Very basic question about reducing voltage

Discussion in 'The Projects Forum' started by kevinwilson, Jan 14, 2007.

1. kevinwilson Thread Starter New Member

Jan 14, 2007
1
0
Hello,

I'm an absolute beginner, so I'm hoping somebody can help me with this basic question. It's also my first post!

I have built a simple circuit (from instructions I found at http://www.heatsink-guide.com/content.php?content=control.shtml) which controls a small cooling fan, using a thermistor to maintain a constant temperature.

I'd now like to take a cheap LCD digital thermometer which I bought a while ago, and build it in to the same enclosure.

The digital thermometer runs from a single 1.5V battery, so I need to know how to connect it up to the 12V DC supply which is powering the fan controller circuit. Apart from that, the thermometer and the controller circuit will be completely separate.

After a bit of searching, I think I might need to use two resistors in series (a voltage divider?). I don't really understand how this works - would I need to know the current that the thermometer draws? Is there a better way of reducing the voltage?

If anybody could help I'd be very grateful.

Thanks,
Kevin

2. Distort10n Active Member

Dec 25, 2006
429
1
For a simple DIY heatsink, a voltage divider would work. Use a 10k and ~1.5k. You will need to look at the supply current specification to ensure that the current through the voltage divider meets the Icc spec.
I doubt that using 10k and ~1.5k will limit the supply current to that extent, but that depends on what your 12V source is.
You could use an op-amp attenuator, but a better device would be an linear drop-out regulator.

3. thingmaker3 Retired Moderator

May 16, 2005
5,072
6
The "quick and dirty" way would be to use another of those multi-turn pots.

Hook one end to ground and the other to +12V. Important: Adjust pot to read 0vdc on the wiper arm.

Next, hook wiper arm to Battery + of your thermometer and hook Battery - of thermometer to ground.

Adjust pot until wiper arm measures 1.5v and daub with glue.

The "quck and clean" (but more costly) way would be to use something akin to this: http://www.recom-power.com/pdf/innoline/R_5xxxPA_DA.pdf