Want to start using some electrical equipment, but what a circus.

[rstofer]

And design the thing for a constant voltage with a series resistor.  Have an appreciable voltage drop across the resistor.  Don't stack 10V worth of LEDs in series on a 12V supply.  There is no rule and, mathematically, it makes no difference but a common 2V LED at 20 mA on a 5V supply uses a 150 Ohm resistor and drops 3 of the 5V available.  In a long series stack, the resistor should probably do something similar - drop about 1/2 of the supply.  So, for a 12V supply, no more than 3 LEDs, maybe 4, but no more.  Yes, there is a lot of wasted power in the resistor.  That's a cost of doing business.

If you are running all the LEDs in parallel, you just need one dropping resistor per LED.  Take that 150 Ohm resistor above.  If the LED shorts out, the maximum current in the loop is 5V / 150 Ohms or 33 mA.  There is no condition under which the LED can get more than 90 mA.  Even if the voltage went to 10V, given a 2V LED, the max current would be (10-2)/150 or 53 mA.  Not ideal but still withing the rating of the LED.

It gets even better if you design for 10 mA.  In that case, for the 2V LED and the 5V source, you would use a 300 Ohm resistor.  If the PS spiked to 15V, the maximum current would be 43 mA.  Well under 90 mA for which the LED is characterized.  Heck, it could spike to 27V and only pass 90 mA.

This just isn't a problem!

[Decoman]

Thanks for the info guys!

[Decoman]

I see that there is a Rigol DP711 & DP712 variable power supply for some affordable price ($300 new on eBay). The differences seem to be mainly that one is 30V/5A and the other is 50V/3A.

Would it make sense to suspect that one version is more accurate, or more careful when working with LEDs? Though, I suppose there just isn't enough info to even suppose such.

At least these units are labeled EAC, which according to Wikipedia seem to indicate that there is some safety requirements/standards to adhere to with such a labeling on products.
https://en.wikipedia.org/wiki/Eurasian_Conformity_mark

[rstofer]

The other way to do things is to string a bunch of LEDs in series and drive them with a much higher voltage (perhaps as high as 24V) and control the current with a 2-transistor constant current driver like:

http://www.electroschematics.com/9383/led-constant-current/



The current source control will act very fast so overshoot is a non-issue and the version linked above can work on up to 24V.

One problem with long strings of LEDs is that they are all about the same brightness.  That may not be the best lighting scheme.

There are lots of current sink circuits and the above is just the first I looked at.

Do pay attention to the power dissipation in the upper transistor.  A heatsink might be a good idea depending on the dissipation.

It seems a shame to waste a perfectly good power supply on driving LEDs.  You can use a 12V wall wart to power 4 or 5 LEDs with this circuit.  The 20 mA isn't an issue for the wall wart so several strings can be connected in parallel.  Like 10 strings in parallel if the wall wart can provide an honest 2A.  Half that many if it is overrated.

[Decoman]

*reads about 'burden voltage' re. DMM's.*

Haha, so my digital multimeter will be inaccurate for reading off mA measurements with the DMM probes, because it also lower the voltage in the same process as measuring the current at low current values?

*reads about the uCurrent product*


Edit:

It seems a shame to waste a perfectly good power supply on driving LEDs.  You can use a 12V wall wart to power 4 or 5 LEDs with this circuit.

Yes, yes. I would want to use a variable power supply when working with the setup, and then finally connect the LED to some kind of generic 9V or 12V power supply.

[rstofer]

*reads about 'burden voltage' re. DMM's.*

Haha, so my digital multimeter will be inaccurate for reading off mA measurements with the DMM probes, because it also lower the voltage in the same process as measuring the current at low current values?

*reads about the uCurrent product*

Even experts get caught out from time to time.  First, watch the video on current sources I linked above and then watch the follow-up.  There is a lot to learn from this video:



So, if you are measuring voltage, you need to be sure your meter doesn't load the circuit.  If you are measuring current, burden voltage comes into play.  If you are measuring resistance, you shouldn't hand-hold the parts because skin resistance enters in.  In fact, there's a reason that bench meters offer 4-wire resistance testing (hint: it is to eliminate the effect of voltage drop in the test leads):

https://www.allaboutcircuits.com/textbook/direct-current/chpt-8/kelvin-resistance-measurement/

Nothing about electronics is a simple as it seems.