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Topic: Basic Ohms law problem (Read 2 times) previous topic - next topic

smeezekitty



He said the wall wart is putting out over 6v not 5.5.


Right you are. I gotta get to sleep... I'm seeing things. Where on earth did I get 5.5 from???

Probably from my post. I mentioned 5.5v as the maximum that TTL circuits will behave normally.
Avoid throwing electronics out as you or someone else might need them for parts or use.
Solid state rectifiers are the only REAL rectifiers.
Resistors for LEDS!

MaJiG

Before going too far, most wall warts have a significant internal thevenin resistance and put out a higher voltage with no load (read current).

Be sure the wall wart's voltage is measured with a reasonable load on it.

Just sayin'...

MarkT

The lesson here is that a wall-wart is unlikely to be suitable for powering digital electronics unless
used with a proper voltage regulator, or unless it is already "fully voltage regulated" internally - the
good news is that mains _USB_ wall warts are _required_ to be fully regulated to 5V - this is what you
should have gone for I think.

A resistor won't solve this problem since the load is not constant - most useful devices with a microcontroller
are varying loads - a voltage regulator adjusts itself to deal with a varying load, a resistor cannot.

If it were a simple fixed load a resistor could be used.
[ I won't respond to messages, use the forum please ]

fungus


Now I know I could of used and 5v regulator. But it's to late. So I was wondering if I could use a resistor to lower the voltage. Is so can someone show  me how to figure it out.


A diode would be better. Silicon diodes drop the voltage by about 0.7V when it passes through them. Two of them in series would drop it by 1.4V, etc.

Pretty much any diode will do, it doesn't have to be a Zener or anything special. If you want a part number, try a 1N4001.

Transistors are also diodes, a fully open BJT like a 2N2222 will also drop the voltage by about 0.7V.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

kculm


The lesson here is that a wall-wart is unlikely to be suitable for powering digital electronics unless
used with a proper voltage regulator, or unless it is already "fully voltage regulated" internally - the
good news is that mains _USB_ wall warts are _required_ to be fully regulated to 5V - this is what you
should have gone for I think.




And a very important lesson it was. And i am glad I learned it now and not after making a real project. :)

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