DIY LED driver for reef lighting

[LOUIEB]

Sorry 10.7 ohms, I'll see if i can find more, Can i use some 1/4w resistors, I have lots of them Oops 1/4w will not be able to handle the load. I'll keep looking.

 

[DustinB]

If you are looking at using this to power VIN, you could probably even get away with an 1/8th watt resistor. I'll measure it to make sure, but unless I'm mistaken from the datasheet, VIN pulls between .8mA and 8mA. So you are looking at no more than .04watts through the VIN circuit at full power.

 

[der_wille_zur_macht]

The recommended operating range for Vin is 3 to 5.5v. All the readings you are getting from that supply are within that range. I don't think you need to worry about getting EXACTLY 5v from it.

 

[TheFishMan65]

We are supposed to read the data sheet to see what the limits are? What a crazy idea.

Thanks for coming up with the answer - don't know where my brain was.

 

[perikaruppan]

hi in continuation to your Vin, Vout and LED's Current discussion.... i have this issue
I decided to regulate my power using the PWM in the Arduino, so i am currently using 560 ohms RSense equivalent to 970mA...

got 6 Edision LEDS with specs stating voltage @ 4-4.2Volts, and current upto 700mA...

so i start the SMPS @ 24V, this was the observation...

Vin_________@LED_________Total LED Vin_________Current
24_________3.7 _________ 22.2 , _________0.58
25_________4.1 _________ 24.6 , _________0.63
26_________4.1 _________ 24.6 , _________0.72 (Cat gets slightly hotter here, but bearable, the heat is not increasing though)
27.7_________4.12 _________24.7 _________0.93 (Cat gets slightly hotter here, but bearable, the heat is increasing) - due to more voltage drop...

so what should i do now... i though maintaining 26V Vin give me the required 3Watts... but the CAT has got a max Vin @ 25.5, so will there be a problem?

@25.5 i am getting 4.1V and 660mA... should i just stick to that? or up the Vin to the specs that i require?

i currently have made a 8 in 1 board. with individual PWMs for each channel....

 

[LOUIEB]

Thanks to all, I thought it needed to be exactly 5v because it was mentioned it should be regulated. Ahh the data sheet 3v-5.5v . Great now I can order some and use them in my LED project which I'm just getting started with.:spin1:

 

[der_wille_zur_macht]

Peri, in several of your measurements you are overdriving the IC. Above 25v input, we have no garuntee of performance or correct current regulation. If you cannot reach your desired current withunder 25v input, I think you need fewer LEDs in your strings.

Also at one point you said your LEDs had a 700mA max but you also said your Rset choice resulted in 970mA and that you were going to use a pwm signal to regulate power. You CANNOT do this. PWM signals do not reduce the nominal current, they just pulse it rapidly. In other words, even if you adjust the pwm signal to give an apparently safe operating current, your LEDs will still be seeing the nominal current in a short pulse. In other words, if your LEDs have a 700mA max, then you should choose a value for Rset below 700mA regardless of wether or not you will be using pwm.

 

[perikaruppan]

yikes then it shall be driving about 5 LEDs, that i need to redo my entire LED wiring and it gets really really complicated...
i get your point WRT the PWMs... will do it at lower current ratings.

Is there anyway to overcome this issue of stringing them with 6LEDS instead of 5?

 

[perikaruppan]

but it is mentioned in the specs that the peak pulse current max rating is 1000mA? does this play a role with PWMs? or i need to restrict it to the DC forward Current of 700mA, as in specs sheet?

http://www.edison-opto.com.tw/Datasheet/Edixeon/Edixeon S series_Eng_v.5.pdf

They advice to drive the LEDs @ 4.1-4.3v @ 700mA to get 3W output

 

[janne68]

This might be a stupid question, but is it really a problem if the power is above 25V over the LED string?
I can’t see any possibility for the CAT4101 to “see” that voltage.
The spec says “Up to 25 V Operation on LED Pin” and I think that is not the same as the voltage over the whole LED string.

 

[perikaruppan]

thats exactly another question that i had. i have been running the system @ 25.5V for the last 6 months!!!

 

[der_wille_zur_macht]

but it is mentioned in the specs that the peak pulse current max rating is 1000mA? does this play a role with PWMs? or i need to restrict it to the DC forward Current of 700mA, as in specs sheet?

http://www.edison-opto.com.tw/Datasheet/Edixeon/Edixeon S series_Eng_v.5.pdf

Look at the spec for the peak current - it specifies an extremely short on time (<100us) and 25% duty cycle. You're exceeding that. The maximum current you are supposed to use for ontimes or duty cycles higher than that is 700mA.

It's probably obvious but worth pointing out that many times, these LEDs can be run above their specs - though performance and lifetime will be hard to predict. In the end, IMHO, if you are targeting something low and safe like 660mA, then you should use Rset to get that value, instead of using a higher Rset and PWM'ing to get that value.

This might be a stupid question, but is it really a problem if the power is above 25V over the LED string?
I can't see any possibility for the CAT4101 to "œsee" that voltage.
The spec says "œUp to 25 V Operation on LED Pin" and I think that is not the same as the voltage over the whole LED string.

thats exactly another question that i had. i have been running the system @ 25.5V for the last 6 months!!!

We had that question many pages ago (probably late spring). I forget the exact argument, but it seemed convincing that the voltage used should not exceed 25v. It might be worth an email to OnSemi's tech support, as they've answered several other questions for folks in this thread.

 

[TheFishMan65]

On-Semi answered it but not very well. kcress had a very good answer. If you ignored the PWM then I had the same thought as everyone else. When the PWM comes into play the entire 25 volts is stopped but the ship. The theory was an internal FET IIRC so 25 volts is all it can handle. This is the way I remember it any way.

 

[der_wille_zur_macht]

That sounds right. I remembered the discussion but couldn't recall the details and didn't feel like digging.

 

[janne68]

That sounds like a reason! Didn't think about that case

 

[der_wille_zur_macht]

As peri has clearly shown, it's a limit that can be pushed at least a little bit.

 

[kcress]

Yep 25V. You could probably run higher but only if you used it in some weird configuration that never had it PWMing. Whenever current is flowing there will be voltage drop in the LED string but if the current ever stopped there would be no voltage drop and pfffftzzzpp. Pretty bizarre.

Pretty moot point though because there aren't many bargain supplies around that are more than 24V anyway and once you go much over 36V you enter much more hazardous territory.

 

[TheFishMan65]

pfffftzzzpp is that the same as letting the magic smoke out?

 

[kcress]

Youbetcha - just with a little noise included. (noisy smoke)

 

[TheFishMan65]

What no lights - oh no smoke and noise no wonder the lights don't work.

Ok kcress this is probably your question but anyone with insight help would be appreciated. I have six string of LEDs using the same driver (rewiring to measure on the same CAT4101). I have 5 strings drawing 890 and the sixth upper 400s. I also hooked up 2 sting to a power supply and the low current one draws very close to 50% of what I think of as a good string. I figured I had a bad solder joint, but measured the voltage from wire-LED-wire (to make sure I included solder joints) and they all had the same drop 2.46 plus/minus .01. Any ideas what is rong or what else I can measure.

Another one. I have ten CAT4101 drivers nine supply 850 ma the tenth is in the 500 somewhere all with the same string of LEDs. All RSET measure 602-609. Any thoughts one this one.

Thanks all