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Voltage Regulators

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MrJammin

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These voltage regulators are really useful for IC circuits, but how much energy do they dissipate? For example, right now I am stepping down a 12V battery to 5V using a 7805; how would that compare to if I was using a transformer instead?
 
Well, you can only transform AC. Not DC.

Now an unregulated supply (transformer + bridge rectifier + filter caps) vs. a regulated one may be what you meant.

In that case, remember that ALL transformers have a regulation spec. On smaller transformers it is usually worse than on large transformers; a 15VA transformer might be 20% while a 500VA one might be 5%. Either way, it's sucky regulation.

If you really need a stable voltage, the power loss across the regulator will simply be the voltage in - voltage out difference times the current through it*. But you can use switching regulators if you need higher efficiency, but these are harder to use and more expensive. A switcher will be capable of up to 90% effeciency or so, but they only operate well over say 20%-100% of their rated ouput power. At low % output power their efficiency becomes bad that sometimes a linear reg is better.

Duncan Amp Tools has a program called PSU Designer. Google for it, it's a real nice unregulated power supply simulation program that will let you try a lot of different power supply configs and lets you specify rated output voltage, current, regulation, use different diodes and rectifier configs, etc.

*Plus some super minor quiescent current - hardly significant unless you're using the new super LDO (<500mV dropout) regulators.
 
If you use a 7805 with a 12 vdc supply the 7805 will cause a 7 volt drop (12-5=7). The power dissapated in watts is 7 volts times the current (amps).
 
If you're going to use a transformer, I wouldn't use a 12V transformer, I'd go for a 9V transformer. Also for 1A of output current make sure the transformer is rated for 1.4A
 
Thanks for the replies, they were helpful. One last thing, what is the difference between the 7805 and the 7905 voltage regulator IC?
 
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