Need help with power supply circuit

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G

Graygem

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I'm kind of new at this.
Here, I have a 18v input and three output voltages. 12v, 9v, and 5v off of a voltage divider.
My problem is that without the DAC load on 5v, I get 5v. When I have the DAC hooked up, I only get 1.6v. I used the voltage divider because I didn't really want to use another voltage regulator.

Any help and opinions are appreciated.
Thanks
 

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A voltage divider only works properly if the load resistance is much higher than the equivalent resistance of the voltage divider.

You are getting a voltage drop due to the current draw of the DAC. How much current does it take?
 
To get 0.1A through the 4.6KΩ equivalent resistance of the voltage divider requires 460V, thus it is obvious why the voltage you see is so low.

You need to study ohm's law and the equivalent resistance of a resistive voltage divider.
 
You are right, there is a lot of things I need to study. Thank you for that.
So, three regulators it is.
Which is better?
Thanks,
 

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I have to disagree with that generalization. It depends on which rail you need to most power from. Assuming you aren't using heat sinks, with the first circuit your 5V rail will only be capable of about 150ma. Using the second circuit, you will have a collective output capability of about 300ma, meaning you could get 100ma from 12V and 200ma from 5V, or 300ma from 5V. If you only have one heat sink, put it on the 7812 and use circuit #2.
 
Exactly!
Depending on the current requirements and heatsinking capacity there could be circumstances where the second circuit is better.

JimB
 
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