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Using a resistor to reduce voltage

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RF Lighting

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PostPosted: Thu Jan 13, 2011 10:08 pm Reply with quote

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
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westie101

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PostPosted: Thu Jan 13, 2011 10:15 pm Reply with quote

Resistor

V/I=R
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DetlefSchmitz

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PostPosted: Thu Jan 13, 2011 10:32 pm Reply with quote

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?
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wingcoax

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PostPosted: Thu Jan 13, 2011 11:26 pm Reply with quote

Current transformer?
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ban-all-sheds

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PostPosted: Thu Jan 13, 2011 11:59 pm Reply with quote

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MrTinker

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PostPosted: Fri Jan 14, 2011 12:34 am Reply with quote

Would CT = cathode tube icon_question.gif icon_question.gif icon_question.gif
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 icon_question.gif
Getting a trany for buttons would surely be better
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riveralt

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PostPosted: Fri Jan 14, 2011 1:32 am Reply with quote

Resistors in series is the way to go according to this site:

http://www.play-hookey.com/dc_theory/voltage_divider.html

Using 230v, 12v and 12mA got two resistors of 16k and 3.5k but had trouble calculating the voltage drop.

Good learning/ remembering experience though.
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Beelzebub666

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PostPosted: Fri Jan 14, 2011 2:32 am Reply with quote

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.
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ccam108

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PostPosted: Fri Jan 14, 2011 7:14 am Reply with quote

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
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bernardgreen

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PostPosted: Fri Jan 14, 2011 7:59 am Reply with quote

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.
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echoes

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PostPosted: Fri Jan 14, 2011 4:17 pm Reply with quote

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)
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RF Lighting

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PostPosted: Fri Jan 14, 2011 6:13 pm Reply with quote

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.
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Beelzebub666

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PostPosted: Fri Jan 14, 2011 6:17 pm Reply with quote

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The following user says thank you to Beelzebub666 for this useful post:
electronicsuk (14 Jan 2011)
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electronicsuk

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PostPosted: Fri Jan 14, 2011 6:18 pm Reply with quote

RF Lighting wrote:

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


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.


Last edited by electronicsuk on Fri Jan 14, 2011 6:21 pm, edited 1 time in total
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echoes

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PostPosted: Fri Jan 14, 2011 6:19 pm Reply with quote

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)?
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