LED help

GaryB · Post by **GaryB** » Wed Apr 12, 2017 5:12 am

I'm looking to get a Cree Single-Die XM-L 10Watt, 3Amp LED chip
http://www.ebay.com/itm/221240243864
Specs:
• Maximum Drive current: 3000 mA(10w)
• Maximum LM: 1000LM at 3000 mA
• Typical LM: 300LM at 700 mA
• Voltage: 3.2-3.7V.

I have a 12v 2A psu and one of those 12-24v 8 Amp ebay dimmers.
http://www.ebay.com/itm/Manual-Dimmer-S ... Sw3utY6m6v

So the issue is to drive the 3.5v LED via the dimmer which is designed for 12+volts. To get the LED running at around 3.xVolts I'd need a 4 Ohm resistor if my Ohms law is close...

12V-3.5V=8.5V/2.0 Amps=4.25 Ohms. Rounded down to a 4.0 Ohm 10 Watt resistor.

If this is more or less correct, can the resistor be added to the output of the dimmer? because the dimmer doesn't function at low voltage, and does the resistor need to be 10 Watt? My plan is.. 12V PSU > Dimmer > resistor > LED > happy!

Would that work? This stuff makes my brain do a divide by zero, which is why I ask here.

PS: this is to replace the 6v 10 watt halogen in my Zeiss standard.

aidanmoore · Post by **aidanmoore** » Wed Apr 12, 2017 9:15 am

Hi Gary,

Driving high power LEDS is easy if you find the right power supply, but it can be a mess if you try hacking your own solution.

You are pretty close, but not quite right on with the resistor value....4 ohms will drop enough voltage if the current is 2A to give you a (2) * 4 = 8V drop, which leaves 4V to possibly overdrive your LED.

But the real problem is the power dissipation of the resistor. If you drop 8-9V at 2A across the resistor you have closer to 15-20W peak power dissipation. Ceramic resistors in that power range are big and expensive, and you need to heat sink them to keep them from cracking near maximum power dissipation. So the heat issue really precludes this solution.

Much better to see if there is a dimmable 3.2, 3.3 or 3.4V power supply out there, or consider something like this:

https://www.amazon.com/DROK-Converter-V ... B06VWT6CYF

To use a buck DC/DC converter, you would need to either fix the voltage at 3.2-3.4V or remove the trimmer potentiometer and add an external pot with a few series resistors to keep the voltage limited.

If someone here can advise you of a proper power supply for this type of Cree LED it would be the ideal solution.

siliconGary · Post by **siliconGary** » Wed Apr 12, 2017 9:55 am

I am using the same LED on my microscope illuminator. I am driving it with a 2100mA BuckBlock driver http://www.ledsupply.com/led-drivers/bu ... led-driver that I think Pau put me onto. That is powered by an old iPad power supply, and is dimmable with a 20k potentiometer. They don't make a 3000mA one, but driving that LED at 2100mA I found plenty bright enough for my microscope (BH2 BF/DF epi-illuminatior)

dolmadis · Post by **dolmadis** » Wed Apr 12, 2017 10:08 am

That is powered by an old iPad power supply

What is the ?volt/?m'amp (?)spec on that please if it is to hand.

Thanks

BR

John

Pau · Post by **Pau** » Wed Apr 12, 2017 11:12 am

siliconGary,
yes I think it was me but who put me in that way was Charles Krebs. It works very well with lots of light, enough for DF, X Pol and DIC.

GaryB wrote:PS: this is to replace the 6v 10 watt halogen in my Zeiss standard.

So you want to put it inside the microscope base under the collecting lens *.
I think that in that position you will have cooling issues with a so powerful LED, I mount it outside the microscope at the position of the halogen 100W lamp house or inside the 60W tungsten lamp house glued with thermal paste to an aluminum heat sink.

I think you will be better served with a less powered Cree XT-E

The only issue with the Buckblock drivers is the minimum intensity, likely too high for BF at low power. A strong ND filter at the base or under the condenser corrects this issue

aidanmoore wrote:But the real problem is the power dissipation of the resistor. If you drop 8-9V at 2A across the resistor you have closer to 15-20W peak power dissipation. Ceramic resistors in that power range are big and expensive, and you need to heat sink them to keep them from cracking near maximum power dissipation. So the heat issue really precludes this solution

When testing with my friend Francisco with a power supply and resistors like you say we did burn all small ones, big ceramic ones went so hot that they began fuming and the wire soldering fused. Much better to avoid all that issues and use a constant current driver like the Buckblock

* like: http://www.fluorescencemicroscopy.it/en ... o_led.html
or: http://www.lenaturaliste.net/forum/view ... t=0#p33364 but don't do it without a heat sink

GaryB · Post by **GaryB** » Wed Apr 12, 2017 2:21 pm

Thanks for the replies all, it's good stuff to ponder.

Why do LED's always end up a pain in the rear. I wish someone would make one of these modules with a pot attached so when V and I are set, it can just dim from there. It seems like there's a market for it but strangely nobody has done it. The LED market is mature enough now that this would seem an obvious step.

I already have a Cree XT-E 5W and a 10W Cob. The 10W works fine with my current setup but it's a 9 LED array, so not ideal. The Cree XT-E is giving me the same headache as described in the first post because it's 3.xV 900 mA and I have no way to dim it right now.

mawyatt · Post by **mawyatt** » Wed Apr 12, 2017 2:48 pm

Gary,

Check my post on the Voice Coil Driver here.

http://www.photomacrography.net/forum/v ... c&start=15

Simply remove the DAC. Replace Rd with a 10K Pot with the wiper connected to the + Op amp input and the other end connected thru a 20K resistor to Vcc. Replace the Voice Coil with your LED (observe polarity) and remove the snubber network. Set Vcc to 8 volts and you are in business. The Op-Amp can be any low voltage single supply type. Use a modest heat sink on the power transistor.

If you want to get a little fancier you can create a voltage reference for the pot with a Zener diode.

Total cost probably less than $2 for the parts!

Best,

Mike

Saul · Post by **Saul** » Wed Apr 12, 2017 2:59 pm

I'm using 12v
http://www.cree.com/led-components/prod ... lamp-xhp70
and
http://www.ebay.com/itm/DC-12V-8A-LED-L ... SwZQRYZ~ol
with this type of the 12v 20a power supply
http://www.ebay.com/itm/Universal-DC-1A ... c_Gjkr3PoQ
on the
http://www.photomacrography.net/forum/v ... 209#198209

works great

Pau · Post by **Pau** » Wed Apr 12, 2017 3:26 pm

GaryB wrote:Thanks for the replies all, it's good stuff to ponder.

Why do LED's always end up a pain in the rear. I wish someone would make one of these modules with a pot attached so when V and I are set, it can just dim from there. It seems like there's a market for it but strangely nobody has done it...
...The Cree XT-E is giving me the same headache as described in the first post because it's 3.xV 900 mA and I have no way to dim it right now.(

There is such solution for the XT-E:
http://www.ledsupply.com/led-drivers/bu ... ed-drivers
Just select a model with the adequate mA current pot for your LED and built in dimmer and connect i to a DC supply (12 V will work*, voltage is not important as you are directly controlling the intensity). Reference: 3023-D-E-1000P
Current white XT-E have a maximum 1400mA, so the 1000mA will be ideal to drive it

*I use two of them , 700 and 1000mA, at my fluorescence illuminator fed at 12V and working nicely

GaryB · Post by **GaryB** » Wed Apr 12, 2017 4:08 pm

Once again, great info all

Looking at my options, I think I'll go with Pau's solution. It looks like the least amount of headache and will work with what I have and should fit in the base of the stand and use the same hole as the original brightness knob, and have enough room for decent heatsinking and a fan.

Thanks guys!

GaryB · Post by **GaryB** » Wed Apr 12, 2017 8:59 pm

Well I decided to kill *one* big problem, I just bought one of the field lens/prism assemblies to replace the underside mounted no prism assembly so now I'll be able to mount whatever light I want with all the cooling I need outside the base and not worry about internal light mounting or heat issues.

I can setup my own collimation and dump the diffuser

dolmadis · Post by **dolmadis** » Wed Apr 12, 2017 11:32 pm

connect it to a DC supply (12 V will work*, voltage is not important as you are directly controlling the intensity).

I am hoping that I have selected the right spec, 12v DC 1000ma?

https://www.amazon.co.uk/Adapter-Power- ... 12v+1000ma

Thanks

John

Pau · Post by **Pau** » Thu Apr 13, 2017 3:41 am

Gary, this seems a good decision, I did it long time ago with my first Standard.
To mount the lamp I use the original tube (the model without lenses inside) and the collimating lens of the lamp house 100* or the lamp house 60 modified with a rod made by member 75RR**

* http://www.photomacrography.net/forum/v ... p?p=201937
** http://www.photomacrography.net/forum/v ... p?p=174301

Ichthyophthirius · Post by **Ichthyophthirius** » Thu Apr 13, 2017 6:36 am

GaryB wrote:I just bought one of the field lens/prism assemblies to replace the underside mounted no prism assembly

There were two common versions (there were in fact even more), 46 70 57 (for Phomi III and Universal) and 46 70 58 (that's the one you want; for Standard) which differ in their collector lenses (the reason is the difference in the size of the bases of the different models) but the number is rarely written on the Diaphragm insert unit. So you either have to ask the seller where the Diaphragm insert came from, or be lucky.

GaryB · Post by **GaryB** » Thu Apr 13, 2017 8:45 am

Pau,

nice setup you have

Yeah, I thought it would help make life easier. It opens up a lot more possibilities for fiddling in the future too.

Ichthy,

I've seen a few different versions, this has no number on it but it looks like the right fit, if not I can modify it to suit. I have a feeling it may be from an old GFL. I took mine out and did a visual comparison of the mounting and it looks good. If it is from a GFL the distance from lamp to field lens *may* be different so I have some experimenting to do. If it's from an old WL I can use existing drawings to make the aluminum mounting rod. I'll have to see when it arrives and do some bench testing.

I couldn't resist, it was $15 with free shipping