# How to obtain 12V DC from 48V DC

#### Sougan

Joined Jan 23, 2023
1
Hello everyone,
I am a student, and as a project we have to build an electric race car.
To power it, we are using a 48V battery.
We need to add a horn and the horn we have needs 12V DC and 1.5A so we need to convert the 48V delivered by our battery into the 12V needed for our horn. We are not allowed a secondary power source as everything must be powered by that battery.
We have been looking at different voltage regulators to either step the 48V down to 24V then the 24V to 12V or the 48V directly to 12V.
If you have any clues as to how we could achieve our goal, that would be greatly appreciated.

#### Boggart

Joined Jan 31, 2022
67
There are quite a few off-the-shelf DC-DC converters that will do the job, check ebay or aliexpress. Alternatively, if you have to make the circuit yourself, there should be plenty of circuits on this site and similar sites. Voltage conversion is a very common requirement...

#### Ya’akov

Joined Jan 27, 2019
6,856
Welcome to AAC.

First you’ll need to actually characterize your horn. Don’t depend on the written specifications, measure it. There may be unexpected results from the measurement. Also, don’t just assume you will not find yourself needing 12V for something unexpected, consider how the 12V might otherwise be used.

Once you have a definite requirement, look at the various ways you can take a high voltage and reduce it. Linear regulators are only one option. Look for others, like buck converters for example. Read about efficiency and heat.

Come back with questions as you find information.

#### Ian0

Joined Aug 7, 2020
6,713
Also worth considering is the length of time for which it needs to operate. Is it worthwhile making a complicated and efficient circuit for something that only ever operates briefly. A simple inefficient circuit is not going to waste much power considering how much the horn operates.
(Unless you are going to be driving it somewhere like Mumbai)

#### Papabravo

Joined Feb 24, 2006
19,610
Welcome to AAC.

First you’ll need to actually characterize your horn. Don’t depend on the written specifications, measure it. There may be unexpected results from the measurement. Also, don’t just assume you will not find yourself needing 12V for something unexpected, consider how the 12V might otherwise be used.

Once you have a definite requirement, look at the various ways you can take a high voltage and reduce it. Linear regulators are only one option. Look for others, like buck converters for example. Read about efficiency and heat.

Come back with questions as you find information.
There is more than one way to do this conversion. Regardless of the method you choose you should always start with the immutable rule of DC-DC conversion schemes:

The output power will always be less than the input power. Sometimes it will be much less.

This will help you establish your requirements. It works like this. Assume for the sake of argument that you have a process that is 80% efficient, and that you need 18 watts of output power at 12 volts. Now a quick, back of the envelope, calculation reveals that you need:

$$\cfrac{18\text{ watts}}{80\text{%}}\;=\;22.5\text{ watts, input power}$$

and that means you will need:

$$\cfrac{22.5\text{ watts}}{48\text{ V}}\;\approx\;469\text{ ma}$$

Now you can add up all of the required currents from the 48 V battery to see if it has enough capacity to satisfy all of the requirements. This estimate is for planning purposes only, because it is quite likely that the actual scheme you pick will be able to do better than 80% efficiency. Engineers just love happy surprises. Sort of like a Bob Ross painting with all those happy trees.

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#### MrChips

Joined Oct 2, 2009
27,695
The simple solution is a resistor and zener diode.
Since this would require a 25W zener diode, the next best solution would be to add a power pass transistor.

#### Ian0

Joined Aug 7, 2020
6,713
The simple solution is a resistor and zener diode.
Or, as the load is known, just a resistor, especially as the horn is probably electromechanical and won’t suffer too much if the voltage gets a bit high.
It might need a capacitor as the current is unlikely to be ripple–free

#### Ian0

Joined Aug 7, 2020
6,713
Four horns in series should be reasonably audible
If they are the type which oscillate by a contact attached to the diaphragm breaking the circuit, the effect might be interesting!
I have on that worked when the car was stationary, but as soon as the engine was started the increase in voltage would lock the diaphragm in one position and it would stop working.

Joined Jan 23, 2023
11
Do you have to use that 12V horn? Or could you use a cheap one rated at 48V? I don't recall horns getting used in racing. If you want another drivers attention, you just bump their car

#### Danko

Joined Nov 22, 2017
1,492

#### georgf

Joined Aug 12, 2021
6
Hello everyone,
I am a student, and as a project we have to build an electric race car.
To power it, we are using a 48V battery.
We need to add a horn and the horn we have needs 12V DC and 1.5A so we need to convert the 48V delivered by our battery into the 12V needed for our horn. We are not allowed a secondary power source as everything must be powered by that battery.
We have been looking at different voltage regulators to either step the 48V down to 24V then the 24V to 12V or the 48V directly to 12V.
If you have any clues as to how we could achieve our goal, that would be greatly appreciated.

#### georgf

Joined Aug 12, 2021
6
LM76003 2MHz down stepper can 60Vin and 3A, or older design LM2576HV can do that no problem, the circuit is very simple, pretty small, is high efficiency,
can you do PCB, s?

#### Ian0

Joined Aug 7, 2020
6,713
If the horn is like the one @Danko posted then it‘s a horrid mix of inductor and contact breaker points with no suppression. Not a nice place for a delicate switched-mode circuit to operate! It will take big peak currents at about 500Hz and make nasty voltage spikes when each pulse finishes.

#### georgf

Joined Aug 12, 2021
6
If the horn is like the one @Danko posted then it‘s a horrid mix of inductor and contact breaker points with no suppression. Not a nice place for a delicate switched-mode circuit to operate! It will take big peak currents at about 500Hz and make nasty voltage spikes when each pulse finishes.

#### georgf

Joined Aug 12, 2021
6
Yeah, you're right, I was thinking about that too, there where no real specifics on the horn, if it is this nasty type, it will probably screw a couple other things too, may be that's what the test is about, a electronic horn the better way to go for, tricky thing to solve, even the remote control could loose it using the horn