 Ah battery is now connected and the charging current was measured to actually be 55mA @ 13.3V.  I think I shall be quite happy with this setup. image

  • in reply to billabott

    I think this will work long time.