Using a resistor to reduce voltage

RF Lighting · 14 Jan 2011

That looks ideal.

Yep it's to power the backlight and also the LCD display.

I presume the supply will run straight through a bridge rectifier allowing either DC or AC to be used.

riveralt · 14 Jan 2011

In learning mode here - With the Maplin transformer the ouput is 100mAmps - isn't your requirement for 12 mAmps?

electronicsuk · 14 Jan 2011

riveralt said:
In learning mode here - With the Maplin transformer the ouput is 100mAmps - isn't your requirement for 12 mAmps?

Just because you have a 60, 80 or 100A supply to your house, it doesn't mean that the loads in your house consume 60, 80 or 100A. Same applies here. The important thing is that the voltage is correct, and the current rating of the transformer is equal to or greater than the current required by the load.

echoes · 14 Jan 2011

If you go for that Maplin one, it's centre tapped giving 9-0-9, so you only use half of the seconday winding if you supply the CT with a.c. (i.e. 0-9, or 9-0).

If you're planning on rectifying and smoothing, then fig. 3 here should give you about 11 to 11.5 V d.c. with a bit of ripple (allowing 0.7 V across each diode).

EDIT: wrong figure

and apologies if I'm stating the bleeding obvious.

ban-all-sheds · 14 Jan 2011

FGS, just buy a so****g regulated power supply, plug it in, and get on with your life.

Look at it this way - if someone asked you to build a power supply for them from components, and offered to pay you what the difference is between buying the components and buying the supply, would your reaction be:

1) Love to, I'll start right away

or

2) **** ***

?

Or Plan B - rummage through the box in the loft where you've kept all the power supplies, wallwarts and chargers for all the items which broke/wore out/got lost/got thrown away over the years - can't believe you won't find one which can do 8V - 12V AC or DC at a few 10's of mA....

echoes · 14 Jan 2011

Now where's the fun in that b-a-s?

RF Lighting · 14 Jan 2011

I don't have room for a plug in PSU and a socket to plug it into. That's the whole point of trying to find a smaller way of powering it, such as with a resistor or as I now know would be best, a subminature transformer as echoes has kindly directed me towards.

flameport · 14 Jan 2011

http://www.rapidonline.com/sku/Elec...ct-PCB-mounting-power-supplies/119271/84-2628

echoes · 14 Jan 2011

flameport said:
http://www.rapidonline.com/sku/Electrical-Power/Power-Supplies/AC-DC-Converters/3W-and-5W-Ultra-compact-PCB-mounting-power-supplies/119271/84-2628

Thanks for that link flameport - it's just what I need for something else and was struggling to cram the PS components into a little box.

RF Lighting · 14 Jan 2011

That looks beautiful

gman76 · 15 Jan 2011

ban-all-sheds said:
FGS, just buy a so****g regulated power supply, plug it in, and get on with your life.

Look at it this way - if someone asked you to build a power supply for them from components, and offered to pay you what the difference is between buying the components and buying the supply, would your reaction be:

1) Love to, I'll start right away

or

2) **** ***

?

Or Plan B - rummage through the box in the loft where you've kept all the power supplies, wallwarts and chargers for all the items which broke/wore out/got lost/got thrown away over the years - can't believe you won't find one which can do 8V - 12V AC or DC at a few 10's of mA....

Love "Plan B" that's exactly what I have in my loft!

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Using a resistor to reduce voltage

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