# Suitable resistor for LED using current source?

Discussion in 'Homework Help' started by iCosine, Mar 11, 2013.

1. ### iCosine Thread Starter New Member

Mar 11, 2013
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0
Hi, I'm currently assigned a project by my company to test the current carrying capacity (CCC) of the company's product. Let's call it UTI. UTI has a very low resistance of 35mOhm at room temperature and since it is very small, I'm afraid there might be short circuit. Thus, I decided to add in a LED to act as a load. However, I'm unsure of the resistance value I should use to protect the LED. I'm not using a voltage source since I'm testing the CCC of UTI and as such I cannot use the resistor-voltage calculator available on the internet.

So I would like to know which resistor should I use to protect my LED if I were to test UTI using a current range of 0A to 10A supplied by a current source. Thanks!

P.S Sorry for the wall of text.

2. ### TheComet Member

Mar 11, 2013
88
12
A current source from 0A to 10A? That will annihilate your LED. Your LED will only exist for a fraction of a micro second before it will explode, creating a miniature mushroom cloud (which looks pretty nice btw, at least until the fire alarm kicks in).

First of all, ask yourself a simple question: If you are using a current source, why would you need to limit the current at all? That's exactly what a current source does in the first place.

Current sources can be short-circuited. In fact, current sources like it when you short circuit them. Think about it, what they do is they try to output a fixed amount of current. They do this by changing the voltage so it matches the current load resistance (Ohm's law). So for example, if you'd like to output 1A of current, and you have a 0.035Ohm load, the current source will try and output 35mV. If you short circuit the current source, it will still output the 1A of current, but there's 0 resistance, so you'll just have 0V on the output (which means the power dissipation is P=V*I = 0V*1A = 0W).

You can think of current sources to be the exact opposite of voltage sources. It is dangerous to not connect anything to current sources, just as it's dangerous to short circuit voltage sources.

In other words, you don't need anything to limit the current.

TheComet

3. ### iCosine Thread Starter New Member

Mar 11, 2013
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Thanks for the reply! Yea, I know that such big current will annihilate my LED. My task it to test how much current can the UTI handle before burning out. Do you think it's enough to have only the UTI as the load even though it has a very small resistance? Basically my circuit diagram will be a current source connected to the UTI and back to the ground, is that it?

4. ### TheComet Member

Mar 11, 2013
88
12
That's it, yep. Only the given amount of current will flow. The resistance needs to be small for a current source to work, so that's fine.

TheComet

5. ### Evil Lurker Member

Aug 25, 2011
116
24
First thing you need to do is aquire a sample lot of UTI's. Next thing you need to do is to contact the engineering dept of whoever designed the UTI and get the specs for how much current it was designed to handle, either continuously and/or for transient periods. Then you set up a variable constant current source with ammeter in series with an optional temperature sensor on the device under test and slowly ramp up the current till the device fails and record the results. Then you repeat the process to eliminate errors caused from variables in quality from UTI to UTI. From that you can determine the average current failure rate along with the mean current failure rate (which in many cases may be more important than the average failure rate).

Load is unimportant if all you are testing is pure current handling capacity thanks to Kirchhoff's circuit laws, however if you wish to see exactly how a device performs in its intended application you should attach an identical load, be it resistive or inductive. Better yet blow up several lots of them with and without loads, and actual devices if they are intended as a component of something else. The more UTI's you blow up and the more sample lots you use from different production runs the more accurate your tests results will be.

Thorough documentation of all testing devices along with the methodology used is a must in case of legal problems arising from product liability claims, and for justification to management as to why you are destroying so many UTI's. If a UTI is meant to be part of an expensive device, mission critical, or for medical purposes you may wish to use precision NIST calibrated and certified equipment, for example as your current source a good lab grade power supply. You may also wish to video record the testing process, and if you do, keep a copy in secret for your own records in case something happens later on down the road so your company cannot scapegoat you for someone else's screw up.

Hope this helps.

6. ### iCosine Thread Starter New Member

Mar 11, 2013
3
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Alright, thanks for the replies! Now I'm really clear on this. Thanks again!