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mphotis

5 volt regulated power supply HELP!!!!

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Hi there,

im not very good at this sort of thing but im trying to get better.
im in desperate need of a design for a 5 volt regulated power supply. THe trick is that the design should not involve a 5 volt regulator chip. The regulator has to be built from discreet components. Im using a transformer that is stepping the voltage down to 7.2 volts AC.

Any thoughts or designs would be very appreciated.

BTW thanx for the help on my last topic, was highly appreciated.

Thanx,
mike

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Hello mphotis

You can build a LM7805 regulator using discreet components following the internal circuit diagramm of it. Here it is:



Also i found this page: http://www.usmicrowaves.com/voltreg/usm7805.htm

this circuit diagramm:



which is also an equivalent 780x regulator

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now, you are now allowed to use a regulator chip....
but what about using a switching chip, with some extra components?????
or building a switching with descrete components?
if you can use a simple circuit with a zenner and a transistor.


mixos,
if you look at the second circuit you have posted at a connection called 'sense' you will see that all the resistors connected at the bottom right of the schematic are actually shorted....so.....why are they there?

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the schematic with only the zenner and without the transistor can not suply you too much current.
a 1.3W zenner cand suply a few miliamps(sorry, no time to do the numbers). if you want 5V, you might consider using a 5.1V zenner in place of the 4.7V one.

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I do not think this is what they were referring to. I think it is a question about the LMXX schematic.
However, I have not seen very many tansformers with a secondary of 7 volts that have much current rating. If current is a consideration, then perhaps we should know the current needed of this circuit. If you need to limit yourself to no more than 5 volts such as used with microprocessors, then use the zener/resistor/caps schematic as is. If you need at least 5 volts, then yes, a 5.1 volt zener is better to use.
Bogdan, why don't you also post your schematic with a transistor so that there is another choice?

MP

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ive got another problem-maybe you guys can help me out.... im attaching a schematic of my supply so far. it works pretty well for medium loads 1k - 1000k. but as soon as the load goes quite low, like 10 ohms or lower, the output voltage stops regulating and starts to drop drastically. the current, which isnt allowed to be more than 500mA shoots right up to bout 2 or 3 A.

any suggestions?!!!

thanks kindly

post-1195-14279141594793_thumb.jpg

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This supply can't give you large current. If you try, for example to sink 500mA, the power on the transistor would be 10W!! This will destroy the transistor! You must change th 2N2222A with BD179 for example. The BD179 can handle 30W, with a heat sink.

'You have a mistake in the schematic. The earth in 230Uin isn't the same with the earth in the circuit. You must remove the first earth, if you want to do simulation.'

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A zener diode is being used as the regulator in many of your circuits. If you use the zener diode as a voltage reference only, then it becomes more accurate because of the small change in current. If you use it as a regulator, then it's less accurate. Also, if you use it as a regulator, put it right on the load instead of back behind the transistor.

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