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Very basic question about reducing voltage

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kevinwilson

New Member
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
 

ljcox

Well-Known Member
Kevin, there are 4 ways to do it:-

1. a resistor divider as you said. You do need to know what current the LCD digital thermometer draws in order to calculate the resistor values.

2. a resistor in series with 3 diodes. The LCD digital thermometer would be connected across the diodes. It would still be helpful to know the current.

3. a LM217 voltage regulator and 2 resistors configured to provide 1.5 Volt.

4. A DC - DC converter. This would be the most energy efficient, but also the most complex.
 

kevinwilson

New Member
Thanks for your reply Len. I don't have access to a multimeter, so I presume there's no way for me to find out what current the thermometer draws?

With that in mind, perhaps options 2 or 3 would be the best way to go. If you've got time, do you think you could explain them in idiot-proof terms for me? :)

Thanks again
 

Russlk

New Member
The thermometer draws microamps so if you design the voltage divider with milliamps of current, the microamps can be ignored. I have a digital thermometer in my van, it has run for several years on one AA cell. I think the cell will die of old age before it runs down!
 

ljcox

Well-Known Member
kevinwilson said:
Thanks for your reply Len. I don't have access to a multimeter, so I presume there's no way for me to find out what current the thermometer draws?

With that in mind, perhaps options 2 or 3 would be the best way to go. If you've got time, do you think you could explain them in idiot-proof terms for me? :)

Thanks again
If the LCD thermometer does require only microamps (I have not measured one but I expect that Russlk is right), then for option 2 you could use a 10k resistor and diodes such as 1N4148, 1N4004, etc.

For option 3, connect the +12 Volt to the input of the LM317, a 270 Ohm resistor between the output and adjust pins of the LM317 and a 680 Ohm resistor to gnd. This will give you about 1.5 Volt between the output and gnd but it will draw about 4.4 mA from the 12 Volt supply.
 

Hero999

Banned
How well regulated is the 12V supply?

If it's not reguluated at all it might be as high as 18V when the fan isn't on so if you'vbe set up your divider to divide by 8 then it would give 2.25V whict might damage your thermonitor.

Three diodes might also product a higher voltage than it can handle, I'd go for two, then if it doesn't work add another one.
 

kevinwilson

New Member
Thanks for all your replies so far.

Hero999 said:
How well regulated is the 12V supply?

If it's not reguluated at all it might be as high as 18V when the fan isn't on so if you'vbe set up your divider to divide by 8 then it would give 2.25V whict might damage your thermonitor.

Three diodes might also product a higher voltage than it can handle, I'd go for two, then if it doesn't work add another one.

I'll be using a regulated plug-in transformer, so I suppose it'll be fairly well regulated?
 
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