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How to calc this resistor ?!

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Whenever a power supply is labeled with a voltage and a current (like 5V, 1.2A) this means it can (ideally) provide either 5V or 1.2A, whichever is reached first.

So when I load such a supply with a load of 100 Ohms, the supply delivers 5V/100Ohm = 50mA of current. The voltage of 5V is reached before the current of 1.2A is reached.

If I'd load such a supply with 1 Ohm, the supply delivers 1.2A * 1 Ohm = 1.2V of voltage. The current of 1.2A is reached before the voltage of 5V is reached.

In normal day use, you can read the label as "This is a 5V supply that can deliver up to 1.2A of current", though in theory (not always in practice) the opposite is true as well.

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Detailed explanation:


There are several types of power supply, but the type most often seen in the consumer and electronics world  is constant potential (constant voltage). The current specification is the MAXIMUM current it will supply at the specified voltage (give or take... sometimes the voltage will drop a bit before the current rating is reached, sometimes there is a little leeway), but the voltage is, nominally, fairly constant for most types.


Most important is the type of supply...


Presuming that the supply is a DC supply, three main types exist. Unregulated, linear regulated, and switching.


Most wall wart types are unregulated or switching, thought there are a few linear regulated in the wild.


Unregulated may provide much higher than rated potential at lower than rated load, and are generally not very stable, with a lot of noise (typically at 100 or 120 cycles per second, depending on line frequency, and up to 50% or more voltage at full load), and should not be used with devices requiring regulated supplies.


Switching are pretty tightly regulated, and generally cut out (drop to near zero output) if the rated current is exceeded by more than a few percent. Within the rated current, they drop very little with load and have moderate to low noise -- maybe less than a percent -- at high frequency (10 KHz to 1MHz). Some types may not regulate with less than a few percent load, so ALWAYS load these at 10% or so current when testing them, and be careful when using to power low loads. For low loads -- less than about 10% rated -- check with a dummy load to be sure that it regulates. Use an oscilloscope, as the overvoltage may be pusles to pretty high potential, with average potential looking ok on a meter. This isn't as big a problem with modern supplies as with the units from the 70's through the 90's, but some of the no-name and counterfeit units from the far east are pretty bad.


linear are typically the least noisy, but not as efficient as switching types. They may have a little more variation over load than switching, in the simpler ones, but can be significantly more stable than switching in the higher quality designs. They tend to drop off more gradually at overload, and usually are conservatively rated, tolerating a few to 20% overload, at least for a brief period, without dropping from specified voltage. They generally regulate well down to zero load, but some may have stability issues with loads that are principally inductive or capacitive. Then again, so can switchers.




If the original supply was regulated, and the new one is (switching or linear), no issue.


If the original was unregulated, or the new one is, then you need to do some work to be sure, but are likely ok. If the old one was, the new one should be fine as long as the device powered wasn't relying on the supply limiting current.


If the new one is unregulated, DO NOT use it. You will likely have a problem.


If old and new are both regulated, no dropping resistor needed and all should be good.




Additional info: many older (1970's and 1980's) supplies were unregulated and were designed to work with a particular load. The transformers in much consumer grade product used saturation limiting in the transformer for protection from overload (often no fuse or thermal protection was used... sat limiting met the requirements for certification by UL and other ratings agencies if the primary wire could safely burn on a primary short... still find devices like this even today) The saturation limiting also provided a controlled drop under load, and a nominal 5V supply might show 8 or 9V open circuit, dropping fairly linearly to 5V at rated load, and limiting current at not much more due to transformer saturation. These types of unit ARE NOT suitable for modern electronices, are not that safe, and should not be used with anything other than the devices they were designed for. Back in the 90's a ton of Coleco and other similar wall warts came on the surplus market that fall into this category, as well as a bunch of cord-and-brick supplies. Lost a lot of gear to unclueful power suppply replacement. Mostly other people doing it, as after the first time, I learned (but still managed to lose a really nice frequency counter to my not looking at the supply. Right connector, so I went with it. Poof!)


More info, since I have diherria of the typing finger (and it is frigid and windy here):


The other type of supply is constant current. They tend to be seen as blocks in circuits (op amps use them by the bucketful internally), but are sometimes seen in high power applications. There are parts of the world (eastern europe and rural US, in particular) where costant current street lighting is still used. The lamps then don't need individual ballasts, but must fail electrode shorted at end of life.


Yet more info: MIG and fluxcore (wire feed) welding is constant potential. Stick (SMAW/MMA) and TIG (GTAW) welding are constant current. Neither is true constant, and true constant potential or constant current makes for a very difficult to control process. Real welding sources have some 'droop', to allow for an equilibrium to be established with inherent negative feedback for control. This holds true for both manual and automated processes, though automated processes are often held to a flatter curve.




Cat got off my lap, so I'll stop now....

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