# 12v drop to 9v

Discussion in 'General Electronics Chat' started by Laserdog, Jun 30, 2008.

1. ### Laserdog Thread Starter New Member

May 3, 2008
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I have a purchased 9 volt rated timer circuit and would like to use it on my boat that has a 12 volt battery. Would rather use the 12v boat battery than purchase 9 volt batteries. Is there a divider circuit or some regulator circuit that could be easily built to provide the 9 volts from the 12 volt battery? If So could someone provide info.

2. ### silvrstring Active Member

Mar 27, 2008
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Does your timer say how many Watts it uses?

3. ### Laserdog Thread Starter New Member

May 3, 2008
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It is an Adjustable Cyclical Timer . The only electrical
Specifications:Supply voltage: 9V DC / 100mA

4. ### silvrstring Active Member

Mar 27, 2008
159
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That means your timer uses .9 W of power (P=IV), and the resistance of your timer is about 90 ohms because R=(V^2)/P.

If you place a 30 ohm resistor in series with the timer, your timer will get about the 9V that is recommended for it; the resistor will take the other 3V. Just make sure your resistance is rated about 2 times the power it will be dissipating. Your resistor would be using about .3 W of power, so make sure it is rated at or above about .75 W to be safe.

5. ### silvrstring Active Member

Mar 27, 2008
159
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You could also use a zener diode in parallel with the timer, but resistors are easier to find.

6. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
1,461
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You should use one of these:

http://www.fairchildsemi.com/pf/LM/LM7809.html

The problem with using a resistor is that the voltage is entirely load dependent and if the current drawn by your timer changers then so will the voltage. Also, your boat battery is only nominally 12V and will be several volts higher when the engine is running.

Last edited: Jun 30, 2008
7. ### silvrstring Active Member

Mar 27, 2008
159
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Yes, the regulator would be best--especially considering a running engine.

8. ### Laserdog Thread Starter New Member

May 3, 2008
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Thanks to all for alternative solutions to my problem. I think I will
go with the regulator