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Basic Electronics » Why 5 volts is most widely used?

March 31, 2011
by rajabalu21
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I would like to know why 5V is most widely used as the supply voltage for digital electronics. Is there an engineering reason behind this practice?

I guess we are seeing increasingly 3.3 volt circuits because of Li-Ion battery usage with 3.7 volt. But, I am not sure I know the reasons for 5 volt selection in the first place.

-Raja

April 01, 2011
by mongo
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Good question!

I think it has something to do with having a stable operating voltage from battery power. Since most battery combinations can be 6 to 12 volts unregulated, there needs to be a voltage that can be stable regardless of the input voltage as the batteries are discharged. Powered from a 9 volt source and regulated to 5 volts, a circuit can run stable for a long time, where if the circuit were designed to run at say 6 volts unregulated, fresh batteries will do fine. But as the batteries lose their charge and the voltage drops, weird things can happen from frequency shifts to sensitivity issues.

April 01, 2011
by rajabalu21
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Thanks Mongo.

That was my initial guess too. But, I was thinking may be it has something to do with semiconductors and their characteristics. I was thinking there should be something like the reason behind 4-20 mA operating range selection in control systems.

-Raja

April 01, 2011
by Ralphxyz
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That of course leads to the question of then why not 4 volts. It is a very interesting question.

My related question is always efficiency. The Nerdkit User Guide really gulls me when it ask about "what happens to the reduced voltage" when talking about the voltage regulator.

Frankly I do not know what happens to the reduced voltage I assume it is lost as heat which wraps back to my question of efficiency.

How efficient is a voltage regulator? When you drop from 9 volts to 5 volts where does that 4 volts go? Is it conserved (still in the battery)?

So thanks Raja for a interesting question, I really do not have a answer but once Mongo ventured a answer that prompted my question.

Ralph

April 03, 2011
by mongo
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That 4 volts goes into heat. So, the regulators are not really all that efficient. The best you can get is to stay as low as you can on the input voltage. Switching regulators on the other hand, are a lot more efficient but generally require a little higher input voltage because they also require an operating voltage. It's not as bad though, but they do take up more space and have a higher component count.

April 03, 2011
by Ralphxyz
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Ah, thanks again mongo, once again you have enlightened me to the foibles of electronica.

"The four volts is lost to heat" that is what I was afraid was happening. That is really irksome but thought provoking.

There must be a better way.

Specifically I am thinking about solar collectors, which have a premium on energy generation in the first place. But then If I used a voltage regulator on the accumulated energy (12 volt battery) I would be compounding the inefficiency. Would it be more "efficient" to use 6 volt storage rather than 12 volt? I know it would depend on the load, most solar appliances use 12 volt or higher.

So unless I stored 5 volts, I would be throwing the efficiency calculations out the door to use a 12 volt storage medium (battery) to energize a Nerdkit @ 5 volts.

All of which is veery interesting.

Thanks for the reply, stuff like this keeps me awake at night.

Ralph

April 04, 2011
by mongo
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12 volts is the lower end of the solar power stuff. (Unless you go custom, which can get expensive.) The best alternative, if there is anything of any real current requirements, is a switching regulator. The 7805's are fine for low currents under 1 amp and at the 7 volts lost to heat, well, it is only 7 watts out of 12... That's where the inefficiency comes in. To run a 5 watt device, still takes 12 watts input. A switching regulator can drop that to about 6 watts input and still give you 5 watts out.

April 04, 2011
by bretm
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A primary characteristic of digital systems is their immunity to noise. There is a range of voltage levels around Vcc that counts as "high". When you use a smaller voltage you narrow that range.

As far as i know, battery voltages were not a major consideration. They could have chosen 10V and devices would just use more batteries. Remember that a 9V battery is just six small 1.5V batteries in a single outer package. Note that this would not necessarily have doubled power consumption.

There is a movement to 3.3V since power consumption in many large scale integrated circuit architectures is proportional to the square of the voltage, but designers of those systems have to work harder to keep signal noise under control.

April 07, 2011
by Hexorg
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"How efficient is a voltage regulator? When you drop from 9 volts to 5 volts where does that 4 volts go? Is it conserved (still in the battery)?"

Yes, in a voltage regulator, the power is conserved (except for minor losses in heat). The power conservation in the voltage regulator is similar to that in AC power transformer - if the circuit requires 100mA @ 5V, and the input is 10V, the input current will be approximately 50mA (500mW out = 500mW in)

April 07, 2011
by bretm
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That's approximately true of a switching regulator, but not a linear regulator like the 7805 that comes with the Nerdkit. Linear regulators just convert it to heat. Most would consider it waste--I just think of it as using a battery to take some of the load off my furnace.

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