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### Topic: Basic Ohms law problem (Read 3587 times)previous topic - next topic

#### kculm

##### 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

#### smeezekitty

#1
##### Feb 02, 2013, 04:47 am
What is the nominal current at 5v? 5v devices do not generally tolerate >5.5v too well.
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!

#### emdee

#2
##### 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.

#### kculm

#3
##### 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

#### kculm

#4
##### 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?

#### JimboZA

#5
##### Feb 02, 2013, 05:33 amLast Edit: Feb 02, 2013, 05:44 am by JimboZA Reason: 1
Or throw a 7805 in there....

(We learn everyday... I had to Google "Larson scanner", now I know what it is....)
Johannesburg hams call me: ZS6JMB on Highveld rep 145.7875 (-600 & 88.5 tone)

#### Krupski

#6
##### Feb 02, 2013, 06:58 am

Or throw a 7805 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?).
Gentlemen may prefer Blondes, but Real Men prefer Redheads!

#### smeezekitty

#7
##### Feb 02, 2013, 07:05 am

Or throw a 7805 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.
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!

#8
##### 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

Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### Krupski

#9
##### 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???
Gentlemen may prefer Blondes, but Real Men prefer Redheads!

#### smeezekitty

#10
##### 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.
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

#11
##### 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'...

#### MarkT

#12
##### 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.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### fungus

#13
##### 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.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

#### kculm

#14
##### 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.

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