# Arduino Forum

## Using Arduino => General Electronics => Topic started by: kculm on Feb 02, 2013, 04:35 am

Title: Basic Ohms law problem
Post by: kculm on Feb 02, 2013, 04:35 am
Need some help guys & gals. And try to hold back the laughter.

What I have is a basic Ohms law problem.

Ok, I made a small project as a Joke for a co-worker. It is basically a Larson scanner  incased in acrylic.

The issue I am having is figuring out the supply voltage. You see as I was making it I was using my Switching Power supply set at 5v. I used  5 Volts because of the atTiny85 and I was planning on using a 5v wall wart that I have.

Everything was working great on until I went to use the 5v 550mA wall wart.  Witch in reality puts out 6.24vdc and makes my scanner blink crazy.

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.

Thanks
Title: Re: Basic Ohms law problem
Post by: smeezekitty on Feb 02, 2013, 04:47 am
What is the nominal current at 5v? 5v devices do not generally tolerate >5.5v too well.
Title: Re: Basic Ohms law problem
Post by: yogimarkmac on Feb 02, 2013, 04:47 am
The resistor would have to precisely match your current draw to get the right voltage drop. Given your predicament the easiest route might be to just burn the extra power with a 5v zener diode to lower the voltage.
Title: Re: Basic Ohms law problem
Post by: kculm on Feb 02, 2013, 05:16 am

The resistor would have to precisely match your current draw to get the right voltage drop. Given your predicament the easiest route might be to just burn the extra power with a 5v zener diode to lower the voltage.

That sounds cool, but over my head. Can you explain.

Thanks
Title: Re: Basic Ohms law problem
Post by: kculm on Feb 02, 2013, 05:17 am

What is the nominal current at 5v? 5v devices do not generally tolerate >5.5v too well.

How do I figure that out?

Title: Re: Basic Ohms law problem
Post by: JimboZA on Feb 02, 2013, 05:33 am
Or throw a 7805 (https://www.sparkfun.com/products/107) in there....

(We learn everyday... I had to Google "Larson scanner", now I know what it is....)
Title: Re: Basic Ohms law problem
Post by: krupski on Feb 02, 2013, 06:58 am

Or throw a 7805 (https://www.sparkfun.com/products/107) in there....

(We learn everyday... I had to Google "Larson scanner", now I know what it is....)

A 7805 needs 2 to 3 volts differential in order to start regulating (i.e. 7 to 8 volts in minimum). It won't work for this application.

There's no reason why a 5 volt board won't work at 5.5.... but if it's really necessary to throw away that 1/2 volt, why not just use a series 1N4001 diode and drop 0.7v off the end of the wall wart?

The real reason that the OP's board is going nuts is not the extra 1/2 volt, but probably something else like maybe lousy filtering in the wall wart (AC ripple?).
Title: Re: Basic Ohms law problem
Post by: smeezekitty on Feb 02, 2013, 07:05 am

Or throw a 7805 (https://www.sparkfun.com/products/107) in there....

(We learn everyday... I had to Google "Larson scanner", now I know what it is....)

A 7805 needs 2 to 3 volts differential in order to start regulating (i.e. 7 to 8 volts in minimum). It won't work for this application.

There's no reason why a 5 volt board won't work at 5.5.... but if it's really necessary to throw away that 1/2 volt, why not just use a series 1N4001 diode and drop 0.7v off the end of the wall wart?

The real reason that the OP's board is going nuts is not the extra 1/2 volt, but probably something else like maybe lousy filtering in the wall wart (AC ripple?).

He said the wall wart is putting out over 6v not 5.5.
Title: Re: Basic Ohms law problem
Post by: CrossRoads on Feb 02, 2013, 07:10 am
There are other low dropout regulators that will work with a 6.24V input. 7805 is not the only choice.

http://www.digikey.com/product-detail/en/MC33269DT-5.0G/MC33269DT-5.0GOS-ND/1479179
Vout = 5V with Vin  >=6.1V

Title: Re: Basic Ohms law problem
Post by: krupski on Feb 02, 2013, 07:13 am

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???
Title: Re: Basic Ohms law problem
Post by: smeezekitty on Feb 02, 2013, 07:37 am

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.
Title: Re: Basic Ohms law problem
Post by: MaJiG on Feb 02, 2013, 10:39 am
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'...
Title: Re: Basic Ohms law problem
Post by: MarkT on Feb 02, 2013, 12:14 pm
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.
Title: Re: Basic Ohms law problem
Post by: fungus on Feb 02, 2013, 12:32 pm

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.
Title: Re: Basic Ohms law problem
Post by: kculm on Feb 02, 2013, 04:55 pm

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. :)
Title: Re: Basic Ohms law problem
Post by: kculm on Feb 02, 2013, 04:58 pm
Wow, I am really impressed on all the input I got. This goes a long way to helping me learn.

Thanks  everyone.
Title: Re: Basic Ohms law problem
Post by: kculm on Feb 02, 2013, 05:01 pm

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?
Title: Re: Basic Ohms law problem
Post by: retrolefty on Feb 02, 2013, 05:07 pm

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
Title: Re: Basic Ohms law problem
Post by: JimboZA on Feb 02, 2013, 05:39 pm
I'm giving serious consideration to buying the latest edition of Practical Electronics for Inventors (http://www.amazon.com/Practical-Electronics-Inventors-Paul-Scherz/dp/0071771336) for me and my daughter to share as we continue our electronics journey.

Title: Re: Basic Ohms law problem
Post by: CrossRoads on Feb 02, 2013, 07:11 pm
Or get a LDO regulator such as
http://www.digikey.com/product-detail/en/MC33269DT-5.0G/MC33269DT-5.0GOS-ND/1479179
Title: Re: Basic Ohms law problem
Post by: kculm on Feb 02, 2013, 08:23 pm

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

Title: Re: Basic Ohms law problem
Post by: Grumpy_Mike on Feb 02, 2013, 09:15 pm
A book to last a lifetime is this:-
http://www.amazon.co.uk/Art-Electronics-Paul-Horowitz/dp/0521370957 (http://www.amazon.co.uk/Art-Electronics-Paul-Horowitz/dp/0521370957)
Title: Re: Basic Ohms law problem
Post by: kculm on Feb 02, 2013, 09:46 pm

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

Thanks
Title: Re: Basic Ohms law problem
Post by: fungus on Feb 02, 2013, 10:02 pm

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.
Title: Re: Basic Ohms law problem
Post by: dhenry on Feb 02, 2013, 11:03 pm
Quote
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)
Title: Re: Basic Ohms law problem
Post by: Grumpy_Mike on Feb 02, 2013, 11:19 pm
Quote
AVRs can work comfortably at 6.24v.

Sure they can that is why the data sheet says
Quote
Maximum Operating Voltage ............................................ 6.0V

and then goes on to say
Quote
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
Title: Re: Basic Ohms law problem
Post by: AWOL on Feb 02, 2013, 11:23 pm
dhenry,
Stop posting arrant nonsense, or be banned.

Is that clear enough for you?
Title: Re: Basic Ohms law problem
Post by: nickgammon on Feb 02, 2013, 11:23 pm

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.
Title: Re: Basic Ohms law problem
Post by: CrossRoads on Feb 02, 2013, 11:40 pm
I'm  counting at least 4 moderators now that think dhenry be banned for this crap.
Title: Re: Basic Ohms law problem
Post by: retrolefty on Feb 03, 2013, 12:27 am

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
Title: Re: Basic Ohms law problem
Post by: AWOL on Feb 03, 2013, 12:28 am
Just in case anyone is any doubt, dhenry is banned.
Title: Re: Basic Ohms law problem
Post by: smeezekitty on Feb 03, 2013, 04:38 am
To be honest. I do not think that banning a member for making a false remark is really correct.
Afterall, mistakes do happen.

Of course it is up to the moderators in the end.

That said, microcontrollers generally will not work properly at 6.24v and can even be damaged in some cases. IIRC the absolute maximum rating for Atmel chips is 6.0v but don't quote me on it.
Title: Re: Basic Ohms law problem
Post by: nickgammon on Feb 03, 2013, 04:50 am
Quote
To be honest. I do not think that banning a member for making a false remark is really correct.

It is the latest in a very long line of false remarks, and generally unhelpful posts.
Title: Re: Basic Ohms law problem
Post by: kculm on Feb 03, 2013, 05:20 am

Just in case anyone is any doubt, dhenry is banned.

Hey guys, I did not mean for my question to get anyone banned .
Title: Re: Basic Ohms law problem
Post by: CrossRoads on Feb 03, 2013, 05:27 am
@kculm,
It had nothing to do with you.
Go into Members and view dhenry's posts, you'll see what brought it on.
Title: Re: Basic Ohms law problem
Post by: JimboZA on Feb 03, 2013, 05:40 am

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

A third edition is anticipated late this year, according to this article (http://en.wikipedia.org/wiki/The_Art_of_Electronics) in Wikipedia. Must be true if it's in Wikipedia  8). Current, second edition is dated 1989.....

I really like the look of Practical Electronics for Inventors (http://www.amazon.com/Practical-Electronics-Inventors-Paul-Scherz/dp/0071771336), up-to-date 3rd edition (2013) already in the shops.
Title: Re: Basic Ohms law problem
Post by: JimboZA on Feb 03, 2013, 05:50 am
Quote
Hey guys, I did not mean for my question to get anyone banned .

It's not the question, it's the answer that did that... and seemingly a long line of answers, at that.