# Current limiting for constant current source

I have 6 LED strips, each of them requires 1080mA & 36V DC.

I've found a Constant Current driver which can supply up to 8.9A & 36VDC. As there is no resistor on the strip so I think it should be current driven for high efficiency.

The driver is: Meanwell HLG-320H-36B

LED Specs:
- Forward voltage: 33.6V - 36V
- Forward current: 0.9A - 2.7A


I planned to wire all 6 LED strips in parallel and use a current limiter circuit to protect each strip. So if one the strips is down, the others won't sink more current.

I want to ask is this a good approach ? And how to build that current limiter ? Any suggestion or instruction ?

As the driver is being shipped to me, I have to use it anyway. So is there any other solution that can help me light up my LED with the driver ?

Thanks.

p/s: I don't want to use separated driver for each strip because of the price & shipping fee.

• "I've found a Constant Current driver which can supply up to 8.9A..." makes no sense. If it's a constant current driver it supplies a fixed current by varying the voltage. Please post a link to its datasheet. – Finbarr Aug 29 '17 at 15:07
• I think the misconception here is about the nature of current limiting circuits and constant current drivers. You can't have one constant current driver hooked up to 6 current limiting circuits in parallel. That makes no sense. The current limiting circuit is the constant current driver. (Or at least an integral part of it.) – Dampmaskin Aug 29 '17 at 15:08
• If you want to do your own current limiting, then you don't want a constant current power supply. – JRE Aug 29 '17 at 16:09
• Possible, but kind of dumb. – JRE Aug 29 '17 at 17:12
• The constant current source will run its output voltage up to the max in an attempt to make the rated 8.9A flow. Then, the limiters you want to build will try to limit the current to the 1A you want to run your LEDs. – JRE Aug 29 '17 at 17:16

The best solution would have been to add 6 Mean Well LDD-1000 (\$3.50 ea) to drive each string and they all can be powered by the HLG-320-36.

Another solution is to use TI's LM3466 Multi-String LED Current Balancer for Use with Constant-Current Power Supplies

Going for a separate driver for each strip will increase reliability and prevents all sorts of issues.

If the strips are not 100% equal then the current will not divide equally between all strips. The strips might be equal now but they will not be after some weeks of usage. This will result in:

• The strip which gets the most current will wear out first
• Strips not being equally bright
• once a strip develops a fault either it will take all the current and destroy itself or fail open and all other strips will get more current (the total current remains 8.9 A) putting stress on them making them fail sooner.

Current protection: that's not a solution as disconnecting the current to one strip will force the other strips to take more current and making them fail sooner. A complex current monitoring system could be made which would shut off everything in case of a fault but I guarantee you that that will cost much more than an individual driver for each strip.

So all in all, using one 8.9 A driver is asking for trouble.

• As I mentioned, is it possible to install a current limiter for each strip? Because I believe that the limiter will guarantee the sinks current of each strip. So if one of the strips is down, the others won't sink more current. – Jack Fury Aug 29 '17 at 14:53
• What current limiter ? Show me where these can be bought. If you don't know Jack about how limiting a current works then do not suggest that this is the solution. Limiting the current won't help as the total current is determined by the driver. Limiting the current in one strip will increase the current in the other strips. There is no way around that unless you design your own driver. These LED systems are current driven and not voltage driven where things are much more simple like adding a fuse. – Bimpelrekkie Aug 29 '17 at 15:05
• No, I'm not suggesting the solution. I've seen some current limiting circuits from this link. Are they applicable for my scenario ? – Jack Fury Aug 29 '17 at 15:18
• Your whole scenario is false so no, those circuits are not applicable. And even if they were such circuits are to be used with voltage driven loads, and not current driven loads like your LED strips. These current limiters open the circuit when the current gets too high and an open is not what you want and need. – Bimpelrekkie Aug 29 '17 at 17:39
• TI has a LM3466 Multi-String LED Current Balancer for Use with Constant-Current Power Supplies. About 50¢. – Misunderstood Nov 10 '17 at 17:20

The driver linked above has an output voltage range of 18-36V, in which it will behave as a current source whose current is set by a pot between 4.45A ~ 8.9A.

The 36V limit is a problem. If the LEDs are "36V nominal" then it is likely their voltage will vary in an interval like 32-38V so without seeing the LEDs datasheets, it is unclear whether they will all light or not.

Now let's suppose you're lucky: all your LEDs have a Vf of 36V. You wire them in parallel... but there is no voltage headroom to install a "current limiter" (whatever that would be) on each LED.

Now, if your LEDs have built-in resistors or drivers, and are able to regulate their own current, then it should work. With the supply set to over 6 amps, then it will simply turn into a constant-voltage supply (instead of constant current) and deliver 36V (with unspecified precision)...

So... this whole arrangement seems a bit half-baked... please give the LED info.

EDIT

OK, the LED Vf is 33.6-36V which means that:

• If they are in parallel, the one with lower Vf will hog all the current and burn, so an individual current limiter should be installed on each...
• But you will not have enough headroom since the max Vf is 36V and the max voltage delivered by the supply is also 36V.

So you need a low dropout current source of 1A, something like that with a RdsON 0.02ohm MOSFET and a resistor like 0.1R you should have a dropout of 0.12V at 1A which should be just fine, MOSFET dissipation would be quite low (max 2.4W per FET so still need a heat sink TO220) and no efficiency problems...

just need to select proper component values and FET.

• I've just updated the LED specs, please check it. – Jack Fury Aug 29 '17 at 17:01
• ok, got a suggestion, see above – peufeu Aug 29 '17 at 17:13
• I'm confused that you mentioned that "With the supply set to over 6 amps, then it will simply turn into a constant-voltage supply (instead of constant current) and deliver 36V (with unspecified precision)..." How does the driver become a constant voltage ? – Jack Fury Aug 29 '17 at 17:27
• Look, the constant current source will try to push 8.9A through the circuit. When you have additional current limiting circuitry in series with it, the current limiting circuitry will not allow that to happen. You now have two components that are fighting each other, which is a bad thing. To prevent this, you could adjust the current source to a lower value than the series limiting circuit, and that might conceivably work, but it would be a clunky, unreliable and inefficient solution. – Dampmaskin Aug 29 '17 at 18:02
• Check datasheet of power supply page 5 on the bottom "Driving method of LED module" the text there explains everything. "Mean Well's LED power supply with CV+CC characteristic can be operated at both Constant Voltage mode (with LED driver, at area (A) and Constant Current mode (direct drive, at area (B)." So, if you want to use the individual opamp current source on each LED, basically set the current to MORE than what will be used, which makes the PSU switch to CV mode, this turns it into a 36V supply with a current limit... simple ;) – peufeu Aug 29 '17 at 18:09

At its simplest, an appropriate resistor in series with each strip will probably do what you want. It'll be a trade-off between adjusting the system to produce the same current through each strip, and ending up with a resistor that dissipates too much power.

Given that the power supply only goes up to 39V, you'll be limited to a resistor of about 2 to 3 ohms, Make sure you get sufficiently chunky ones (perhaps 5W rated) to avoid overheating.

Once it's all set up, adjust the power supply to give the correct current through each strip.

• Resistors will not help with a constant current source will not help. it will only waste electricity. Constant current + constant resistance = constant voltage. The resistor will do nothing to balance the load. – Misunderstood Nov 10 '17 at 17:14