question about a schematic

i built this… and the first time i turned it on, the batteries started to overheat and the LED wont light up. is there something wrong? i’m a beginner with electronics. i haven’t taken any classes yet… so i don’t know what’s wrong.

Built this what? The part you drew in?

Well, are you sure you have the regulator pins right (In-Gnd-Out)? You have an LED on the output, but NO resistor - always NFG.

Looks like you have no current limiting resistor wired to the led. Once the applied voltage is at or above the Vf rating of the led it acts like a short circuit and will try and draw very high current which may then cause the voltage regulator to go into overcurrent protection mode.

Lefty

yes, i built the whole thing plus the part i drew in. do i need a resistor? and why to the batteries still overheat?

KatjaD: do i need a resistor? and why to the batteries still overheat?

Yes, it needs a resistor - that's why I mentioned it.

So, is it OK without the parts you added or what? Is the rest of it a kit?

Short circuits are the usual cause of over-heating.

retrolefty: Looks like you have no current limiting resistor wired to the led. Once the applied voltage is at or above the Vf rating of the led it acts like a short circuit and will try and draw very high current which may then cause the voltage regulator to go into overcurrent protection mode.

Lefty

interesting. i didn't know that. what kind of resistor do i need? and will that fix everything? (if there's no other short circuits or any other problems not related to the schematics)

KatjaD:

retrolefty: Looks like you have no current limiting resistor wired to the led. Once the applied voltage is at or above the Vf rating of the led it acts like a short circuit and will try and draw very high current which may then cause the voltage regulator to go into overcurrent protection mode.

Lefty

interesting. i didn't know that. what kind of resistor do i need? and will that fix everything? (if there's no other short circuits or any other problems not related to the schematics)

Depends on the Vf specification and the maximum continuous current rating for the led is. Given that information it's a simple ohm's law calculation to determine the proper size of the resistor required.

Lefty

thank you very much

A voltage regulator is a lot of trouble to go to to run an LED. You could run it from the 18V.

[quote author=Runaway Pancake link=topic=165927.msg1237804#msg1237804 date=1368318838] So, is it OK without the parts you added or what? [/quote] You didn't say whether everything is OK with all of that disconnected.

the circuit should be perfect with that disconnected.. but how can you run an LED from 18v? wouldn't it like.. blow up or something? that's a lot of voltage.

KatjaD: .. but how can you run an LED from 18v? wouldn't it like.. blow up or something? that's a lot of voltage.

What - you were going with no resistor at all.

Here's how: I'll assume this is a red LED and 5mA will do it. So, it if you place a 3300? (1/4W) resistor in series with the LED then you can run it from 18V.

ok. so.. the resistor thing didn't work. the LED is still not lighting up and the batteries are still heating up.

on the input of the 5v regulator, voltage is reading positive. but it reads nothing on the output...

Get rid of the regulator. Use the LED and the 3300? resistor - unless you need the 5V for something else.

ok. so 3.3k ohm would do the trick? i'll try it

Yes 3300? orange-orange-red

the batteries still heat up and the LED still doesn't work. i've confirmed that it works perfectly without the LED in the circuit. but with the LED, it over heats. is there any where else i can put the LED in the circuit?

If your connection is +18----3300?-----A_K----Gnd then there's no good reason for it not to work let alone cause [u]anything[/u] to overheat.

Are you certain what connections you are making? How about taking a picture of what you're doing?

here’s a picture. while the solder job might be crappy, i’ve made sure there is no short circuits or anything.

Oh, good grief. What am I supposed to...?

Where's our 3300? and the LED in question? Can you just take the 18V and run the LED-3300? circuit by itself ?

Do you have a meter that you can take readings on the LED? I wonder if it is shorted from the initial hook up?