# Choosing power supply requirements

Discussion in 'Power Electronics' started by Brennon, Jan 11, 2013.

1. ### Brennon

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Jan 11, 2013
Hi all,

I am very new to electronics and trying to understand some basics. I have searched the forums for a previous Q&A that may help but I don't appear to understand in full what I get back.

My question is, when designing a circuit, how is a power supply requirement figured out? I see some designs that require 12V others 5V but I dont understand why the difference.

Is this a case of adding up all the component requirements in some other value (amps etc). The main point of confusion comes from being able to run components in parallel and in series with the same power supply (etc - 2 or more 5V servos running of a single 5V supply)

Any help or link to educated me would be greatly appreciated.

Cheers

2. ### GreenGiant

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Feb 9, 2012
well it depends on what parts you are using, if you have parts that operate off of 12 volts you will need at least a 12V supply, you have to look at your highest voltage required device to determine the voltage of the supply, and then add the total power consumption of the circuit then go a little over that.

For example if you had a circuit that has 5V parts, and 12V parts with a total power consumption of 10W based on all of the components, you should probably get a 12V 1A power supply, then reconfigure your circuit to add a voltage divider/regulator to get the 12V down to 5V where needed

With your example of the servos you would want to run the two in parallel so that both see 5V, if you put them in series (assuming both are identical) they would each only see 2.5V off a 5V power supply.

Look on Google for "calculating voltage and current in series and parallel circuits" or something along those lines

3. ### Brennon

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Jan 11, 2013
Thank you Mr Giant.

So to clarify, the power requirement is based on the highest voltage component requirement Max down and then regulated to suit each component. The amp requirement based on the sum of all component draw values?

Thanks again.

Cheers

7,682
1,686
Jan 5, 2010
OMG!

Bob

5. ### Brennon

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Jan 11, 2013
Is there a problem Bob?

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,809
Jan 21, 2010
Bob is referring to a post by another member that was deleted by another moderator for the following reason "total rubbish one more post like this and you will be banned -- you have had warnings".

If you didn't see it, all the better. It contained negative information (you would know less after reading it than you did before).

But relax, it's a terribly complex thing. There are many different types of voltage ratings for a start. Some are for power supply values (i.e. an IC which requires 5V +/- 0.5V), or a maximum reverse voltage (say, 6V for a LED), or a breakdown voltage for a transistor (say 60V), or a Vgs(th) for a mosfet (say 2.5V).

All of these components could be used in a circuit powered from 12V.

It is up to the designer to know what all of these mean, and to make sure that they (the designer(s)) do the right thing (which is different for each and every one of the above examples).

Example 1 . IC with power supply rating of 5V +/- 0.5V

This might be a TTL chip. They require a 5V power supply that remains within 10% of the rated voltage.

If you have a circuit running from 12V, you would need to use a voltage regulator to reduce the 12V to 5V. In this case the voltage rating is something that you need to remain at, or close to, and you must do whatever is needed to do this.

Example 2. A LED with a maximum reverse voltage of 6V

This is an example that you can most often ignore. Frequently a circuit will not apply a reverse voltage to a LED. If you have a circuit which does apply a reverse voltage, then you need to take steps to ensure that it does not exceed this value (and preferably remains well below it).

A LED operating from 12V via a resistor is an example of a circuit where this parameter is not a concern (unless you connect the power up backwards!)

Example 3. A breakdown voltage for a transistor of 60V

This is an example of a maximum voltage that can be placed across the component. In a circuit operated from 12V, this is unlikely to ever be exceeded (but be careful, there are times when it can!).

Many component have voltage ratings like this, and you generally try to select a component so that its rated maximum voltage is comfortable (1.5 or more times perhaps) higher than the maximum applied voltage.

You can easily find circuits where the voltage rating of a component is less that the voltage applied to the circuit. This is because the voltage at most points of a circuit is less than the full voltage applied to it.

Example 4. Vgs(th) of a mosfet of 2.5V

In this case, the voltage rating is one where the mosfet does something. In this case, a voltage of less than 2.5V means the device is effectively turned off, while a voltage above 2.5V marks the point at which it starts to turn on.

Circuits using this part would need to ensure that the voltage is either below this figure, or significantly above it (if the mosfet is used as a switch).

So a circuit powered from 12V could use all of these components.

But you've seen circuits that are labelled for 5V or for 12V. What is the difference?

One possible difference is that the 5V circuit used something which required a 5V power supply and for which the circuit does not include a separate regulator.

Another reason is that resistors have been chosen to provide a certain current from a 12V supply (say a resistor associated with a LED). Operating it from 12V may be ideal, 10V to 14V might be OK, but 5V might make the LED so dim as not to be visible.

Can you operate a circuit from a different voltage? Well, that depends on the circuit. You need to look at each one. With experience you can make a judgement. With that experience, you might also see what you need to change, add, or (in some cases) remove to make it operate from another voltage.

That's the long answer. The short answer is "show us and we'll try to tell you".

7. ### Brennon

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Jan 11, 2013
Steve, may I thank you for your very detailed and super helpful response.

I have been a moderator for around 10 years with Microsoft forums, so I know the challenges and I certainly know how time consuming it is to help people. Your efforts for a complete novice in the field of electronics are greatly appreciated.

I think the trouble stems from books and kits that tend to show a concept and provide already designed circuits, which explain each component but rarely the rationale behind the choices and in particular the power supply.

Some of the components I am using are quite expensive, such as gyro's and multiple servo's, so I am wanting to dive deeper into understanding electronics so I select both the right components and don't destroy them at the same time (me included).

Many thanks to all.

Cheers