Basic Ohms law problem

Krupski:

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.

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'...

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.

kculm:
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.

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.

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

Wow, I am really impressed on all the input I got. This goes a long way to helping me learn.

Thanks everyone.

fungus:
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.

Would I just put it inline with the Pos. side of the power?

kculm:
Wow, I am really impressed on all the input I got. This goes a long way to helping me learn.

Thanks everyone.

The problem with that is you seem to be (or wanting to) learning basic electronics by memorizing a series of 'rules', rather then studying basic electronics DC theory which covers ohm's law and once completely understood would allow you to answer all those questions yourself once you are then ready to be introduced to semiconductors. Trying to learn by dealing with semiconductors (leds for example) and basic voltage/current/resistance at the same time is not a good way to start one's education into electronics, at least that is my opinion. There is a reason for the basic order of the subjects presented in learning electronics fundamentals and deviation from that is certainly an invitation for having gaps of understanding and substituting 'rules' for proper understanding of the principles and theory.

Lefty

I'm giving serious consideration to buying the latest edition of Practical Electronics for Inventors for me and my daughter to share as we continue our electronics journey.

See this thread.

Or get a LDO regulator such as

retrolefty:
The problem with that is you seem to be (or wanting to) learning basic electronics by memorizing a series of 'rules', rather then studying basic electronics DC

Lefty

Now that's that kind of Input I am looking for. I have posted all over what would be the best way to get started in all this. I was not really getting any good input. Most would tell me to do the Arduion examples.
Even though that just shows me how to do things and not why.

I agree with you, I need to study the Basics first.

My issue is, I learn better by doing. I have found a few books on the subject but it's hard for me to retain unless I see it in action. That is way I have been doing some real odd ball projects.

If you have any recommendation on Books, I would be very grateful.

Thanks

A book to last a lifetime is this:-

Grumpy_Mike:
A book to last a lifetime is this:-
http://www.amazon.co.uk/Art-Electronics-Paul-Horowitz/dp/0521370957

Thanks

kculm:

fungus:
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.

Would I just put it inline with the Pos. side of the power?

Yep.

Witch in reality puts out 6.24vdc and makes my scanner blink crazy.

If your arduino runs off an r/c oscillator, its frequency is more dependent on the voltage, but not that dependent.

AVRs can work comfortably at 6.24v. If you are really concerned about it, you can put a diode or an led in serial + a resistor to pull down the voltage.

Moderator edit: dhenry was banned for making this post. The quoted figure of 6.24V exceeds the maximum rating of the chip. This is not the first time that dhenry has made completely false assertions. Strikeout added by me. (Nick Gammon)

AVRs can work comfortably at 6.24v.

Sure they can that is why the data sheet says

Maximum Operating Voltage ............................................ 6.0V

and then goes on to say

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent dam- age to the device. This is a stress rating only and functional operation of the device at these or other conditions beyond those indicated in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.

Oh but I forgot according to you data sheets are a conspiracy and only to be used if you are designing equipment for NASA

dhenry,
Stop posting arrant nonsense, or be banned.

Is that clear enough for you?

dhenry:
AVRs can work comfortably at 6.24v. If you are really concerned about it, you can put a diode or an led in serial + a resistor to pull down the voltage.

dhenry, you keep telling people to read the datasheet, then just make stuff like this up.

I'm counting at least 4 moderators now that think dhenry be banned for this crap.

CrossRoads:
I'm counting at least 4 moderators now that think dhenry be banned for this crap.

I'm actually kind of surprised that because of how big this forum has become that there isn't a army of dhenry types, but he seems to be pretty unique among the membership around here.

But I will give dhenry this, he is like the relative that came to dinner but won't leave. I wonder what he gets out of posting here. Understanding his motivation might be a key to the puzzle that is dhenry. Or is that too flattering on him?

Just rambling I guess, go 49ers.

Lefty