pcb

Discussion in 'The Projects Forum' started by vultac, Mar 6, 2009.

  1. vultac

    Thread Starter Active Member

    Mar 2, 2009
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    Hi there, I was given a PCB board with voltage regulators etc attached to it and basically what the board does is read a 64pixel image. The problem now is that i am supposed to get a power cable for the power plug but im not sure what power ratings should i use.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Do you have any information at all as to the manufacturer and model of the board?

    Failing that, does the board have a transformer on it?

    Failing that, do the voltage regulators have part numbers on them that you can read?
     
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Is there any information on the white silkscreen text on the board, or as letters etched in the copper side with any information related to input?

    Also, SgtWookie's question on part numbers, presence of diodes/bridge rectifier, transformer, etc.
     
  4. vultac

    Thread Starter Active Member

    Mar 2, 2009
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    Hi guys, thanks for the reply, I was just given the board which has been done by someone else and has no access to schematic whatsoever. From the PCB board there are 5 voltage regulators on the board all using G3/V5L9VA78M33CTI and im not sure about the two other components it could be transformer and has C5 79 printed on it...
     
  5. vultac

    Thread Starter Active Member

    Mar 2, 2009
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    I assume its 5V but what power ratings should i use? How could i find out the current value? needed?
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    5V is too low.

    UA78M33C is a 3.3V 500mA positive regulator. 5 would give a total of 2.5 Amps at 3.3V regulated so far. Minimum input voltage for 3.3V out is 5.3V In.

    http://www.datasheetcatalog.com/datasheets_pdf/U/A/7/8/UA78M33C.shtml The other letters/numbers you read were date code, lot number, and TI for Texas Instruments.

    Is the transformer large enough to be used at 60Hz? Usually relatively heavy due to iron laminated core, and at least a 2" cube for very low power.

    Should be some mid sized electrolytic capacitors if the power supply is over 2A at 3.3V, what is the voltage rating on the capacitors before and after the regulators? There might also be a +5V Regulator as well as other voltages. If what looks like a transformer isn't for line power, It could be part of an onboard switching power supply, more likely if it is toroidal/donut shaped.

    Other hints to look for are: Size of traces, power tend to be wider than data. Voltage and Size of electrolytic or tantalum capacitors. Follow the traces going into the leftmost pin (with the tab up, both surface mount and TO-220 style), that is the input pin, and may show other paths the input voltage takes to regulators.
     
  7. vultac

    Thread Starter Active Member

    Mar 2, 2009
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    Hi mate, I have attached the pic of the board...could u help me see whether thats a transformer or not! thx!!
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    Nope- the black thingie to the left of the IDC cable you're holding is the power connector for a wall transformer. There are on diodes, so the input has to be DC. The regulators are 3.3 volt and 5 volt (that's the MO33 or MO5 designator). The jumpers (the small rectangular plastic sleeves slid down over a pair of pins standing up off the PCB with JPxx labels) suggest that the voltage selection may be either/or.

    There are several power connectors that are different only in the diameter of the center pin. Take the board to some place that sells wall warts and see if you can find one to fit. Typical diameters are 2.1 mm and 2.5 mm. Hard to tell by eye, and my identification is a guess based on visual similarity.

    The fun part is deciding voltage and current. You will need at least 2 volts over the regulated output, and the wall wart has to have a current rating high enough to stay in regulation. But how much is enough? Go for at least one amp to play safe.

    Do you have any more information about this apparatus? A name and model number would be immensely helpful in running down pertinent information. It is not obvious if only one of the two regulated voltages has to be applied, or both. Guess wrong and smoke appears.
     
  9. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    The Tantalum capacitors are 10V 10uF, those are at the low voltage side/after regulators.

    All regulators on board are 5V or 3.3V, with a minimum required input of 7.5VDC, next common value up is 9VDC. The center post in the jack is positive, the outside shield is negative/connected to ground plane.

    The regulators have their tabs connected to the large ground copper plane on top for heat dissipation, the extra grid of holes near each one is for air circulation from below.

    Current abilities: Wall wart power supplies aren't often used above 10 Watts.

    Suggested: 9VDC/1 Amp supply, Center positive 2.5mm or 2.1mm plug (whichever fits).

    What does this board supply? Is there a processor on the other side of it?
     
  10. vultac

    Thread Starter Active Member

    Mar 2, 2009
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    Hye guys, thanks for replying again...

    to:beenthere, there are no more additional information given to me...im just given this board and have to figure out wad power supply to get for myself...y do we usually choose 2v more than refgulators? any particular reason?

    to:thatoneguy, yah its an image sensor, its a 64pixel ads image sensor...
     
  11. beenthere

    Retired Moderator

    Apr 20, 2004
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    Linear regulators like the 78XX series, can't output as much voltage as they are fed with. With a 2 volt margin, though, they are happy regulating voltage to your load. More voltage will work, but the excess power shows up as heat in the regulator.
     
  12. vultac

    Thread Starter Active Member

    Mar 2, 2009
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    Hi thanks for the help. But im still concerned about the value of current. Should i use a lower value current for the supply? say 400mA?
     
  13. beenthere

    Retired Moderator

    Apr 20, 2004
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    A load will use only the amount of current it requires. It is only important to supply the correct voltage. The regulation keeps the voltage constant if the load current varies. We have no idea what the overall current might be, but it seems to be spread among 4 regulators. So the supply of something on the order of 7 volts and capable of 1 amp is a reasonable guess.
     
  14. vultac

    Thread Starter Active Member

    Mar 2, 2009
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    Hi, I looked at the datasheet and it says that the recommended max output current is 500mA so shouldnt i choose a lower value for the current>?

    got the datasheet from earlier post
    "http://www.datasheetcatalog.com/datasheets_pdf/U/A/7/8/UA78M33C.shtml"
     
  15. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    An adapter with a 1A rating is capable of 1A output, but it doesn't force 1A into the circuit. You can always use a higher rated capacity adapter when regulation is present, and the voltage is correct.

    A circuit that needs 750mA, and only has a 500mA supply will quit due to the voltage dropping below what the regulators can work with.
     
  16. vultac

    Thread Starter Active Member

    Mar 2, 2009
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    ok thx buddy!
     
  17. vultac

    Thread Starter Active Member

    Mar 2, 2009
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    hi,

    thatoneguy: would like to know how do you know whether my plug for the power supply was has a center positive thx!

    Everyone : Another question is that for the grey colour cables(uploaded few posts ago) they are 34 of them how would i know which 1 of them connects to ground? or do they go by a pair? thx!
     
  18. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Center positive was deduced by looking at the picture, the pin out of the back of the jack (center) goes to the inputs of all the regulators, and the side tabs of the jack are connected to the large copper areas, which are the ground plane.

    You may want to verify with a resistance meter before applying power.

    --ETA: The grey color cables, are you referring to the ribbon cable?
    The red stripe usually denotes pin one, and the connector should have an arrow on it denoting pin 1. Sometimes, if you look at the other side of the board, pin 1 will be a square copper pad, while the rest will be round.
     
  19. vultac

    Thread Starter Active Member

    Mar 2, 2009
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    thx for the info on power plug, yah im refering to the ribbon cable...well the pin header got 16 pairs (32 pins) the ribbon cable im currently using has got 40 wires....what im confused is the pairings....how do i determine them from the ribbon cable and do each pair has got 1 wire which goes to gnd(ground)?
     
  20. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    I have no idea. It could be differential signal, using two wires for each signal.

    The information silkscreened on the board doesn't cover all the wires present, so it's impossible to say what should go where, as there isn't really a standard for such things.
     
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