resistor value

Discussion in 'General Electronics Chat' started by fareedreg, Dec 2, 2008.

  1. fareedreg

    Thread Starter New Member

    Nov 25, 2008
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    I m a newbie in electronics... My modem power supply had problem I opened it and found it that transformer not working properly. It was 9vdc 1amp transformer... I go to electronic shop and purchase new one but instead of 9vdc 1amp shop guy give me 12vdc 1amp.. and after fixing it with bridge I got 12v dc ... but i required 9vdc... My question is CAN I DROP OUTPUT VOLTAGE WITH RESISTOR IF YES HOW MUCH OHM RESISTER I HAVE TO FIX TO GET 9V DC OUTPUT.

    Thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Your "quickest fix" would be to use a 7809 voltage regulator. It will require capacitors on it's input and output, and you must use a heat sink.

    A simple resistor divider network will not do the job for you.

    If you want a replacement "wall wart" 9V 1A unregulated DC supply, MPJA.com has one for under $5:
    http://www.mpja.com/prodinfo.asp?number=17564+PD
    They ship quickly, I've bought from them a number of times and have yet to be disappointed.
     
  3. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Welcome to a great discipline.

    As a matter of interest how did you do this? How do you know the transformer is not working?

    Your shop supplier may have been correct. Most 9 volt mains adapters supply 12 - 14 volts when measured off load.

    Drawing the rated load current reduces this to the rated voltage (9 volts in this case).

    This is why manufacturers state you should only use the correct supply, although I would agree with anyone who said they tend to overcharge for replacements.
     
  4. fareedreg

    Thread Starter New Member

    Nov 25, 2008
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    I check transformer output with multimeter and found nothing in output end. Is there is anyother way to check transformer functionality.
     
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Did you check the DC output of the unit, or did you check the AC output from the internal transformer?

    Your mains adapter could function in either of 2 ways.

    The most likely for a modem is that the internal transformer is a step down transformer, converting your mains voltage to between 6 and 9 volts AC. This is then rectified and smoothed to provide the DC output required by the modem. These units often have an internal thermal fuse or cutout. Lack of output could be due to this failing.

    I am only posting this for interest as these units are not worth the labour cost of repair.

    Did you understand my point about your new one?
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    What kind of modem? A LOT of dialup modems had AC power supply units, which were rated at whatever voltage the modem needed. I don't know why, but the rectification and regulation was done in the box.

    Being a sysop of a local BBS I still have a box of them, modems and power supplies alike. There really isn't any standardization, I suspect the market is almost dead nowdays.
     
  7. leftyretro

    Active Member

    Nov 25, 2008
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    Yes, I have a large assortment of 'Hayes' type modems (300 to 56k baud) that I found in thrift stores for pennies. I bought them mostly for their fine metal cases and external AC transformers. Most of the AC power modules indeed provided a 3 wire center-tapped low voltage AC power to the case.Inside on the single PCB they would rectify/filter and regulate and end up with +5vdc and + and - 8vdc or more depending on model.

    Lefty
     
  8. fareedreg

    Thread Starter New Member

    Nov 25, 2008
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    Friends this is not the answers of my question.. I only want to know how can i drop voltage...
     
  9. leftyretro

    Active Member

    Nov 25, 2008
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    SgtWookie in post #2 already gave you the two best solutions. :rolleyes:

    Here is a link to a data sheet for a 7809 9 volt 1 amp voltage regulator.

    http://www.fairchildsemi.com/ds/LM/LM7809.pdf

    figure 10 on page 23 shows you how to wire it up.
     
  10. studiot

    AAC Fanatic!

    Nov 9, 2007
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    I realise you may be using a slow dialup connection but we need more information.

    We asked about both the power supply and the modem.

    Is the psu AC or DC?

    Either way the power to be dropped will be in the range 3volts x 0.5 to 1 amp = 1.5 to 3 watts.

    We have already said that the supplied replacement may actually be correct with its output falling to 9volts on load.
    Have you tested it?

    If your psu is AC you need a resistor calculated from Ohm's law R = 3/currentdraw. I would suggest a 5 watt resistor. The resistor will get quite warm in use, so you will need to make arrangements to cope with this.

    If your psu is DC, Wookies 7809 solution is likely to be unreliable because the supply at 12 volts is too close to the required 9. You would also require to heat sink the device.

    In any case the modem will probably happily accept a 10 volts supply.
    I would suggest wiring three or four rectifier diodes of better than 1.5 amp capacity (eg 1N5401) in series with the supply. this will drop enough voltage to make the supply usable on your modem. They will also fit safely into the power supply case, and not require heatsinking.

    You could also use the resistor as per the AC solution above.

    I am still not convinced you have correctly measured the transformer output. Are you sure you had the multimeter on the correct range?
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    The other point that I was making is, if it was a true AC unit, the regulation and rectification is already done in the box.

    We are working in a vacuum, a lack of information. Without it, we take our best guess as to what you're asking. Communication is two way for it to be effective.

    AC units aren't like DC units (and I've tried DC in AC powered modems, didn't work).
     
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