Two led circuit questions

[cyrusthevirus]

First:

If I have a string of 12 LEDs which have a Fv of 3.5V @ 700mA and 3.7 @ 1000mA and have a hypothetic constant power supply that is exactly 42V and 1A what will the Fv of the LEDs be? Are they limited by the voltage or the current?

Second:

My LCD has a LED which has a forward voltage of 4.2V and draws 300mA. The power supply supplying my PIC and LCD is 5V/1A. Other than a resistor for the LED do I need to reduce the current to the LED? Why or why not?

 

[mneary]

Sounds like a homework question. Tell us your answer, and how you reached it.

Someone will help you if needed.

 

[cyrusthevirus]

Sorry it comes across that way, but it really is not. If you search for me on the forum you will see I am building a lighting system for my aquarium and am not confident in my design.

Here is a link to my post about my aquarium and you can see the usernames match "cyrusthevirus".

**broken link removed**

 

[mneary]

Glad to help. We don't want to deprive students of the opportunity to learn.....

OK, 42V/12=3.5V, so 700mA is close to the current you'll get. Remember that the Vf is never exact, and has a negative temperature coefficient (Vf gets lower as the LED gets hot), so the 1A current limit is still important.

For the LCD back light, a resistor is all you need. 0.8V / 0.3A = 2.6 ohms. You aren't obligated to supply the maximum rated current to the back light. I would go with 4.7 ohms or higher as long as it's bright enough.

 

[cyrusthevirus]

Thanks I realized that by adding the word hypothetical I made it sound like homework. I was trying to avoid a large discussion around resistors.

So how can I measure what current the LEDs are using? I assume I can see the voltage used by measuring the voltage of the string.

 

[mneary]

To measure the current you insert a meter (milliammeter) in series with the LEDs. (Break the LED circuit and insert the meter so that the current must flow through the meter.) The default location for the meter'ss probe leads probably goes up to 199.9mA. Since you are expecting 700mA, you will probably have to change the place your meter's red wire is inserted.

 

[Hero999]

The LEDS need a constant current supply.

To power from a lower voltage, you need a boost constant current converter.

The constant current supply will put 700mA or 1A into the LEDs and adjust the voltage as required so at 700mA you get 12*3.5 and at 1A you get 12*3.7.

Assuming you're using a 12V supply to power the converter which is 80% efficient, the input current will be 2.94A when the output is 700mA and 4.2A when the output is 1A.

 

[cyrusthevirus]

Sorry for the delay in the response. But if the supply is only 42V will the current be limited to 700mA? How can I tell how efficient my PS is?

 

[Hero999]

The LEDs forward voltage increases with the current.

The the supply voltage is 42V and 1A and you connect 12 LEDs, the current might be 500mA or it could be 1A because the forward voltage isn't very well controlled and can vary depending on the temperature. If you want to be sure of 1A then your PSU needs to be able to give 3.7*12V.

The efficiency can be determined by connecting a known load and measuring the power taken from the supply and can be calculated by dividing the power out by the power in.

 

[cyrusthevirus]

I am more concerned about the power being 700mA than 1A or worse greater than 1A. I can add in an LM317 to limit the curret to make sure it does not go over 1A.

 

[audioguru]

A current limiter needs to have extra voltage. An LM317 current limiter needs at least 3.25V more.

 

[cyrusthevirus]

Ahh that's right I forgot about that. Any other options to limit the current?

 

[audioguru]

A series resistor limits the current but also needs some extra voltage.

 

[Hero999]

Is the power supply voltage or current regulated?

You need a constant current (current regulated) supply to power high powered LEDs efficiently. I thought your supply was a 1A constant current supply with a maximum voltage of 42V.

 

[cyrusthevirus]

Okay, looks like we are moving out of the theory to the actual. So here is what I have:

I am running a 58V, 1A Volgen Power Supply. It can be found here: 58V, 1A VOLGEN POWER SUPPLY-MPJA, Inc.

Attached to this are a string of 16 3W Cree LEDs with a Fv of 3.5V @ 700mA and 3.7 @ 1000mA. So either 3.5*16=56V or 3.7*16=59.2V.

In addition I have a 2W 1.2ohms 5% resisor in series.

The specs on the coll white leds are here: https://www.cree.com/products/pdf/XLamp7090XR-E.pdf

 

[Hero999]

You're not giving yourself enough voltage headroom.

I wouldn't recommend a resistor, for 700mA, you could use an LM317 with a 1.8R resistor (Google for LM317 constant current for the schematic) and a 60VDC power supply.

For 1A you need a 64V supply.

 

[cyrusthevirus]

You're not giving yourself enough voltage headroom.

I wouldn't recommend a resistor, for 700mA, you could use an LM317 with a 1.8R resistor (Google for LM317 constant current for the schematic) and a 60VDC power supply.

For 1A you need a 64V supply.

My plan is to run the LED at about 700mA. If I run it lower than 700mA it is okay, I just do not want to run it higher than 1A.

I already have the Power Supplies so I could reduce the string of LEDs to 15 for more voltage headroom.

If I run 15 how how will the LM317 get?

 

[Hero999]

What do you mean how will the LM317 get?

The LM317 will get too hot if you don't use a suitable heatsink.

You need a >56V supply to power 15 LEDs using an LM317.

The resistor value is the same, 1.8R, it doesn't change because it's constant current so it doesn't matter how many LEDs are in series with it.

 

[cyrusthevirus]

What do you mean how will the LM317 get?.

Sorry, How HOT will it get? You already answered that.

So, if I choose to not use an LM317 and run 16 LEDs on a 1A 58V PS, what problems could/will I have?

 

[Hero999]

The temperature of the LM317 will depend on the ambient temperature, the power dissipation (in this case 3.5*0.7 = 2.45W).

As stated many times previously, LEDs need a constant current supply, the problem with running them from a constant voltage supply is the forward current is unpredictable and can change by a large amount as the temperature changes or the output voltage changes by a fraction of a volt.