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LED Efficiency - where to aim for with the driver current

A project log for Yapolamp

An experimental torch/flashlight intended to be safer for eyes, completely inspired by and built upon the TritiLED project

Simon MerrettSimon Merrett 04/01/2019 at 21:139 Comments

Updates:

Getting closer to an LED  "Arb" Rating

For this log, the main information is the result of an efficiency test for the "yellow" (orange) LEDs I'm using in Yapolamp. In previous logs I have described why these are a nice colour for a night light (warm, don't spoil night vision, comforting if you grew up with a high pressure sodium street lamp shining through your bedroom window, don't damage your eyes much...). We're going to stick with them for the foreseeable. 

The LEDs in question are the Chanzon jobs:

They have a claimed spec of:

Test bed

@Ted Yapo properly characterised LEDs for #TritiLED in Arbs but I have used more of a lash-up rig to find the peak efficiency of these Chanzon yellow LEDs. I taped the LED to the TSL2591 light sensor with a 5mm piece of acrylic/HIPS between them and covered the pair in black electrical tape to reduce interference from outside the LED. An INA219 was pressed into "high precision, reduced range (400mA max)" mode to measure the current from 0.8mA to 26mA, set by a trimpot and fixed resistor in series.

Which yielded the following results:

So rather strangely the light output (yellow data points) started to drop off after 13mA and the peak efficiency (blue data points) appears to be in an elongated region between 6 - 12mA. This is in line with Ted's observations that peak efficiency is balanced with "cost of die area" in LED economics to produce the higher recommended current ratings; 60mA in this case. The broad magnitude range of 6-12mA for the high efficiency zone bodes well for achieving the "peak sawtooth" efficiency that Ted calculated for Tritiled. 

Sorry, I didn't have time to convert results into the internationally recognised unit of Arbs that such a lash-up measurement rig deserves.

Please jump into the discussions for any handy observations or tips about how you go about calculating or designing that "peak sawtooth".

Super Power

You may have guessed but the plan for this version of Yapolamp is to use supercaps as the energy store. Initial tests and spec sheet leakage figures seem like they'd fit the use cases of short run times at high brightness and longer term low-glow levels so the torch can be found in the dark. By adding a transistor based constant-current charging circuit and an anti-backfeed Schottky, we could let the (young) user recharge in under 30 seconds using a USB battery pack and run for many minutes at full brightness, even after a few days in "always on" standby mode. 

Here's an unfettled image of the test board I ordered, from the Kicad 3D viewer.

PS, the purpose built LED driver ICs are ~$0.05 each in qty 1-10 so they are potentially a really good option for many other LED circuits.

Discussions

richard wrote 05/17/2021 at 10:11 point

I had some success with the MT9284 driving a 1W 9V Bridgelux LED.  Efficiency was measured between 1mA and 20mA and is in line with the datasheet. However, the datasheet does not explicitly state what the maximum LED current is. The graphs hint that 20mA is the maximum but I'm trying to get 100mA. Has anyone had any success at getting 50mA or more through their LED string?

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Simon Merrett wrote 05/18/2021 at 07:06 point

Hi @richard sorry but I have not tried that driver at higher currents. 

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richard wrote 05/19/2021 at 05:18 point

I just successfully pushed 80mA through a single Bridgelux device. No issues noted.

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Simon Merrett wrote 05/19/2021 at 06:32 point

Thanks for the update @richard . What inductor are you using? 

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richard wrote 05/19/2021 at 08:09 point

This 22uH inductor is what I used : https://lcsc.com/product-detail/Power-Inductors_Yanchuang-SCCD43-220KT_C532863.html

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Simon Merrett wrote 05/21/2021 at 21:32 point

@richard thanks for the inductor link

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Ken Yap wrote 04/01/2019 at 23:38 point

>PS, the purpose built LED driver ICs are ~$0.05 each in qty 1-10

Could you provide some links to vendors of the MT1284?

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Simon Merrett wrote 04/02/2019 at 07:30 point

https://lcsc.com/product-detail/LED-Drivers_MT9284CS6_C181786.html is what I have. Although this offering looks to be pretty much the same: https://lcsc.com/product-detail/_RYCHIP-Semiconductor-Inc-RY3730_C370887.html and is closer to the price I mentioned (may have been thinking in £GBP rather than $US).

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Ken Yap wrote 04/02/2019 at 08:52 point

Thanks!

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