Choosing power supply requirements

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).

Brennon, your reply to GreenGiant indicates you really don't fully understand.

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".