Using a resistor to reduce voltage

Discussion in 'Electrics UK' started by RF Lighting, 13 Jan 2011.

  1. RF Lighting

    Joined:
    31 Mar 2006
    Messages:
    18,441
    Thanks Received:
    1,118
    Location:
    Leeds
    Country:
    United Kingdom
    Evening.

    I've just bought my self a CT meter off ebay.

    The display is an LCD screen with backlight, so requires a power supply.

    It requires ~12mA at 12V

    The stated input range is between 8V and 12V AC or DC.

    Would a resistor be suitable to reduce the voltage for this, and if so, what size might I need, or am I being crazy and ought to use a small transformer?

    Thanks in advance
     
  2. westie101

    Joined:
    3 Jan 2011
    Messages:
    2,497
    Thanks Received:
    312
    Location:
    Cumberland
    Country:
    United Kingdom
    Resistor

    V/I=R
     
  3. DetlefSchmitz

    Joined:
    17 Aug 2010
    Messages:
    885
    Thanks Received:
    95
    Country:
    United Kingdom
    If you mean drop the voltage from 230V then yes, you're being crazy. You would need a 3W resistor, which is highly inefficient, but worse, if the device took less current than expected, it would experience an over-voltage. A proper power supply is only £3 or so.

    Maybe you meant something different though.

    EDIT: PS. What's a CT meter?
     
  4. wingcoax

    Joined:
    1 Mar 2009
    Messages:
    1,945
    Thanks Received:
    157
    Location:
    Blackpool
    Country:
    United Kingdom
    Current transformer?
     
  5. ban-all-sheds

    Joined:
    27 Aug 2003
    Messages:
    52,034
    Thanks Received:
    2,032
    Location:
    London
    Country:
    United Kingdom
  6. MrTinker

    Joined:
    14 Dec 2010
    Messages:
    220
    Thanks Received:
    26
    Location:
    Merseyside
    Country:
    United Kingdom
    Would CT = cathode tube :?: :?: :?:
    Am i getting this right you want to drop 230 down to 12 with a resistor?
    well mmm well mmm well, yes it'll work ish but why :?:
    Getting a trany for buttons would surely be better
     
  7. riveralt

    Joined:
    4 Jan 2004
    Messages:
    4,018
    Thanks Received:
    596
    Country:
    United Kingdom
  8. Beelzebub666

    Joined:
    3 Dec 2009
    Messages:
    62
    Thanks Received:
    9
    Location:
    Hertfordshire
    Country:
    United Kingdom
    The resistor size required would depend on the resistance of the appliance, and whatever rating it turns out to be I'm guessing you'd set fire to the carpet. Go have a look on ebay for a 12v power supply.
     
  9. ccam108

    Joined:
    2 Jan 2009
    Messages:
    289
    Thanks Received:
    12
    Location:
    Lanarkshire
    Country:
    United Kingdom
    Yes you can, assuming the load will be fixed.

    It's not obvious what you mean when you say the input is 8-12V AC or DC, as clearly 8V DC would be below the required input for the screen. Even 8V AC will only give you ~11.3V peak.

    Also, if the input is as variable as you say, and AC only, I would use a transformer to up the voltage to around 15 and then rectify and regulate. A linear regulator would be fine for such a low current.

    For an 8V DC supply you will need a step-up inverter to get 12V out.

    Colin C
     
  10. bernardgreen

    Joined:
    3 Nov 2006
    Messages:
    12,347
    Thanks Received:
    1,042
    Location:
    Bedfordshire
    Country:
    United Kingdom
    AC/AC Multi-Voltage 500mA Power Supply only £13.99

    A simple wound transformer with tapped secondary

    http://www.maplin.co.uk/ac-ac-multi-voltage-500ma-power-supply-35927

    Virtually any plug top power supply with the correct ouput voltage will do. I have even used transformers from fairy lights in emergency.

    But avoid switch mode supplies as the 12 mA load will be too low for them and they may become unstable and the voltage become un-regulated and go too high.
     
  11. echoes

    Joined:
    27 Mar 2007
    Messages:
    956
    Thanks Received:
    108
    Location:
    Surrey
    Country:
    United Kingdom
    Voltage divider circuits are only good when the voltage at the junction supplies a high-impedance input, which isn't the case here.

    If the load is a constant 12mA, then a 3W, 18K resistor in series would work, but it would get very toasty, and if it failed s/c then the mains would fry your display.

    A small, cheap transformer with a 9V to 12V secondary would be ideal (£4 maplin)
     
  12. RF Lighting

    Joined:
    31 Mar 2006
    Messages:
    18,441
    Thanks Received:
    1,118
    Location:
    Leeds
    Country:
    United Kingdom
    A CT meter is used to measure current. It is done by passing the wire you want to monitor through the centre of a current transformer (CT). This generates a voltage proportional to the current flow which is measured by a meter and converted to display the current flowing in the wire.

    I'm not sure that the load will be entirely fixed. It might vary as the reading on the display varies.

    I was partly trying to do this on the cheap, and partly wanting to limit the amount of space taken up by the components.

    If someone could provide a link to a tiny little transformer that would be brilliant.
     
  13. Beelzebub666

    Joined:
    3 Dec 2009
    Messages:
    62
    Thanks Received:
    9
    Location:
    Hertfordshire
    Country:
    United Kingdom
    • Thanks Thanks x 1
  14. electronicsuk

    Joined:
    7 May 2007
    Messages:
    3,316
    Thanks Received:
    275
    Location:
    Lincolnshire
    Country:
    United Kingdom
    Are you wanting just a transformer, or an AC-DC power supply? There are loads of small PCB mount transformers available, but you'll still need to add at least a rectifier and smoothing capacitor. Plenty of slightly larger (but still relatively small) chassis mount transformers are also available. Again, rectifier and smoothing required.

    EDIT: I have also used these before as part of a job at work. Pretty compact and no other components required. They are PCB mount, but that's easily resolved with a bit of strip board and some terminals.
     
  15. echoes

    Joined:
    27 Mar 2007
    Messages:
    956
    Thanks Received:
    108
    Location:
    Surrey
    Country:
    United Kingdom
    clicky

    There is a 9V or 12V version.

    EDIT: Is this just to provide power to a backlight (assuming since either AC or DC is OK)?
     

Share This Page