# Preset Lower Voltage Circuit.

#### Khaleef

Joined Mar 22, 2017
78
Good day,

I want to get 3 preset voltages from a 12v battery to use for a fan control. I want to make 9v,10v and 11.8v sources from a 16v Battery. The fan draws about 3A on start and continues at about 800ma.

Can someone help with a circuit diagram I can follow for this?

Thanks.

#### Papabravo

Joined Feb 24, 2006
20,122
Good day,

I want to get 3 preset voltages from a 12v battery to use for a fan control. I want to make 9v,10v and 11.8v sources from a 16v Battery. The fan draws about 3A on start and continues at about 800ma.

Can someone help with a circuit diagram I can follow for this?

Thanks.
Can you define what kind of circuit diagrams you can follow? Would a switch mode power supply be within the realm of your capabilities?
If so this one will be complicated by the startup requirement. They are flexible, but can have difficulty with load transients.
One more thing. You refer to both a 12V battery and a 16V battery. I've not seen any 16V batteries, but that doesn't mean they don't exist.

#### Khaleef

Joined Mar 22, 2017
78
Can you define what kind of circuit diagrams you can follow? Would a switch mode power supply be within the realm of your capabilities?
If so this one will be complicated by the startup requirement. They are flexible, but can have difficulty with load transients.
One more thing. You refer to both a 12V battery and a 16V battery. I've not seen any 16V batteries, but that doesn't mean they don't exist.
As a self-learner of electronics, I can follow any well part-labelled diagram.

I want to power a 12v DC fan and want it to have 3 speeds, so I'm thinking Low,Medium and High and would be powered from 3s 18650 battery of 12.6v or 4s 16.8v.

So a simple circuit that could take the 12.6v or 16.8v and output 3 different voltages, I'll use a 3 pole fan switch to select the different voltages.

#### crutschow

Joined Mar 14, 2008
32,032
You could use low a dropout adjustable regulator such as the MIC29302WT.
It's similar to the LM317 but with a lower drop-out voltage (minimum input voltage to maintain the output regulated) of about 300mV @ 1A. and 600mV max. @ 3A.
That will allow you to get 11.8V output with a 12.6V battery.

Use the switch to select one of three external resistors with values selected to give you the desired output voltage.

Note that the device may need to be on a small heat-sink, depending upon the fan current at the lower voltages.

#### Papabravo

Joined Feb 24, 2006
20,122
The applicable circuit is called a buck regulator. The wiki article is here. Follow the references for more details. You can google "buck regulator" or "buck converter" to find OTS (Off the Shelf) units that you can buy.

If you want to try to design and build one you should know that in a long career I have never seen anybody get the design of one of these things right the first time. So you should expect a couple of board turns.

If you want to use a linear regulator, pay head to the power dissipation. Going from 16.8 VDC to 9 VDC @ 3A startup current will mean that the part will dissipate 23.4 watts. When the current drops to 800 mA, that is still 6.25 watts and I'm guessing you'll a pretty big heatsink: liquid cooling even.

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#### crutschow

Joined Mar 14, 2008
32,032
Here's the LTspice simulation of an example circuit using an adjustable linear regulator.
It shows output voltages of 9V (green trace), 10V(yellow trace), and 11.8V red trace) for the three positions of switch SW.

I only had a model for the LM317, so I had to increase the input to 14V for simulation purposes, but it should work down to 12V with an MIC29302WT.

Edit: Modified so that output voltage doesn't go to supply voltage when all SW contacts are momentarily open during switching.

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#### Khaleef

Joined Mar 22, 2017
78
Here's the LTspice simulation of an example circuit using an adjustable
Here's the LTspice simulation of an example circuit using an adjustable linear regulator.
It shows output voltages of 9V (red trace), 10V(yellow trace), and 11.8V green trace) for the three positions of switch SW.

I only had a model for the LM317, so I had to increase the input to 14V for simulation purposes, but it should work down to 12V with an MIC29302WT.

View attachment 227343
Thanks @crutschow for coming through everytime. I understand the circuit and can't wait to try it out.

Thanks to everyone for the help.