A word of caution regarding Steve's LEDs LED Drivers

[slinger]

I apologize if this information has been posted before. I couldn't find mention of it...

 

 

PLEASE NOTE: I am not trying to bash Steve's LEDs LED driver circuit with this post. It is a perfectly capable LED driver and will work great so long as you realize some of the caveats of using a linear topology driver like this.

 

Anyway, I was doing a little analysis of the driver circuit that Steve's LEDs sells. Sometimes the hyperbole on that site is a little much, and I realized that this custom driver is actually just an ON Semiconductor CAT4101TV chip soldered on board with 5V regulator providing power and a setting resistor to control maximum current.

 

The issue with the CAT4101TV is that it is a linear topology driver with no voltage conversion capability. What this means to anyone using this driver is that the voltage delivered to the series-connected LEDs is exactly what is delivered by the power supply, so if you're running this driver at 1000 mA, the power delivered by the power supply is its voltage level multiplied by its current, regardless of how many LEDs you have in series. As a result, any power that is not dissipated by the LEDs themselves is dissipated through the driver.

 

For example, if you use a 25V power supply with this driver and run the LEDs at 1000mA, the power supply is delivering 25 watts. Each LED is going to dissipate roughly 3 watts (it's Vf drop multiplied by the current), so if you only have 4 LEDs in series, 12 watts will be dissipated by the LEDs leaving 13 watts that get shunted through the driver. Those 13 watts are pure heat that must be dissipated by the heat sink, and if you have multiple drivers set up like that you are dumping a lot of heat (and wasting a lot of power). Note that on the web page describing these drivers there are two very important points:

 

1 - "Thermal Dissipation- capable of dissipating over 6 watts when attached to a large heatsink*"

 

2 - "To Obtain Max Efficiency - You must use an adjustable voltage power supply to obtain maximum efficiency and to guarantee that the driver is outputting the specified current rating."

 

Note that point 1 relates directly to what I've just described. The driver chip is capable of dissipating over 6 watts when connected to a large heatsink, which presumably means it can dissipate the 13 watts in our example, but I think you are asking for driver failures trying to bleed off that much heat.

 

Point 2 is the rub. In order to combat the problem of point 1, you really need to match your voltage to number of LEDs you are driving. In our example, if you had used a 13V power supply instead of a 25V power supply, you would only be dissipating 1 watt of power through the heat sink - a far more manageable number.

 

In conclusion, Steve's LEDs driver will work just fine for your project so long as you are cognizant of the need to match power supply voltage to the sum of the LED Vfs and the necessary operating headroom. A step-down or buck driver would alleviate this issue, but they require more components on the board (including inductors) and are more complicated and expensive to implement.

[Wizzy]

Good Post

 

Very clear, even for someone without any electrical expertise (such as myself).

 

And if no one's done it yet...

[Milad LEDGroupBuy.com]

cliff notes version

if you dont use exactly the right power supply with those drivers you are better off lighting a MH because you are wasting an insane amount of power.

[slinger]

Are you trying to say that I'm excessively wordy?

[nor_cal_nano]

I stopped at caveats.

[jedimasterben]

 

I'm a bit confused by this. I use five strings of LEDs powered by these drivers and a Meanwell power supply running at 25v. One driver is a "double" (two strings of 1A on a single driver board), a triple at 1A, two singles at 700ma, and a double at 700ma. Most of those are not using all 25v supplied by the power supply.

 

So what you're saying is that if I run all five strings at 100%, no matter what, it will pull 25v and 7.8A, which is ~195w. By my calculations according to Vf of my LEDs on each string, it should only pull ~150w at maximum power draw. My Kill-a-Watt meter says 158w from the wall, which lines up with the efficiency of the drivers and power supply.

[pwreef]

Does the same thing apply to a driver like this:

http://www.micro-bridge.com/data/CRpowtech/PT4115E.pdf

Have some of these on order, which look perfect for our purpose:

http://www.ebay.com/itm/370604722551?ssPag...9#ht_500wt_1180

[slinger]

I'm a bit confused by this. I use five strings of LEDs powered by these drivers and a Meanwell power supply running at 25v. One driver is a "double" (two strings of 1A on a single driver board), a triple at 1A, two singles at 700ma, and a double at 700ma. Most of those are not using all 25v supplied by the power supply.

 

So what you're saying is that if I run all five strings at 100%, no matter what, it will pull 25v and 7.8A, which is ~195w. By my calculations according to Vf of my LEDs on each string, it should only pull ~150w at maximum power draw. My Kill-a-Watt meter says 158w from the wall, which lines up with the efficiency of the drivers and power supply.

 

There's always the possibility that I'm completely wrong and have just made an ass of myself on the internet (again)...

 

But, honestly, I don't think this is one of those times.

 

Take a look at the data sheet for the CAT4101:

 

http://www.onsemi.com/pub_link/Collateral/CAT4101-D.PDF

 

In particular, check out "Figure 1. Typical Application Circuit". That figure shows the driver operating in current sink mode (it's normal mode of operation). In that picture, you can see that the LED string is fed the full voltage output of the power supply, and the current is sunk through the driver to ground. The driver is acting as a sort of current-controlled variable resistor that sits in series with the LED string, changing the ground path resistance to make sure that the current stays constant. From that wiring diagram, you can see that the LED string is delivered the full output power of the power supply at the desired current (25 watts in your case with a mean well power supply at 25 volts). Any power delivered to the LED string that is not dissipated through the LEDs themselves must be dissipated by the driver.

 

Anyway your calculations are correct. 5 drivers running at 1 amp and 4 running at .7 amp, with a power supply outputting 25 volts, equates to 195 watts. I'm not sure why there is such a discrepancy between your watt meter and those calculations, but I would be inclined to do a little in-circuit measuring to see if the power supplies really were outputting 25v, as well as using and ammeter to measure current at certain points. If you have the means to make those measurements, I'd really be interested in hearing about them.

[slinger]

Does the same thing apply to a driver like this:

http://www.micro-bridge.com/data/CRpowtech/PT4115E.pdf

Have some of these on order, which look perfect for our purpose:

http://www.ebay.com/itm/370604722551?ssPag...9#ht_500wt_1180

No, this wouldn't apply to that driver as it is capable of stepping down power supply voltage to match the combined drop of the LED string.

 

That Arduino driver shield thing looks pretty darn interesting. Is it using the driver you listed in each of the individual channels?

[jedimasterben]

We all make asses of ourselves on the internet, I do it all the time!

 

If my power supply was not outputting 25v, then my white, royal blue, and violet strings would not light up at all, as they are each pulling roughly 23.5v.

 

 

I don't have access to a multimeter, unfortunately, but my Kill-a-Watt device I did measure when I purchased it and it is spot on between 100 and 750w (outside of that the margin of error goes up to about 10% or so).

[pwreef]

That Arduino driver shield thing looks pretty darn interesting. Is it using the driver you listed in each of the individual channels?

Yes. The seller told me it does. Should get them in a couple of weeks I guess, but its my first time with Arduino, so will probably take a while, especially since I am trying to set up a touchscreen controller.

[syncro]

Yes. The seller told me it does. Should get them in a couple of weeks I guess, but its my first time with Arduino, so will probably take a while, especially since I am trying to set up a touchscreen controller.

Let us know how it works. Looks interesting.

[jm82792]

Hey I totally understand your point!

I made your conclusion awhile ago, and I was guessing it was a cat4101. They seem to be great for price, especially since they do PWM dimming. At under $5 a DIY driver which can drive 7 or so leds it's especially good for people who have a bunch of channels or such.

I have a few of the chips and they work great for what they are made for if the input voltage is watched.

[slinger]

I don't have access to a multimeter, unfortunately, but my Kill-a-Watt device I did measure when I purchased it and it is spot on between 100 and 750w (outside of that the margin of error goes up to about 10% or so).

By any chance do you have the potentiometers installed on the drivers? If they aren't turned up all the way they could be lowering your output current and therefore the power utilization of your fixture. Table 6 in the data sheet I referenced shows the relationship between the RSET value and the LED current. Going from 1000 mA to 900 mA is just a matter of going from 549 ohms to 604 ohms resistance, something that can happen just by connecting the potentiometer to the driver.

 

I'm curious... When you got the drivers, was there a jumper wire installed between pins 5 and 6 (the dimmer pot pins)? I'm assuming that for the dimmer circuitry to work, the 5K ohm pot is installed in series with the RSET resistor to create a resistance range of 549 ohms to 5,549 ohms, but a series connection would require that pins 5 and 6 be jumpered when a dimmer is not installed. Also, I noticed that Steve's LEDs includes a linear taper potentiometer. Given the logarithmic nature of Table 6, I would have though you would want a log taper pot.

[jedimasterben]

By any chance do you have the potentiometers installed on the drivers? If they aren't turned up all the way they could be lowering your output current and therefore the power utilization of your fixture. Table 6 in the data sheet I referenced shows the relationship between the RSET value and the LED current. Going from 1000 mA to 900 mA is just a matter of going from 549 ohms to 604 ohms resistance, something that can happen just by connecting the potentiometer to the driver.

 

I'm curious... When you got the drivers, was there a jumper wire installed between pins 5 and 6 (the dimmer pot pins)? I'm assuming that for the dimmer circuitry to work, the 5K ohm pot is installed in series with the RSET resistor to create a resistance range of 549 ohms to 5,549 ohms, but a series connection would require that pins 5 and 6 be jumpered when a dimmer is not installed. Also, I noticed that Steve's LEDs includes a linear taper potentiometer. Given the logarithmic nature of Table 6, I would have though you would want a log taper pot.

Yes, there are potentiometers installed onto each driver for dimming. Power numbers I gave are for 100%, though.

 

On the drivers, there are two dimming functions - one via PWM and one via potentiometer. The PWM pins are the ones that are jumpered by Steve because most people don't use PWM dimming, the pot pins aren't jumpered.

[avbarr11]

Thank you so much for posting this information! I'm planning on doing an LED build for my future office pico and am looking at buying from Steve's LEDs. This has definitely saved me a headache...

[slinger]

Yes, there are potentiometers installed onto each driver for dimming. Power numbers I gave are for 100%, though.

 

On the drivers, there are two dimming functions - one via PWM and one via potentiometer. The PWM pins are the ones that are jumpered by Steve because most people don't use PWM dimming, the pot pins aren't jumpered.

Well, shoot... I can't figure out how he's achieving potentiometer dimming without inserting the pot in series with RSET, and the would require a jumper when no pot was connected.

 

Unless...

 

Does the driver work if there is no pot connected? If it doesn't that would explain why there is no jumper between pins 5 and 6.

 

Anyway, there's a mystery here and I think it behooves you to take one for the team and buy a multimeter / ammeter to test this!!!!

 

I'm kidding of course....

[jedimasterben]

Well, shoot... I can't figure out how he's achieving potentiometer dimming without inserting the pot in series with RSET, and the would require a jumper when no pot was connected.

 

Unless...

 

Does the driver work if there is no pot connected? If it doesn't that would explain why there is no jumper between pins 5 and 6.

 

Anyway, there's a mystery here and I think it behooves you to take one for the team and buy a multimeter / ammeter to test this!!!!

 

I'm kidding of course....

Yes, the driver works normally when no pot is attached.

 

 

And yes, I need some hooves!

[slinger]

Thank you so much for posting this information! I'm planning on doing an LED build for my future office pico and am looking at buying from Steve's LEDs. This has definitely saved me a headache...

You're welcome. As I said before they are perfectly good drivers - just try to match the power supply voltage to the needs of your LED strings.

 

 

Yes. The seller told me it does. Should get them in a couple of weeks I guess, but its my first time with Arduino, so will probably take a while, especially since I am trying to set up a touchscreen controller.

Why is it called a "driver shield"?

[pwreef]

Why is it called a "driver shield"?

Thats Arduino terminology: http://arduino.cc/en/Main/Hardware

[OneTrickPony]

Just to make sure I understand correctly

If I was ruining 7 LEDs - (Forward Voltage is 3.0V @ 1000ma) @ 1000ma with one of his drivers and power supply's

I would need to adjust the Voltage down to 22 or so volts

 

Thanks

[slinger]

That's correct. You want to overcome the combined voltage drop of the LEDs (7 x 3.0V) and the dropout voltage of the driver itself (.5V), so you would need at a minimum 21.5V. Setting the driver to 22 volts would be perfect.

[pdavidc]

Not sure that I understand all of this (which is sad since I plan to build my own led fixture using Steves led's, drivers and power supplies) but steve's power supplies are adjustable + or - 20% of stated voltage via an onboard adjustment screw.

 

So...if I understand correctly, the 24Vdc power supply can run as low as 19Vdc or as high as 28.8Vdc. He gives instructions on his sight as to how to adjust the power supplies.

 

I think this means that the 24Vdc power supply will work for as low as six 1000ms led's and as many as 9 per driver chip (array). Please correct me if I am as wrong as I think I probably am!

 

Dave

[slinger]

Not sure that I understand all of this (which is sad since I plan to build my own led fixture using Steves led's, drivers and power supplies) but steve's power supplies are adjustable + or - 20% of stated voltage via an onboard adjustment screw.

 

So...if I understand correctly, the 24Vdc power supply can run as low as 19Vdc or as high as 28.8Vdc. He gives instructions on his sight as to how to adjust the power supplies.

 

I think this means that the 24Vdc power supply will work for as low as six 1000ms led's and as many as 9 per driver chip (array). Please correct me if I am as wrong as I think I probably am!

 

Dave

 

You're not wrong - you are exactly right! The adjustable power supply makes it very easy to dial in the correct voltage for the most efficient operation. Most of what I've said in this topic is of concern to people who are using fixed voltage power supplies.

[ruffyruff]

Sorry for reviving an old thread.

 

I ordered Steve's LED Biocube 29G stock lighting with the typhon controller.

 

I tried to ask an engineer to explain to me what are mA, volts and watts, but that got me to understand a little. My question is, am I safe with the setup?

 

Is this the right calculation?

 

If i have a PSU with 200w 25v, it goes to the 28 LED (each LED is 3W = 1000mA X 3 volt) hence i need either 84W or 84v, when I only have 25v to share... so I would be under driving the LEDs?

 

Thanks!