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### Topic: LM317T Driving me nuts (Read 15903 times)previous topic - next topic

#### SgtOneill

#45
##### Jul 05, 2011, 09:42 am

If i cant find that 0.82ohm 2watt resistor i could try and use the 1.2ohm 5Watt resistor i bought.
that would give me 1.02ohm 5Watt in parallel  [   R= (1,2*1,2)/(1,2+1,2)    R=1.02ohm  ]

that would give me around 1.2amps of current. a little low but could give it a shot.

I really thought it would be easier to drive these LED's decently hehe. At least im learning a bunch of new stuff.

#### mmcp42

#46
##### Jul 05, 2011, 09:48 amLast Edit: Jul 05, 2011, 09:51 am by mmcp42 Reason: 1
er wrong formula there
two resistors in parallel if they're both the same is simpy R/2
so 1.2 in parallel with 1.2 will give you 0.6 ohms

be careful grasshopper

actually right formula, you just did the sums wrong!

R = (1.2*1.2)/(1.2+1.2) = 1.44/2.4 = 0.6
there are only 10 types of people
them that understands binary
and them that doesn't

#### robitabu

#47
##### Jul 05, 2011, 10:14 amLast Edit: Jul 05, 2011, 10:17 am by robitabu Reason: 1

R= (1,2*1,2)/(1,2+1,2)    R=1.02ohm

Wrong calculation. R = (1.2 * 1.2) / (1.2 + 1.2) = 1.44 / 2.4 = 0.6ohm !!!

And you get I = 1.24/0.6 = 2.07 amp which I think it's even better.

#### SgtOneill

#48
##### Jul 05, 2011, 10:28 amLast Edit: Jul 05, 2011, 10:49 am by SgtOneill Reason: 1
*walk of shame*
Yup. 2amps seems to be a nice current for a LED that takes an absolute maximum of 2.8amps.
guess those resistors are going to have a purpose after all

Tonight, i'll put it all together. If im lucky, ill have my new Variable Power Supply (15V /2 Amps) i bought on ebay waiting for me

Just remembered that i actually have 4 of them LED's to connect. Would it be as simple as connecting the other 2 in series with the first two?
3.6*4 = 14.4V wich is over the top for the 12v lead-acid battery. Guess thats not going to happen.

Circuit Final Version:

#### dshay

#49
##### Jul 05, 2011, 02:25 pm
Go back to the drawing board on your schematic.

The resistors you have in parallel, the .6 Ohm ones, are not meant to use as the Adjust R1 and R2 to set your voltage on your Voltage Regulator.

Use those only in series (in a parallel configuration) with your LEDs.

You'll need two separate R1/R2 resistors to set your voltage.

Oh, and an LM317 cannot source 2 Amps, you need a different voltage regulator.

#### SgtOneill

#50
##### Jul 05, 2011, 03:24 pm

Go back to the drawing board on your schematic.

The resistors you have in parallel, the .6 Ohm ones, are not meant to use as the Adjust R1 and R2 to set your voltage on your Voltage Regulator.

Use those only in series (in a parallel configuration) with your LEDs.

You'll need two separate R1/R2 resistors to set your voltage.

Oh, and an LM317 cannot source 2 Amps, you need a different voltage regulator.

Im using the LM338 as a Current Limiter, not voltage. I followed the circuit on hte datasheet.
it says LM317 because the software doesn't let me change it to 338, but i placed a label above it saying 338.

This circuit should be ok for 2 Led's.

#### robitabu

#51
##### Jul 05, 2011, 03:25 pm

The resistors you have in parallel, the .6 Ohm ones, are not meant to use as the Adjust R1 and R2 to set your voltage on your Voltage Regulator.
...
Oh, and an LM317 cannot source 2 Amps, you need a different voltage regulator.

You missed something in the thread. SgtOneill finally considered using an LM338 which is capable of 5Amp and he's not using it as a voltage regulator but as a current limiter instead.

#### floresta

#52
##### Jul 05, 2011, 03:25 pm
@dshay;

You've been away from the thread for a few days, he's not building a voltage regulator any more.  Also, some versions of the LM317 can indeed source 2A.

Don

#### floresta

#53
##### Jul 05, 2011, 03:29 pm
@Sgt:

The LM317 may be driving you nuts but your reference to the LED current is driving me nuts.  Voltage appears across a device, current flows through a device.

Don

#### SgtOneill

#54
##### Jul 05, 2011, 06:18 pm

@Sgt:

The LM317 may be driving you nuts but your reference to the LED current is driving me nuts.  Voltage appears across a device, current flows through a device.

Don

whats the difference?
voltage goes accross a device, meaning it comes in one side and goes out the other,
current goes through a device, meaning it comes in one side and goes out the other.

might be a language problem here, english isn't my main language, i might get confused understanding stuff, specially with technical terms ^_^;

#### floresta

#55
##### Jul 05, 2011, 07:04 pm
Quote
whats the difference?
voltage goes accross a device, meaning it comes in one side and goes out the other,
current goes through a device, meaning it comes in one side and goes out the other.

You are less likely to get confused or in trouble when you use the proper terminology.  Voltage does not go in one side and out the other, whereas current does.   Since voltage appears across a device you measure voltage by connecting a voltmeter across the device.  Since current flows through a device you must open the circuit and connect the ammeter in series with the device in order to measure the current.  If you connect an ammeter across a device to measure the current 'across' the device you will destroy the ammeter instantly (or blow it's fuse if it has one).

Quote
might be a language problem here, english isn't my main language, i might get confused understanding stuff, specially with technical terms

That's why filling in your your location in the user profile helps....

Don

#### SgtOneill

#56
##### Jul 05, 2011, 07:18 pm
So current flows through a device but voltage doesn't?
Should i be thinking of voltage like potential energy, and current as cinetic energy? meaning one flows and the other is just there.

#### mmcp42

#57
##### Jul 05, 2011, 07:21 pm
yup
you can even refer to a potential difference of x volts!

and current is like water - it flows

so voltage is like the pressure in a tap, even with your finger onit the pressure is still there, even though no current flows

and like a tap, when current flows, the pressure sometimes drops a little

HTH
there are only 10 types of people
them that understands binary
and them that doesn't

#### robitabu

#58
##### Jul 05, 2011, 07:54 pm

so voltage is like the pressure in a tap, even with your finger onit the pressure is still there, even though no current flows

There's an even more intuitive analogy with water ... think of a waterfall, the voltage value being the height of the water (and the water itself being the positive charge carrier). Water flows from high to low (from a higher voltage level to a lower voltage level).

You set a reference voltage level (the GND) being the zero volt. Other nodes in the circuits may have a positive voltage value or a negative voltage level. It all depends if those points are "higher" or "lower" than GND. When two points in the circuit have different voltage levels, there you may have a current flow (from V+ to V-) (*). Just like water flows from a higher altitude to a lower level.

It's all about potentials. Here in Italy is difficult to misunderstand the point since we call it ddp="differenza di potenziale" (=potential difference). It couldn't be less clear :-)

As you may see, there's no voltage flowing across anything at all. That's the point.

------------------------------------------------------------------------------------------
(*) actually electrons flow from V- to V+ ... but that's another story.

#### SgtOneill

#59
##### Jul 05, 2011, 10:34 pm
Great, another lesson learned

I must personaly thank floresta, robitabu and Grumpy_Mike you've all been very patient!
As soon as I get all this working i'll take some pictures.

just another small question thats been poking my mind.
Im limiting the current that flows on the LED's so thats my controled variable. However, im assuming each LED will suck whatever voltage they need from the 12volts.
Since i have 2 LED's (each with a vF of 3.6) im assuming there will be 7.2volts on the 338 vOut and 2Amps. If I plug another LED or whatever else with say, 2v vF that would be 9.2v on the 338 vOut.

Is this correct?

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