System Voltage

The designing engineer or the equipment/ appliance data tag.

Factor is the amount of power you required at output.

suppose you have 100W, what will be the system voltage?

Power=voltage*current. You cam take any voltage and currents values such that their product satisfies the equal power.

There's no one correct voltage. It depends on experience and what convention has been arrived at in a particular application. E.g in automotive small cars and 2-wheelers might be 6 volt (more in the past than nowadays), most cars 12 volt, lorries 24 volt and I believe some now on 48. But that's based on a balance of pros and cons, it didn't have to be that way.

You haven't given the application. For a small load like 100W it could be anything (within reason), not just your original list.

I would say it depends on where you are....European house=250vac, American house=120vac, pre 1955 vehicle=6vdc, 1955 and later vehicle=12vdc, big truck=24vdc, forklift 48vdc, older computer bus=5vdc, newer computer bus=3vdc and if you're going to use one of these to power it, you'd probably chose one designed to function there. You might say choice was made by committee, long before you were consulted, you can choose to follow their standards, or maybe not.

What ever Mr. Ohm said it would be. -- JHF

But an LED doesn't obey Mr. (or should it be Herr) Ohm.

Would have been nice to know the load would be an LED in the OP! JHF

Agreed, it would, but he told us later in #9.

Kyom you got my point or you need any further explanation?

Got you... thanks

Are you free to select the voltage for the 100W load? I.e designing it from scratch. If it's an existing device it will be designed for a specific voltage, which must be provided. If you give it the wrong volts it will either not work or maybe burn out. For a resistive load power varies as V^2.

The load is a dc led street lamp..

OK, what's the voltage? It should be written on it, or in the data sheet.

A quick look on wiki indicates an LED does not obey Ohm's law. To limit the current, either a resistor in series is needed, or a more complicated circuit to reduce power loss. For a fairly powerful LED like 100W I'd guess the latter. I would assume (but you need to check) that in either case it's built into the lamp or the lamp base. Maybe some are supplied with built-in rectifier so can be plugged directly into AC mains. You still need to check and supply the volts specified.

http://www.google.co.uk/url?url=http://en.wikipedia.org/wiki/LED_lamp&rct=j&frm=1&q=&esrc=s&sa=U&ei=9pYWVdi4DcOU7Qac0oDYDQ&ved=0CEIQFjAD&usg=AFQjCNEibdV9JjJZcG2hI8jVjLY8SrCS3g

Good link. I hope KYOMA looks at it.

If this is an LED street lamp, doesn't the manufacturer offer a driver to power the lamp? Most lamp drivers will allow multiple voltage inputs to provide the correct power (voltage/current ) to operate the lamp.

When you design something it's a trade off between current and voltage.

High current devices may be more expensive, extra batteries for higher voltage may be expensive or heavy.

High voltage parts may not be available. There is also the influence of what is already designed... like you'd be mad to try and produce a car with a 9v battery.

Like most things it somewhat arbitrary and somewhat a compromise.

Del

Good post (as ever!).

One of the biggest constraints I see when designing ANY system is cost. Especially what it will cost the customer's bottom line....

You can design a completely new car system for example, with perfect power balance between supply and demand, but the batteries might then be 10 times more expensive (assuming batteries are needed for some reason).

The redesign of say the alternator, might multiply the cost of it many times.

Its always a better idea to study what is available, especially parts made by the million that have achieved a high reliability and very low cost....like nominal 12 volt car alternators.....

Even if you have to accept some other restraints just to keep the costs down to an acceptable rate.

A perfect example of such a problem is some of the electric cars around today. The cost (just for example only) is say €40,000 for the complete car. The battery has a usable life of say between 5 and 10 years, when it will need to be replaced.

So at say (early!) 6 years old,the battery needs replacing. The car has a street value of say €15,000 (with a good working battery!). The replacement battery costs say €25,000......

So basically the car has a negative value.......writing off an electric car after say less than 10 years is basically making it a very expensive investment.......

I personally feel that the hybrid cars around, with a far smaller battery (far cheaper to replace!) are probably a better option, especially the ones with a small Diesel engine.....that can charge up while underway, with far easier recharging (fill the tank again), but manage quite interesting MPG in comparison to "normal" cars.....

But thats just my personal take, I am willing to be proved wrong once the costs involved over many years are comparable to a normal car, or hopefully far less.....

I find the Mitsubishi Outlander Hybrid to be a good example of what is possible today, even with a petrol engine. Plug in and recharge for short journeys from the mains, or fill up the tank for longer distances! Best of both world's :-

Mitsubishi Motors plug-in-hybrid-outlander

That is German link, you may need a different language......

In DC systems lead length/size and voltage drop determine the choice of voltage. Your 100W load draws 8.13 Amps at 12V, but only 2.06 Amp at 48V, depending upon how much you want to spend on wire will tip the economics towards one or the other, especially if you're going to do this for multiple lights.

The biggest factor is the supply voltage available.

Would you design a 48v system if only 12v were available?