Microscope Lamp Power Supply and Inc. Bulbs question, AC/DC?

[ericmark]

Bulbs like this one

20mW 6V polarity sensitive at £5 for pack of 5 would likely work OK at 0.005 amp not sure of your bulb base but viewing angles vary and also colour with white and warm white and as Bernard says much will depend on what you are studying. The advantage of LED is when studying items in liquid it has very little or no heat so does not dry it out. They also last a long time.

My interest is general photography and the microscope was the next stage with macro. However it is not as good as expected one because the camera resolution is low, two flat items have different focus edge to centre, three the table will not drop low enough to put items like a watch under it. However bryophytes between two slides keeping them flat does work but the idea of taking samples to place under the microscope goes against the grain I want to where possible take pictures of the items in their environment. Using close-up filters and bellows in the main works better I can take a series of photos with overview and close up showing the environment as well as the item in question.

The ability to close the aperture on the camera means I get a far better depth of field so less stacking of images is required. However this means very little light gets to the camera CCD so I need very high light levels or very long exposure. I have used battery powered LED light strips and long exposure and flash guns. The latter was a problem as the lens tends to shield the light so I use two flash guns a slave triggered from the main one.

The colour spectrum I test with a CD. By angling a CD and looking at the reflection of a light source one can see the spectrum and if colours blend or have sharp lines where they change. The CFL has rather sharp lines but the LED is more gradual.

Your unit is it would seem far more powerful to mine so would need better lighting but since LED bulbs are DC I would look for a DC power supply so you could use LED in the future.

The simple charger like this

will likely work well but the more complex have electronics in so for you cheap is good. But if moving to LED then you need less power so a Wall plug type would have enough power. Do not buy one that says "stage" these are not suitable for what you want. You want a cheap type should according to output required be £10 to £25.

 

[bernardgreen]

Some LED lamps are fixed brightness having inbuilt current control. They cannot be dimmed by changing the voltage.

Eric . I would disagree with that battery charger, I have one and while it is very good for charging car batteries it is NOT a regulated power supply.

It is not un-common for transformers specific to a specific lamp to be wound to match the lamp not only in voltage but in resistance of the windings to limit the in rush current of the cold lamp to prolong filament life.

 

[Gully]

My bulb is obscure and I don't really want to waste time and money on that and it's controller now they have both gone. Maybe another time, but right now I want to be able to use my microscope.

So I ordered a '3W' LED, rated at 150 lm, 0.7A, 3.6V voltage drop. As a basic part I believe it doesn't have any inbuilt current control and can be dimmed. I also ordered a base, some solder and a small lens to focus the wide beam from my LED. I neglected to order a resistor though being unsure of the final power supply I would be using. I was also really tired and sort of thought I would find something suitable until I come up with a final solution... now I'm worried that was a mistake.

But I also don't really want to order one tonight and have to wait an extra day for it so my solution is to use my variable voltage train controller for now. This is based on the theory that an LED is not damaged by too low voltage and does not need a resistor if the voltage is the correct voltage for the LED (Anyone reading this please note I have since found out this is incorrect information). So I thought I'd wire my LED in series with my power supply set at about 3V and slowly work up until the LED illuminates at around 3.6V. Then I will leave it there and be careful not to knock it up! That should work until I have a suitable fixed power supply and resistors for my final set up.

As I've said, ultimately I would like to rehouse my power supply in the original box and use a set of resistors to gain a range of voltages off the rotary switch. I made a little circuit diagram to illustrate this:

Untitled

• Gully
• 21 Mar 2013
• 1

I haven't done a circuit diagram since school, but I think it's quite strait forward and should work. Based on this calculator and a 5V power supply I calculated that my primary resistor (furthest to the left) would need to be about 2 ohm with at least a 1w ratting. If I make each of the others 1 ohm I should get a good range of brightness.

What do you think?

 

[bernardgreen]

So I thought I'd wire my LED in series with my power supply set at about 3V and slowly work up until the LED illuminates at around 3.6V.

NO that is likely to burn out the LED

LEDs must have a controlled current supply, unlike a filament lamp they do not have any significant internal resistance, once the applied voltage is greater than the forward bias voltage the LED will start to conduct current and will take all the current the supply can provide. In effect the LED becomes a short circuit across the power supply.

The current MUST be controlled by the power supply.

LED lamps that run on 12 volts are an LED and a driver in a package, the driver controls the current,

 

[maltaron]

I am not up to date with modern high brightness LED'S but the early ones needed around 2 volts at 20mA max. Generally a 560ohm resister in series with the LED gave maximum brightness off 12 volts. Modern LED's may take currents. It must be remembered that LED's are a current operated device, i.e. constant voltage. In your circuit your resister values should be calculated to drop 3 volts at your selected current.

 

[bernardgreen]

It must be remembered that LED's are a current operated device, i.e. constant voltage.

Just to remove any confusion.

When talking about the LED element.

The constant voltage on the LED terminals is created by the LED when the current through it is within it's safe operating limits. The power supply ( or driver ) feeding the LED element must be constant current. ( or constant average current if in pulse mode control )

 

[maltaron]

Nicely explained, Bernard, For my interest do the modern high brightness LED's operate on higher currents or are they just higher efficiency.

 

[ericmark]

There are a number of ways to control a LED and when you buy just an LED as bernardgreen rightly says you normally need a resistor to limit the current. Not sure about white LED's red ones have a 1.2 volt drop across them which I used to lift a 7812 voltage regulator up to 13.2 volts.

The different colours have different voltages but clearly most people want to use LED's with a standard voltage 5v, 6v, 9v, 12v etc. So inside the package there is some built in voltage reducer. However it may also have other bits by flashing an LED one can over drive it and make it brighter clearly if this is in the package to may it dimmer is a problem.

So although the LED is a current beast the package is likely a voltage beast. The 7805 voltage regulator likely the easiest way get to 5v DC and using diodes with threshold voltage of 0.6 volt the multi position switch should work well.

 

[bernardgreen]

Nicely explained, Bernard, For my interest do the modern high brightness LED's operate on higher currents or are they just higher efficiency.

The short answer is both. Higher current and more efficient conversion of electrical energy into light.

Some LEDS are very efficient producers of ultra violet ( black light ) which is used to excite flourescent phosphers which produce visible light.

One way to get very bright and low energy ( long battery life ) warning lights is to pulse the LED at a high current ( several times normal ) with an ON - OFF ratio of 1 - 9 which means the pulse of light is several times the brightness at normal continuous current. The average current is often less than the normal continuous current but the light appears much brighter.

The eye sees it as continuously bright provided it is not moving across the field of vision. This method of getting a bright warning light is not suitable for normal lighting as it produces a strobe effect on moving objects.

 

[Gully]

Thanks guys. I did some research and finally managed to get my head around how this all works. You'll be glad to know that I have successfully illuminated the LED, without burning it out in anyway. I'm surprised by how quickly it gets hot though, I think I will need to get some kind of heat-sink. I still haven't got all my goods to put it together in the microscope, but once I do I'll be sure to post some pictures

Hopefully I'll have some micro shots to show you too

Thanks again. This is a great forum.