How to protect COBs run in series against single COB failure?

fearnoevil

fearnoevil

Well-Known Member
Hey all, I'm still pretty much a noob when it comes to DIY LED fixtures, so bear with me if I'm asking an obvious question ;?D

So I've read that a fixture with say 5 COBs run in series, can experience catastrophic failure in the event that one of the COBs burns out, causing a cascading power overload. Wondering what to do to protect against this, at first I considered some sort of fuse on each individual COB, but I'm also worried that this might introduce other issues (believe me when I say electronics is not my strong suit, lol).

But I have this idea that there might exist some off the shelf switch to which each of the 5 COBs would be wired, separate from the power supply but with an interrupt circuit on the AC side of the driver, and would monitor current flow.Then if any of the 5 failed, it would immediately cut power to the entire unit thus saving everything else.

Yeah, I have no idea if such a thing even exists, lol, and I suppose that if there was some of the big brains on here would have posted it by now, but maybe I just didn't enter the correct search parameters. But so far I haven't been able to find much if any info on this subject, just that it can happen, but if there is such a thread I would really appreciate that link. Thanks in advance for any input and suggestions ;?)
 
fearnoevil

fearnoevil

Well-Known Member
I still haven't found a thread dedicated to the topic of everything that can go wrong and how to prevent it, lol. I thought if all that info was in one place, it would help noobs like me to avoid all the pitfalls of building your own COB fixture ;?D
 
fearnoevil

fearnoevil

Well-Known Member
PhotonFUD said:
Easier way is to run parallel strings, each of which can handle the load the driver can put out.
Click to expand...
Parallel strings? If I only have 5 COBs on my fixture, what would a string consist of? Or do you mean running each one parallel? Also, I thought I read somewhere that it was easier/better to run them in series, although that may just be an assumption on my part (and I've read so much that I can't remember half of it and the other half is probably wrong ;?).
 
fearnoevil

fearnoevil

Well-Known Member
Yeah, my problem is that when I decide to dive into something completely new to me, I read and read and read until I'm dizzy, and then what I can remember is often jumbled and full of misremembered pieces that make up a wrong whole, lol. Eventually as I stick with it over time, it does begin to fall into place, kinda like absorbing it through osmosis :mrgreen:
 
Airwalker16

Airwalker16

Well-Known Member
Wrong. Series wiring is one after another basically. If one goes out, so do the others because the circuit is broken. PARALLEL is providing pos AND neg leads off of the last cob so that if a cob blows, the power continues through the "say ideal metal holder" through the plastic wire enclosure provided with 2 holes on each side. Or of course split the wire into as many cobs as you have and run them to each cob on both pos and neg main leads.
 
PhotonFUD

PhotonFUD

Well-Known Member
Its ok. Lots of info on the forum.

With constant current, your driver will have a rated voltage range. You can put chips in series so long as the sum of their forward voltages is within that range.

If you run in parallel, the current is divided by the number of circuits. Those circuits can be single cobs or a string of chips in series. Each circuit must be the same voltage (or thereabouts) when running in parallel.

Leds are typically under driven from what they are rated to run at. If you have enough in parallel, they will be more than able to handle the load, basically up to n-1 can fail.
 
Rahz

Rahz

Well-Known Member
Failure should be pretty rare. A little too much thermal paste is better than not enough. Make sure the cooling is good in other respects and series will be very reliable.
 
fearnoevil

fearnoevil

Well-Known Member
So you think I should run this set up in parallel?
I'm planning on building a total of 6 fixtures, hopefull 400w a piece when I'm done. Wonder I could break the 1.2 GpW right away, so as to blow away my old record with halides. Anyway, any sugesstions are totally welcome, don't be afraid to chime on in ;?D
 
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welight

welight

Well-Known Member
you should restart with your first question, what happens if a COB fails and then flowchart it, if in series the string goes out, resolution replace the COB, if in parallel what happens, if you configured the string at your max drive current then the remaining COBs work but their current doubles, can your config handle that, if not since your still need to replace the COB same outcome only you may damage your COBs in the parallel config if you did not allow for current share based on possible failure, in most cases COB failure unlikely but these are the outcomes, given the issues with unbalanced load(no two leds have exact same forward Voltage) with parallel config, series is simpler all round
Cheers
Mark
 
PhotonFUD

PhotonFUD

Well-Known Member
wietefras said:
Parallel is a worse choice. You should be able to find threads discussing "thermal runaway"
Click to expand...

Depends on the situation. Generally speaking, if each of your strings is capable of handling the drive output then it isn't an issue.

Thermal runaway occurs when things get too hot. Running in parallel to under drive makes them run cool.

Also keep in mind most cobs are just parallel runs of series. If it was generally a bad idea, they wouldn't do so.
 
PhotonFUD

PhotonFUD

Well-Known Member
wietefras said:
Sorry, but just no.

At the very least you should include something to prevent thermal runaway.

See how many of those cheap parallel fixtures break down.
Click to expand...

Sure! Stick the emitter to something or hang it dangling where it can get some airflow.
 
PhotonFUD

PhotonFUD

Well-Known Member
Stephenj37826 said:
If running is parallel using a constant voltage driver use adjustable voltage version or (a) in meanwell. Adjust voltage up to exact vf at 50c. If cob failure occurs the driver should reduce current until vf setting is met.
Click to expand...
Very cool using the protection functions of the driver.
 
Airwalker16

Airwalker16

Well-Known Member
Airwalker16 said:
Wrong. Series wiring is one after another basically. If one goes out, so do the others because the circuit is broken. PARALLEL is providing pos AND neg leads off of the last cob so that if a cob blows, the power continues through the "say ideal metal holder" through the plastic wire enclosure provided with 2 holes on each side. Or of course split the wire into as many cobs as you have and run them to each cob on both pos and neg main leads.
Click to expand...
SUPRA! :D :D risen from the dead!
 
fearnoevil

fearnoevil

Well-Known Member
Stephenj37826 said:
If running is parallel using a constant voltage driver use adjustable voltage version or (a) in meanwell. Adjust voltage up to exact vf at 50c. If cob failure occurs the driver should reduce current until vf setting is met.
Click to expand...
By the "(a)" version you mean the non-dimmable models? If so I guess I'll have to try that on my next build as I currently have an order for a Mean Well 320H C-1050B.
 
Stephenj37826

Stephenj37826

Well-Known Member
fearnoevil said:
By the "(a)" version you mean the non-dimmable models? If so I guess I'll have to try that on my next build as I currently have an order for a Mean Well 320H C-1050B.
Click to expand...

You have constant current ordered. No worries of thermal runaway with that. You are good. Only on constant voltage driver. That means no (C) after the H in the model number. Also it will denote the voltage IE ( Hlg 320H-42A)
 
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