What type of resistor?

Discussion in 'General Electronics Chat' started by Bod, Nov 19, 2016.

  1. Bod

    Thread Starter Member

    Sep 18, 2016
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    (I have started off my past 2/3 threads with this ._.) So I made thread about charging a battery while its being used. Thanks for the responses! Alot of you said to use a resistor to lower the voltage from 5V to 3V. Could I just use a 9V to 3V resistor because I have loads as I like to mess around with LEDs? :)
     
  2. Sinus23

    Active Member

    Sep 7, 2013
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    There is no such thing as a Volt resistor. However you can control/split the voltage and current with resistors.

    But the watts that the resistors can handle is another matter.
     
  3. Bod

    Thread Starter Member

    Sep 18, 2016
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    That is quite true, I'm pretty certain I meant current resistors. :D
     
  4. Bod

    Thread Starter Member

    Sep 18, 2016
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    I totally forgot about the wattage of my convertor. :eek:
     
  5. #12

    Expert

    Nov 30, 2010
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    There is no such thing as a current resistor, either. They are just resistors. The only reason I didn't tell you what size is that you have never told what size battery you have or what quality of 5 volt supply you have. As long as you keep these things to yourself, nobody can tell you the answers.
     
  6. Bod

    Thread Starter Member

    Sep 18, 2016
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    The battery is just a little bit bigger than an AA battery and is 3V. The supply convertor
    Is a decently good one, fully water proof. The convertor is about 18 Watts or something similar.
     
  7. ian field

    AAC Fanatic!

    Oct 27, 2012
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    Actually there is - the MOV or metal oxide varistor (sometime called VDRs).

    Some early TVs with series heater chains had a VDR at the bottom of the chain to drop 12V for the solid state tuners. Another at 68V. was used in early CTV degauss circuits. The PTC thermistor in series with the degauss coils still passed enough hot current to make the picture wobble. The solution was to dump the PTC current into a fixed resistor and couple the resulting output to the coils via the 68V VDR. When the PTC was hot; the voltage developed across the fixed resistor sank below 68V and the VDR cut off the current.
     
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  8. ian field

    AAC Fanatic!

    Oct 27, 2012
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    A PTC thermistor nearly comes close to a current source - if you increase the voltage; more current tries to flow, but the PTC gets hotter and the resistance increases.

    There were all manner of weird and wonderful "resistors" in the early days of radio - one was an iron wire in a hydrogen filled glass envelope, I think that may have been a current stabiliser for heater chains.
     
  9. #12

    Expert

    Nov 30, 2010
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    In that case, use a decently good resistor or something similar.
     
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  10. ian field

    AAC Fanatic!

    Oct 27, 2012
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    Something I read somewhere suggested that 5W is about the limit for a typical battery of AA size cells - you don't see too many 5W Cree flashlights using AA cells.

    3V suggests lithium; which hints at maybe more energy density - but you still won't get long cell life.
     
  11. hp1729

    Well-Known Member

    Nov 23, 2015
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    Not enough information. 9 V in, dropped to 3 V means the resistor will drop 6 V. Divided by the current will give you the necessary resistance.
    Adding an LED? say 2 V across the LED at 20 mA. Now we have 4 V across the resistor and have 20 mA. 4V divided by 20 mA gives us a 200 ohm resistor, 4 Volts at 20 mA gives us Watts for the resistor, 1/4 Watt is good.
     
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