Help with understanding schematic

Discussion in 'General Electronics Chat' started by maca_404, Nov 4, 2010.

  1. maca_404

    Thread Starter New Member

    Nov 1, 2010
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    Hi

    I am having trouble working out what is happening in the area of this schematic marked in red. I know pin 8 outputs +9V and seems to be rectified with all the diodes. What I dont get is why the "gnd" seems to come from the netural of the primary side of transformer t101 and the +9 comes from the netural of the secondary side of the transformer not the live. Does this mean the transformer steps down to +9 ?. Sorry for all the question but im having a hard time wrapping my head around this one.

    Any help is much appriciated.

    [​IMG]
     
  2. rherber1

    Member

    Jan 6, 2008
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    You haven't said what the item of equipment is. What is the make and model number and what is it?

    You can't take this part of the circuit in isolation to what is connected on the other end of CN208 and work out what is supposed to happen. The relevance of the R185 (or R105) connection to the AC Neutral line will depend on the resistor value. I would be willing to bet that this resistor is quite a high value - what is it btw?

    I think this resistor has something to do with referencing the AC CLK line.
     
  3. maca_404

    Thread Starter New Member

    Nov 1, 2010
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    thanks for your reply, This is the ac input board on a rear projection tv. Basically this board distributes the power out to the other parts of the tv.

    Based on the info I see would it be safe to say the transformer is stepping down to 9v ?. Also after reasearching rectifiers the diode config here dosent seem to be one, so im even more lost as to whats going on there.
     
  4. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    There are 4 diodes in a full-wave arrangement. The output of which is labeled STB9V. The transformer output is probably on the order of 6.5 to 7 VAC. I can't read the labels clearly, but it appears the top two pins on the connector are simply connected by a switch to turn the 9-volt supply on.

    Is STB9V a standby power supply voltage?

    John
     
    Last edited: Nov 4, 2010
  5. newbies_hobbyist

    Member

    Jun 4, 2010
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    the diode are full wave rectifier as what jpanhal said. Now as you see the other end of the recitifier D183 and D194 are floating and connected to MSW-. The whole rectification will start when MSW+ and MSW- signal is present (a switch will do). jpanhalt is also correct that STB+9V is a standby power supply. All in all circuit will work with the CN208 circuit are present.
     
  6. maca_404

    Thread Starter New Member

    Nov 1, 2010
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    thanks guys, placing the switch in there makes the rectifier function clearer. And you are correct that this is a standby circuit. Im curious though as to how you put the transformers output voltage at 6.5 to 7v when the stb voltage is supposed to be 9v ?
     
  7. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Rectified AC is - AC Volts times 1.4 = DC Volts
     
  8. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    To sum up what others have noted:

    The diodes are a funky drawn full-wave bridge, with C113 as the filter cap. When M.SW +/- are connected together, it completes the circuit by routing the AC voltage from the xfmr to the bridge. It also generates a half-wave reference clock frequency called AC.CLK based on the input line frequency, with the positive amplitude set by zener D111.

    If the STB.+9V output is a nominal 9VDC, then the xfmr voltage is approximately 9V x .707 = ~ 6.3VAC.

    R185 ties system ground to neutral on the line side, although this is usually done with a high voltage capacitor for safety reasons. It would be best not to plug this in backwards. In other words, know which lines are neutral and hot and connect them accordingly (hopefully it employs a polarized plug).
     
    Last edited: Nov 4, 2010
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    This is a minor point, but a diode bridge has a two-diode forward voltage drop. This means that the transformer output needs to be approximately .707*(9+1.4)≈7.4VAC RMS, where 1.4V is 2 diode drops. This can vary by several tenths of a volt, depending on load current and diode size.
     
  10. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    That area is the stby power and will produce close to 9V. M.SW is some kind of safety trigger switch which cut's of stby voltage when triggered. With no stby the uC will shutdown the Main supply voltages.

    But I am skeptic on the resistor value though. The image is not clear enuf.
    The resistor R185 helps in providing the return path of AC.CLK, which is a signal for AC cuttoff, sort of a mute activation when AC is cutoff with out the TV shutting down.
    Take out this resistor and the TV will shut down after powering up.



    The Plug should be polarized as the chassis might become live in case of Live neutral reversal.
     
  11. maca_404

    Thread Starter New Member

    Nov 1, 2010
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    Thanks for everyones replies this makes so much more sense now and everyone was spot on. Fyi the real world resistance of the resistor typing in to the mains netural is 2.2 ohm.
     
  12. maca_404

    Thread Starter New Member

    Nov 1, 2010
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    2.2 Mega ohm
     
  13. maca_404

    Thread Starter New Member

    Nov 1, 2010
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    Quick thanks to everyone who helped me work this one out. I was able to diagnose the fault and the tv is now working good as gold.

    Thanks Again
     
  14. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    I did not know u were trying to fix a TV
     
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