Recreate the power supply for a medical headlight (3W LED)? - Page 1
Forum section: electronics / projects

author: RK_aus_S
author_rank: Contributor
timestamp: December 17, 2023, 09:22:25 am
content: Hello everyoneMy dentist told me that a battery pack for his 3W LED headlight was very, very expensive and he was wondering if I could show him an alternative?In addition, the running time is only approx. 6 - 7 hours, which is too short for a day's use. So you should also increase the battery capacity.I analyzed the device (headlight + control unit on the belt) and found that the headlight is controlled as follows:The head light contains an approx. 2-3W LED (~2.45V forward voltage) and an NTC, which is probably attached to the cooling plate of the LED for temperature control.The battery consists of 2 x 18650 Li-Ion cells. The control for the LED headlight is based on a constant current source, which passes the full voltage of both batteries (~7.2V) to the LED, but with a current limitation of 900 mA.Three brightness levels can be set using a button, which are then implemented via a PWM signal of 250 Hz:High brightness: 100% duty cycleMedium brightness: 66% duty cycleLow brightness: 33% duty cycleSo it is possible to recreate something like this.Due to lack of experience, here is my question:Does anyone know whether there are already prefabricated power LED driver modules or integrated components for power LEDs that work with an adjustable constant current (up to approx. 1 A) and/or PWM for brightness control? They should work with 7,2 V Li-Ion Battery packs - and of course deliver a very reliable, stable brithtness during discharge of the batteries (since its purpose is for medical environments). If these modules even offer some kind of "input" or possibility to connect an NTC temperature sensor for overheating protection, then that would be more than perfect!Best regards

author: RoGeorge
author_rank: Super Contributor
timestamp: December 17, 2023, 10:32:12 am
content: All medical equipment is heavily overpriced.There are ready made headlamps, with zero flickering. Often used as camping equipment, or for speleological exploration. They might make a good donor. Search for headlight torch, or headlight flashlamp, on Aliexpress. As for the powering module only, search for LED driver. Look for those with DC output, zero flickering, NOT the PWM ones.PWM is horrible for long term use anyway, plus some patients might still see the flickering. You don't want an epileptic to look into flickering LEDs, while sitting in a dentist chair, and with a 30 000 RPM piece of spinning metal in the mouth.

author: RK_aus_S
author_rank: Contributor
timestamp: December 17, 2023, 11:50:45 am
content: Thank you for your advice.Of course, I have already googled and also found (cheap and expensive) LED drivers (AliExpress etc.).However, I haven't found a solution yet that doesn't require some "do-it-yourself" work, especially when it comes to including the temperature sensor (NTC) so that the LED switches off if it overheats.So before I start reprogramming the "rest" with a small microcontroller, I wanted to make sure that there isn't something ready-made.As for PWM brightness control, well, as I said, the original part works like this. I don't think you can speak against PWM in general, as long as the frequency (here: 250 Hz) is far higher than what human eyes can perceive...

author: tooki
author_rank: Super Contributor
timestamp: December 17, 2023, 01:05:06 pm
content: PWM is fine if the frequency is high enough. But 250Hz isnowhere closeto high enough*. For most people, 2-3KHz is enough to never flicker, but some people are more sensitive, and there are edge cases where >30KHz are needed!It's often good practice to use a PWM frequency above 20KHz simply to ensure that the PWM frequency is outside of the range of human hearing, since some circuits will audibly whine at the PWM frequency (or some harmonic of it).*No, the 60Hz accepted for moving pictures isn't the same.

author: KerimF
author_rank: Frequent Contributor
timestamp: December 17, 2023, 02:08:08 pm
content: For this, I guess, one may start with a ready-made SMPS whose output is 12V/2A.A linear regulator, as LM7805 (its dissipation power could be 500mW without heatsink), can supply the MCU board (up to 70mA). I usually add a resistor (Rs) in series between the 12V SMPS output and the LM7805 input to share the extra dissipated power with LM7805. I also add a capacitor (Cp, around 100uF) at the LM7805 input and ground.A DC current source of 900mA that uses a medium power PNP transistor or Darlington can be added to supply the LED headlight (from the 12V SMPS). To calculate the maximum dissipation of the PNP, the input voltage the LED headlight needs to be known (around 7 or 3.5 V?); besides knowing the value of the current limiting resistor. For lower currents, PWM could be used by turning on-off the current source with an NPN transistor, driven by an MCU pin.To avoid using PWM, the limiting voltage resistor at the base of PNP could be made of 3 series resistors instead of one. The 3 resistors provide thelowesthighest current. By shunting one of them (using a high PNP and a low NPN at MCU) ahigherlower current is regulated. By shunting two of them (using another high PNP and a low NPN at MCU) thehighestlowest current is regulated.

author: tooki
author_rank: Super Contributor
timestamp: December 17, 2023, 02:13:22 pm
content: Quote from: KerimF on December 17, 2023, 02:08:08 pmGiven that one of the problems OP wants to solve is insufficient battery life, suggesting linear regulation is totally inappropriate, since it's literally burning off lots of power as heat.

author: KerimF
author_rank: Frequent Contributor
timestamp: December 17, 2023, 02:16:55 pm
content: Quote from: tooki on December 17, 2023, 02:13:22 pmSorry, I thought he liked supplying the LED headlight from mains supply instead of battery (as I did to my dentist)

author: Twoflower
author_rank: Frequent Contributor
timestamp: December 17, 2023, 03:06:19 pm
content: I would say that combining PWM-driven light and fast rotating tools is a no-go! The stroboscopic effect might not just be unpleasant but could be dangerous. Especially if precision is wanted.

author: LaserSteve
author_rank: Super Contributor
timestamp: December 17, 2023, 03:45:48 pm
content: The lamp is expensive because it can be easily sterilized. Sterilization is important.That lamp and supporting gear has to be made in a way to not retain biologically active or infectious materials and be body fluid resistant.Test for your homemade gear..Soak whole unit in mouse urine , saline, blood, and solvents for two weeks each. Not easy.On a personal note, malpractice lawyers in the US are a plague. Having to use medical distributors to make sales to doctors will be even more of a plague.FDA approval and FDA good manufacturing practices (GMPs) are eye watering expensive. Record keeping for seven years is required.That lamp is expensive for a reason. By the time you get it through substantial equivalence testing, you'll understand why it is already obsolete.Automotive LED driver chips using Buck Regulators are common.Steve

author: BILLPOD
author_rank: Frequent Contributor
timestamp: December 17, 2023, 09:44:41 pm
content: Good Morning RK_aus_S, I recommend your dentist friend purchase a second complete light, which he can keep charged up and ready to switch with the other when the light starts dimming. Also, I think alternating between the 2 lights every so often keeps wear and tear to a minimum.Besides he should have a back-up if one gives out in the middle of a root canal. I imagine dentist the world around charge big bucks for their services, (mine gets $100 for a simple cavity filling, which takes him about 10 minutes), so the price of another light shouldn't be too hard to scrape up. Besides, work related equipment is a tax write off, at least here in the U.S. it is.

author: RK_aus_S
author_rank: Contributor
timestamp: December 23, 2023, 09:43:05 am
content: Thank you very much for all the answers, even if I don't share some views and other answers completely missed my question...After much thought, I personally came to the conclusion thatthe manufacturer (Zeiss) chose exactly the right, energy-saving design to controll the LED brightness / power in the original circuit; a current limitation in combination with a PWM control.In my opinion, the least amount of energy is converted into unnecessary heat (power loss). That's why I'm going to recreate the circuit exactly like this, using a CN5711 LED driver IC and a 555 timer IC.Some people in the thread will probably be shaking their heads (), but I think that's exactly what a forum is for, even if in the end you don't always have the same opinion. The journey can also be the destination. And the most important thing is to treat each other with respect.Many greetings and happy holidays.Roman

author: inse
author_rank: Super Contributor
timestamp: December 23, 2023, 10:13:01 am
content: If you already implement current limitation (no matter how it’s done) PWM is not really necessary.If the current limitation is only a resistor, take three different ones and select brightness by a switch.A linear current limitation circuit can be made adjustable.The best design would be a constant (adjustable) current buck regulator, saving the last bit of Wh from the battery.85% efficiency may even be achieved with a linear constant current circuit if battery and LED voltages match.

author: MarkT
author_rank: Frequent Contributor
timestamp: December 23, 2023, 03:35:30 pm
content: Quote from: inse on December 23, 2023, 10:13:01 amNot necessarily true for white LEDs which use a blue LED and a phosphor(s) to generate a white (white-ish) light. The phosphor response may not be linear with the brightness of the blue light, so PWM may cause colour and efficiency changes with brightness, PWM is less likely to do this. Also with multielement LED chips in parallel differences in forward voltage mean they won't all illuminate the same at low brightness. So PWM + high current ensures all the chips are working the same.