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Halogen to LED conversion help

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Unread postAuthor: MrCrowley » Fri Feb 10, 2012 9:03 pm

Okay, so for some reason the LED has started turning off instead of dimming when the potentiometer is turned slightly anti-clockwise from its full value.

I've decided to go the route of something like this (.pdf file).

As a reminder, this is the LED I'm using (Forward Voltage: 3.2v-4.0v;
Forward Current: 300mA).

I'm confused as to how the above driver module is able to dim the LED with a pot. and not using PWM; as it seems you can either use a pot. or PWM.

Also, it says the dimming range is 1A to 0.35A. Since my LED works at 300mA, is it just a matter of using a different pot. on the driver module so I don't blow the LED and am able to get a decent dimming range? Would it be easier to use something like this and add my own pot. to it?

I'm currently using a 12v 500mA cellphone charger as the power supply.
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Unread postAuthor: Technician1002 » Fri Feb 10, 2012 10:33 pm

The module is a current regulator. The pot adjusts the regulator current just the same as a simple pot can be used in any op amp circuit to adjust a voltage regulator. The one you linked to is too high current for your selected LED. It will drive your LED at full power at the minimum setting.

To comment on some prior posts, a filter in the webcam with the original lens expects nearly parallel light entering the sensor. When adapted to a microscope with a different lens (that may be much closer to the sensor) the angle of light passing through the filter at the perimeter will be different, which may change the response of the filter. A thin film filter will change response depending on the angle the light passes through it. Your microscope lens is closer then the original lens I presume.

To answer another post regarding the voltage on a diode, the forward breakdown (conduction) voltage is relatively constant regardless of the current, so they need current regulation. Voltage regulation only can allow wide variations in current at any given voltage. The voltage varies by factors such as the type of diode and it's temperature.

This graph shows the voltage and current not to scale showing the current verses voltage for a rectifier diode.

Image

Note from 0 to 0.7 volts the current is nearly nil. From 0.7 to one volt is enough to burn up the diode. The other side of the graph is highly compressed in scale showing the inverse polarity showing the diode in an off state until the peak inverse voltage is reached.

An LED has a higher forward voltage than a rectifier diode. The voltage is dependent on temperature and the color of the diode. An LED also has a much lower PIV, and may be damaged by reverse current so it is not recommended to run them from AC power with only a current limiting resistor. A diode to prevent reverse current is recommended for AC use.

Here is a typical forward current graph for a popular LED. The difference in proper forward current and LED destroyed is a very small voltage change. This is why current is regulated, not voltage. The VF for the diode MrC is using is 3.2-4 Volts.

Image
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Unread postAuthor: MrCrowley » Fri Feb 10, 2012 10:50 pm

Technician1002 wrote:The module is a current regulator. The pot adjusts the regulator current just the same as a simple pot can be used in any op amp circuit to adjust a voltage regulator. The one you linked to is too high current for your selected LED. It will drive your LED at full power at the minimum setting.
So I could use AA0593 which is a 0-350mA Dimmable LED Driver Module and add my own potentiometer? If so, what value potentiometer would be best?

Sorry that I'm not replying to the latter part of your post at the moment, I did read it it's just that I'm seeing if I can get this fixed before the store closes as I might be going away the next few days.
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Unread postAuthor: Technician1002 » Sat Feb 11, 2012 12:24 am

That module will work great. You can use a pot to dim it using the analog input (Pin 2). Using the provided graph on page 3 of the PDF, you can see if the control voltage is at 0V the module outputs full current. As the voltage is raised up to 5 volts, the current drops to zero.

Leave the PWM pin 3 disconnected when using the analog control.

If your power supply is 12 volts a good combination that will work well is a 5K ohm pot with one end grounded, the wiper to pin 2, and the other pot end tied to a 10K Ohm resistor which connects to 12 volts power. The divider of 10K + 5 K pot will provide 1/3 of 12 volts to the pot or 4 volts at one pin and zero on the other. This will allow the pot to provide 0 volts (full bright) to 4 volts (Dim but not out) to the LED. That module is a good match for your selected LED.

If you want to dim the LED all the way to zero, change the 10K resistor to 6.8K. This is not recommended as the dim setting will be quite dim.
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Last edited by Technician1002 on Sat Feb 11, 2012 12:32 am, edited 1 time in total.
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Unread postAuthor: MrCrowley » Sat Feb 11, 2012 12:30 am

Thank you very much for that, Tech.

Your microscope lens is closer then the original lens I presume
Interesting thoughts about why I'm getting the strange colours with the webcam, definitely something I'll have to check out. As for the above quote, the microscope lens is further away from the camera sensor than the original webcam lens was. This is because the only lens on the microscope when the webcam is in use is one of the 10, 20, 40, 100x objectives with the eyepiece optics removed. That makes the lens about 15cm away from the sensor compared to the original webcam lens being probably less than a centimeter away.
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Unread postAuthor: MrCrowley » Thu Feb 16, 2012 7:16 pm

Hi Tech,

I tried wiring it up but I think I haven't followed your instructions correctly. As per the diagram below, I was measuring about 2.5V on a voltmeter at terminals 5 and 6 and less than 3mA. The readings didn't seem to change much when the pot. was turned back and forth.

On the other hand, if I connected terminal 1 straight to the power supply instead of after the resistor (as in the diagram below), I measured about 12.5V and ~350mA that could be adjusted from about 50mA up to 350mA with the pot.

I can't work out what I'm doing wrong but I have a feeling you'll know the answer.
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Sorry about the unprofessional circuit diagram, I'm hopeless at drawing proper circuits.
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Unread postAuthor: dewey-1 » Thu Feb 16, 2012 7:38 pm

Pin of 1 of the module has to go to power supply plus not the pot as you have it drawn.

That is what Tech stated.
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Unread postAuthor: MrCrowley » Thu Feb 16, 2012 7:50 pm

Yeah I tried that originally and I read ~12V and~ 50-350mA (depending on pot.) at terminals 5 and 6 using a voltmeter. The problem being the 12V, as I need it to be 4V.
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Unread postAuthor: Technician1002 » Thu Feb 16, 2012 11:21 pm

It is a current regulator, not a voltage regulator. When a load is connected, the voltage the diode uses will be supplied as an open load will provide supply voltage.

This module can drive several LED's in series. If you use one that runs at ~3 volts at 300 ma, it will run at ~3 Volts. If two are connected in series, it will still provide regulated 300 ma but at ~6 Volts. this is how a current regulator works. You set the current.

A voltage regulator holds the voltage whether you draw 5 mA or 500 mA. A current regulator provides a set current such as 300mA whether at 2 Volts or 8 Volts.


If you want to test a module, use a 10 ohm 10 watt resistor. At 300 mA it will have 3 Volts on it.
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Unread postAuthor: jackssmirkingrevenge » Fri Feb 17, 2012 1:07 am

speaking of which, ordered a couple of this type of LED bulb: http://www.ebay.com/itm/6W-Warm-White-M ... 1e683ec1fd

... just to see what the fuss is about :)
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Unread postAuthor: mobile chernobyl » Fri Feb 17, 2012 1:33 am

I'll admit I haven't read this thread in it's entirety, but the topic is familiar to me.

Have you considered the light output range of the led? most cheap ones (like in your first post) sit in the blue spectrum of white - usually higher than 5000K (more like 6500K and up).

With a halogen your looking at around 3-3500K - so your getting a lot more red in the light output spectrum - this is essential for viewing tissue samples or other parts of living organisms.

They're are readily available LED's down in this spectrum - but your going to have to search outside the cheap-o world of LED's for a decent output model.

I know you were talking about filters but I'm not sure on which end you were using them.

otherwise cool project! I was hoping you were retrofitting halogens used elsewhere though (perhaps a motor vehicle... :D )
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Unread postAuthor: MrCrowley » Fri Feb 17, 2012 1:34 am

Thanks again for that Tech, think I follow you now.

jackssmirkingrevenge wrote:speaking of which, ordered a couple of this type of LED bulb: http://www.ebay.com/itm/6W-Warm-White-M ... 1e683ec1fd

... just to see what the fuss is about :)

Haha, I've been meaning to get something similar for the workbench in the garage; need something with a little more direction and a little more light.
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Unread postAuthor: MrCrowley » Fri Feb 17, 2012 6:21 pm

Technician1002 wrote:It is a current regulator, not a voltage regulator. When a load is connected, the voltage the diode uses will be supplied as an open load will provide supply voltage.

This module can drive several LED's in series. If you use one that runs at ~3 volts at 300 ma, it will run at ~3 Volts. If two are connected in series, it will still provide regulated 300 ma but at ~6 Volts. this is how a current regulator works. You set the current.

A voltage regulator holds the voltage whether you draw 5 mA or 500 mA. A current regulator provides a set current such as 300mA whether at 2 Volts or 8 Volts.


If you want to test a module, use a 10 ohm 10 watt resistor. At 300 mA it will have 3 Volts on it.

Tech, can I ask for a basic circuit diagram?

I tried hooking up a spare LED assuming it would only draw 3v or whatever like you said, but to no surprise it blew as it was receiving 12v. Surely there must be a resistor somewhere in the circuit or something that stops the LED from being blown.

Edit:

If anyone can make a circuit diagram for this it would be much appreciated. It's simply not working the way it's supposed to judging by what you guys have said about the circuit. Even my dad who knows a fair bit about electronics can't figure it out. I appreciate your help Tech but we cannot understand what you're saying without a circuit diagram and my dad is confused by the circuit diagram in the PDF at the top of page 3 which doesn't show the potentiometer in the circuit and has two resistors (6.8k and 15k).

I'll try and drop by Jaycar next week and ask them for assistance as well.
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Unread postAuthor: Technician1002 » Fri Feb 17, 2012 8:11 pm

The diagram in the PDF is for using a 0-10 volt control. The two resistors shown simply divide the 0-10 volt control signal down to 0-4 volts on the module. If you fed +10 volts as shown and replace the 6.8K resistor with a pot to sweep from 0-4 volts that would work also. Since we are not using 10 volt control signal, using the supply 12 volts instead I gave values that are actually sold for a replacement control. (you can buy a 5K pot, but not a 6.8K pot) A fixed resitor to 12 volts instead of 0-10 volts is used. Instead of the second 6.8K fixed resistor, a pot is placed there to feed 0-4 volts to the control pin.

0-10 volt control is used in many lighting systems. The PDF shows the example circuit to interface with a 0-10 volt dimmer console. This is mostly obsolete as DMX-512 is the new control standard. DMX to 0-10 is sometimes used to control other lighting systems such as LED lights.

Give me some time to find a current limiter online so I don't have to try to draw one.

Edit. Found a basic current regulator. They work by functioning as a low voltage regulator with a fixed resistor load. In the image below, the shown LED can be between the transistor and ground as a current regulator can be placed anywhere in a series circuit.
Image
This one is not adjustable as it uses a fixed resistor with a fixed voltage on it to set the current. Adjustable regulators often have and additional comparator circuit such as a second transistor or op amp to make the voltage applied to the resistor adjustable thus adjusting the current.
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Unread postAuthor: dewey-1 » Fri Feb 17, 2012 8:41 pm

MrC

When you refer to spare LED, what does that mean?
If it refers to a typical LED, the maximum current is about 30 mA.

I am making a schematic for you.
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