Electronics > Beginners
6V LED driver - old microscope upgrade
jmp76:
Hi,
THE solution to drive a LED till 12.5 v à 3A is here: http://pichotjm.free.fr/Docu/Microscopie/Eclairage/_Modif_BHS.php
With this solution you will be able to drive LEDs with low, medium and high currents as necessary. Solutions are given for auto fluorescence with green light (red vision) or with Royal Blue light (green vision). Solutions are given for epi scopy (reflection) and dia scopie (pass thru).
Currents are delivered with 1024 step values. And are OK for LPA (polarisation) or DIC (differential contast Nomarski) in bio or metal options. You can do DIC photos @800 ISO and 1/800 s !!! (these functions are in research microscopes, very expensive ones: > $5000)
Text is in french, but you can use Google Translate. May be no need to translate...
Regards
Tsou:
I've made some improvements in the design of my PCB. The trace between R4 and R5 was too long and not large enough, so it has almost 20% of R5's resistance.
Here I did a copper filled zone which include R5, R4 and the source of the transistor. I also shorten and thicken the trace between the opamp output and R4 but I'm not sure if it's necessary...
Here's the new design, if it can help anyone else...
Zero999:
--- Quote from: Tsou on November 09, 2018, 03:40:32 pm ---I've made some improvements in the design of my PCB. The trace between R4 and R5 was too long and not large enough, so it has almost 20% of R5's resistance.
Here I did a copper filled zone which include R5, R4 and the source of the transistor. I also shorten and thicken the trace between the opamp output and R4 but I'm not sure if it's necessary...
Here's the new design, if it can help anyone else...
--- End quote ---
What provision have you make for heat singing the MOSFET? It will dissipate a considerable amount of power and I don't see any room for a heat sink.
The suggestion made by the previous poster, of replacing the 6V power supply with an LED driver was a good one. I understand you want to use the original potentiometer to control the brightness, which rules lots of options out. Do you know the value of the resistor?
Have you considered using a switched mode power supply? It's fairly easy to make an LED driver with a comparator IC, such as the LM393. That would eliminate the heat sink issue.
Here's an example of am :LED driver kit which uses the LM393. It should be fairly easy to modify it to suit your needs.
https://www.velleman.eu/products/view/?id=366712https://www.velleman.eu/downloads/0/illustrated/illustrated_assembly_manual_k8071_rev1.pdf
Tsou:
I'll buy a ready-made circuit to power the LED in case my circuit does not work, but I'd also like to make this circuit, for education purpose, to go from design to final product.
For the MOSFET, if I have 1.3A, a Rds_on of 0.1ohm max, I have 1.3A*0.1ohm = 0.13V then 0.13V * 1.3A = 0.169W. The 'Thermal resistance junction-ambient max' is 62.5°C/W so I have 10.6°C more than the ambient temperature, no? I can also go for a IRF3205 which has 0.008ohm of Rds_on to lower the power dissipation in the mosfet
Zero999:
--- Quote from: Tsou on November 10, 2018, 11:48:46 pm ---I'll buy a ready-made circuit to power the LED in case my circuit does not work, but I'd also like to make this circuit, for education purpose, to go from design to final product.
For the MOSFET, if I have 1.3A, a Rds_on of 0.1ohm max, I have 1.3A*0.1ohm = 0.13V then 0.13V * 1.3A = 0.169W. The 'Thermal resistance junction-ambient max' is 62.5°C/W so I have 10.6°C more than the ambient temperature, no? I can also go for a IRF3205 which has 0.008ohm of Rds_on to lower the power dissipation in the mosfet
--- End quote ---
You've got the power calculation completely wrong.
The voltage across the MOSFET is not determined by the on resistance given on the data sheet, but the supply voltage and voltage drop across the LED.
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