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Driving LEDs. Help needed

 
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Pau
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Joined: 20 Jan 2010
Posts: 4941
Location: Valencia, Spain

PostPosted: Thu Jun 30, 2016 11:40 am    Post subject: Driving LEDs. Help needed Reply with quote

I've just bought some LEDs for my fluorescence microscope project that I've resumed after a long gap

My previous posts on the subject:
http://www.photomacrography.net/forum/viewtopic.php?p=176093
http://www.photomacrography.net/forum/viewtopic.php?p=176091

I've got finished the mechanical part and now I need to deal with the electronic parts being an absolute ignorant on the subject

My question is about how to drive them. I'm aware that is better to regulate by current than by voltage, but after spending countless hours searching online I'm not clear about what the needed parts are

My requirements are pretty simple:
- one LED at once
- maximum power inside safe limits
- Frequency dimmable if doable, I already have a PWM dimmer
- if possible one single source to feed them. I'm thinking in a ATX computer source as I have it and in principle seems a well regulated DC source

The LEDs specifications are:
1- DC Forward Current: 700mA / DC Forward Voltage: 3.4V - 3.8V
2- DC Forward Current (IF): 350mA~1000mA / DC Forward Voltage (VF): 3.2V-3.6V
3- DC forward current 1500 mA / Reverse voltage 5 v / Forward voltage (@ 350 mA, 85 °C) 2.85v-3.4v
4- DC forward current 3000 mA / Reverse voltage 5 v / Forward voltage (@ 350 mA, 85 °C) 2.85 v-3.4v

Could anyone of you provide me direct links to the needed parts?

Thank you in advance
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mjkzz



Joined: 01 Jul 2015
Posts: 1231
Location: California/Shenzhen

PostPosted: Sat Jul 02, 2016 4:19 am    Post subject: Reply with quote

Are you going to use it pulsed (LED flash) or continuous? If continuous, you can get almost any LED driver on the market, just make sure the driver takes DC (in your case 12V DC) or simple take the driver that gets AC.

These drivers deliver constant current at specified rating. For example, if you plan to use an LED with 700ma rating, just get a driver with that spec.
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Pau
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Joined: 20 Jan 2010
Posts: 4941
Location: Valencia, Spain

PostPosted: Sun Jul 03, 2016 10:16 am    Post subject: Reply with quote

mjkzz wrote:
Are you going to use it pulsed (LED flash) or continuous? If continuous, you can get almost any LED driver on the market, just make sure the driver takes DC (in your case 12V DC) or simple take the driver that gets AC.

These drivers deliver constant current at specified rating. For example, if you plan to use an LED with 700ma rating, just get a driver with that spec.


Thank you very much, I just want to use the LEDs for continuous illumination, but will be very desirable to dim them with PWM, specially the higher power LEDs.

So, if I understand well, despite rated for example "DC Forward Current: 700mA / DC Forward Voltage: 3.4V - 3.8V" I can feed it with 12V DC and only 700mA maximum current is relevant?
Can I mount the PWM before the driver? (this would allow to feed several drivers trough a single PWM)

For example, would this driver be adequate for the aforementioned 700mA LED powered by a 12V DC supply?
http://www.ebay.com/itm/DC-2-26V-DC3V-5V-700mA-Dimmable-LED-Driver-/251568344712?hash=item3a92a44e88:g:eZsAAOSwq5lTqHP0
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mjkzz



Joined: 01 Jul 2015
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PostPosted: Mon Jul 04, 2016 5:57 am    Post subject: Reply with quote

The specific driver should work for continuous light. That specific driver can output 3-5V meaning that it can drive any LED with Vfd between 3-5V and deliver constant current of 700mA (730ma at 24V as in item description).

However, it seems that specific driver is dimmable via some sort of control voltage, not PWM. Some LED driver dims light by lowering output current instead of PWM on output, this means the output is "continuous", not a square wave, so theoretically no flickering, or not noticeable for photography. There might be some high frequency flickering (100s of KHz or even Mhz) because most driver circuit uses some sort of high frequency switching to control current, so there might be flickering, but at very high frequency. I only had flickering issue when I built an LED light for high speed video (1000fps)

You can search for a driver with PWM dimming input. This does not necessarily mean the output is PWM, the driver circuit simple converts that PWM signal into lower output current. But output COULD be by PWM and cause flickering, particularly at low output level, it really depends on how those drivers are designed.
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nucleobyte



Joined: 22 Aug 2010
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PostPosted: Mon Jul 04, 2016 6:49 am    Post subject: Reply with quote

Hey Pau, it would help to know the specifications on your PWM device also.
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Pau
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PostPosted: Mon Jul 04, 2016 10:25 am    Post subject: Reply with quote

Thanks, my PWM dimmer is like this one:
http://www.ebay.com/itm/LED-Switch-Dimmer-Adjust-PWM-Cotroller-For-Office-Lamp-Strip-Light-8A-/231999465066?hash=item36043ee66a:g:GyAAAOSwvg9Xd0ek

It came with a microscope illuminator and actually it flickers interfering the camera LV when dimmed, not a major issue for my fluorescence application where the exposure time will be long. I have another one at 21Kz that doesn't flicker
http://www.ebay.com/itm/21KHZ-high-speed-Pulse-Width-Modulator-of-retroDIODE-microscope-illuminators-/262245829643?hash=item3d0f11cc0b%3Ag%3AG30AAOSw%7EgRViHnj&nma=true&si=nNDPOu1DRFxhXNMMsPRMwhOdX8A%253D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557
I don't have its specifications
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nucleobyte



Joined: 22 Aug 2010
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PostPosted: Tue Jul 05, 2016 6:22 am    Post subject: Reply with quote

Hey, this may be more complicated than you were expecting Sad It looks like the PWM dimmer you have does switch the full load. This could be a problem for connecting a constant current regulator down stream (in series) since the CC regulator may not like being fed a PWM power source. It may work fine with LED lamps are run close to the regulated voltage with a resistor for current limiting, but not so well once a CC regulator is placed in-line.

CC regulators like this http://www.ebay.com/itm/401053630162 work by having a cooperating PWM input ( the little header on the far side of the image) which expects signal level (+5) PWM signal to switch the chips output enable on and off. A set of boards like this tuned for each of the different current requirements and a small PWM signal generator may be one inexpensive solution. (you can find those CC boards for as little as $2) and there are lots of 555 timer type circuits to generate the PWM signal.

You could simply try an inline resistor for current limiting, but it would have to be sized for the power requirement. e.g. if you need to drop 5 volts to 3.5 (difference of 1.5V) at 1 amp you would need a 1.5 ohm resistor rated for at least 1.5 W, which would also generate some heat. Your current PWM device should then work. This is impractical if you are using 12V for a 3.5V LED though, since far too much power/heat would be wasted in the resistor.
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mjkzz



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PostPosted: Tue Jul 05, 2016 7:43 am    Post subject: Reply with quote

The board nucleobyte's link should work well (quality aside). This type of constant current drivers are very efficient and generate less heat compared to using a resistor to limit current. Your other PWM generator (black enclosure) MIGHT have a standard TTL PWM signal output to control this driver.
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Pau
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PostPosted: Tue Jul 05, 2016 8:45 am    Post subject: Reply with quote

Thanks both for your kind assistance Very Happy

Unfortunately I don't really understand all terms Sad

This is the idea of the very simple circuit I'm planning (sorry for the naif drawing):


Only one LED will be connected at once

- Would it work?
- Could you link me a specific part for each box? (dimmer and drivers)
- Is better to use dipolar switches?
- Any simple ideas to improve it?

Thanks again!!!
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Charles Krebs



Joined: 01 Aug 2006
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Location: Issaquah, WA USA

PostPosted: Tue Jul 05, 2016 12:47 pm    Post subject: Reply with quote

Pau,
I am an electronics novice, so I suspect there are many more sophisticated ways to solve your problem. But for all my LED projects so far I've utilized drivers from www.ledsupply.com . They are extremely simple to set up and to dim. They are high frequency PWM and I have never experienced any flicker or issues with photography.

You have a varied range of driving currents:

The LEDs specifications are:
1- DC Forward Current: 700mA / DC Forward Voltage: 3.4V - 3.8V
2- DC Forward Current (IF): 350mA~1000mA / DC Forward Voltage (VF): 3.2V-3.6V
3- DC forward current 1500 mA / Reverse voltage 5 v / Forward voltage (@ 350 mA, 85 °C) 2.85v-3.4v
4- DC forward current 3000 mA / Reverse voltage 5 v / Forward voltage (@ 350 mA, 85 °C) 2.85 v-3.4v


One thing you don't mention is whether or not these are the "maximum" values from the specifications or the desired drive values. They may not be the same. Generally in the specifications they provide the output ratings at drive currents considerably less than the absolute maximum value. Depending on the LED there may not be as large an increase in light output as might be expected if the current is increased to the maximum (it is not a linear relationship). A very practical reason to consider your drive current is that increasing the current to, or close to, maximum rating can require a much larger heat sink. (For example, the LEDs I used for my lights have a max rating of 700mA at the forward voltage. But all the performance specs use a drive current of 350mA. In order to keep the heatsinks to a modest/small size I chose one that should be able to handle the heat when driven at 350mA-500mA. When driven at 350mA the heatsinks seem OK (they still get pretty hot). If I go to 500mA they get very hot and I do not like to use them for more than a brief period. At 700mA I would expect even more heat and eventually possible damage to the LED. I could drive them at about 700mA, but I would have needed to use a much larger heat sink and the lights would have become too bulky for their intended use. If you are not concerned about the heatsink size you can approach the maximum continuous rating... but it might depend on the location where the LEDs need to placed and the heat sink size you can accommodate.

LEDSupply sells drivers that would meet your need except for the 3000mA one. They have models that can be dimmed a couple of ways, the simplest being either a 5K or 20k ohm linear taper potentiometer (variable resistor). For units under 100mA the even sell a cheap wiring harness that has the potentiometer already attached. The models you would use could all be operated for an inexpensive 12v DC power supply ( http://www.ledsupply.com/power-supplies or just about anywhere ).

Here are some pages you can look at to see the drivers I am suggesting.

http://www.ledsupply.com/led-drivers/buckpuck-dc-led-drivers
350mA, 700mA, 1000mA

http://www.ledsupply.com/led-drivers/buckblock-dc-led-driver
1000mA, 1400mA, 2100mA
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Pau
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PostPosted: Wed Jul 06, 2016 3:23 pm    Post subject: Reply with quote

Charles, thank you for your advice and links, these drivers seem very adequate but for the 3000mA at maximum power.

The current at the posted specifications of my LEDs are maximum values, so they will work nicely with a lower current or voltage dimmed, in fact I'm surprised that LEDSupply dim them with potentiometer more than with PWM but this will match well the Cree LEDs specifications. I'm rethinking the circuit design to save on dimmers (actually I only need one dimmer for each power because only one LED will light at once), 3 or 4 drivers and dimmers will suffice putting the switches after them.
Even the 2200mA could be sufficient for the 3000mA LED
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mjkzz



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PostPosted: Wed Jul 06, 2016 4:02 pm    Post subject: Reply with quote

Thanks Charles, I forgot about LuxDrive, well built, I used one of their early modules (too) long time ago when I was in US.

It seems the A009 series modules should work well. In the document, they mention PWM of 200hz and still no flickering, it means it is converting PWM into control voltage, not the output.

The fact that a D/A converter can be used to dim output is exactly what I need. Thanks again.
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