Basic question about the PWM

Gundal21 wrote:

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Another reason is efficiency: If you want to dim a light, with a PWM you can turn it full on and off and nearly all energy is used for the light. With a linear amplifier most of the power is used for heating the amplifier for dark dim levels.

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Frank Buss, fb@frank-buss.de
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In engineering, cost, performance, and timeliness are the essentials. There are lots of wrenches in the toolbox to achieve those things--the art is in coming up with a design that does well in all three areas.

As you and others have said, PWM usually saves cost and often power as well, at the price of longer response time and lower precision.

Sometimes that's an excellent trade, sometimes not, depending on the application, the schedule, and other design constraints such as board space. That's where the engineering comes in.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
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Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net

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I use PWM because:

It's cheap especailly if you are using a PIC with it built in.

It's easy to design.

PWM is more noise immune. For example controlling charge voltage or current in a switch mode charger.

PWM signals can be optically coupled to shift between different reference grounds. For example from a low voltage safe area to a high voltage area.

PWM can be capacitively coupled to different reference grounds in

4-20mA systems. For example a single PIC controlling 2 indepenent 4-20mA outputs in a loop powered system.

What exactly is missing from the previous answers? If you can you explain why you are not satisfied it may be easier for us to give you the information you need.

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If you are in the business of making electronic products price is very important.

As others have said PWM gives greater efficiency. This gives two practical advantages. Firstly the more obvious advantage is that less energy is used and secondly there is less waste heat to get rid of.

For a high powered device the increased efficiency could result in significantly lower running costs and these days there is considerable political pressure on manufacturers to make things more efficient to reduce carbon dioxide emissions (a high proportion of electricity is generated by burning coal). For a battery powered device greater efficiency means a longer battery life, same battery life with smaller battery or some compromise between the two. Obviously, a portable product will be much more attractive to a customer if it's smaller, lighter and runs for longer.

If you used DAC and linear amplifier the wasted power would be dissipated as heat in the amplifier. To prevent your amplifier from overheating you need to get rid of this heat. This usually means that you need a larger heatsink for a linear method than a PWM method. This makes your product bigger, heavier and more expensive.

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