LED driver question - Page 1
Forum section: electronics / beginners

author: Red Squirrel
author_rank: Super Contributor
timestamp: October 31, 2019, 02:22:05 am
content: I understand constant voltage PSUs and how they work, and how they will supply any device that works with that voltage and those devices draw whatever current they need. But constant current is a concept I always have a bit of trouble grasping. And before I start I just want to be sure, a LED driver is just another word for a constant current source right? Or do they do anything more special?So my question is, say I have a LED module rated at 300ma with a forward voltage of 12v, if I want to power multiple of these from the same PSU do I just get a 300ma constant current PSU that has a higher voltage? Ex: at least 24v to power two modules. Add up the voltages, but current stays the same? From my little understanding of how these work they simply work the same as constant voltage, but instead of using voltage as feed back, it uses a shunt for current, and it will increase/decrease voltage as needed so that the draw is whatever it's rated at correct?Now if I put 2 LEDs in parallel, then I would need double the current, so need 600ma correct? But this is probably bad practice as if one LED is removed from the circuit for any reason then the PSU will try to deliver 600ma to the 300ma rated LED right?And lastly, lot of drivers tend to just use PWM instead of a smooth DC output, can I safely stick a capacitor at the end to smooth that out or will that mess up the current measurement somehow and cause it to deliver wrong current?

author: Whales
author_rank: Super Contributor
timestamp: October 31, 2019, 04:44:48 am
content: > And before I start I just want to be sure, a LED driver is just another word for a constant current source right?Most off-the-shelf LED drivers are some form of constant current driver.An LED can technically be driven by anything: constant voltage, constant current, constant power & even more complex systems:CV driving:possible, but fiddly. Has to be very accurate and under-drive the LED, otherwise risks blowing it up as the LED warms.CC driving:works, stable, popular.CV + resistor driving:works, stable, popular. Technically not CC, but close enough & requires less parts if you already have a known CV supply.CP & other more complex driving schemes:not very popular, simpler methods typically work fine.> if I want to power multiple of these from the same PSU do I just get a 300ma constant current PSU that has a higher voltage?Yes> From my little understanding of how these work they simply work the same as constant voltage, but instead of using voltage as feed back, it uses a shunt for current, and it will increase/decrease voltage as needed so that the draw is whatever it's rated at correct?Yes.Rather than thinking in terms of "reducing" and "increasing" voltage I suggest you think in terms of "more V and I" and "less V and I". For most loads (including LEDs): increasing either with increase the other.Some CV supplies can be turned into CC supplies by modifying where they measure their feedback voltage from.> And lastly, lot of drivers tend to just use PWM instead of a smooth DC output, can I safely stick a capacitor at the end to smooth that out or will that mess up the current measurement somehow and cause it to deliver wrong current?No you can't just stick on a cap. It will charge quickly to full in the ON period and basically make lowering the PWM do nothing (except increase current draw over short periods, which increases stresses and can cause damage). You will instead want to put a proper filter on the PWM output (ie L + C), which effectively turns it into a switchmode power supply, but this requires some calculations & considerations to get right.EDIT: PWMing LEDs can be anything from harmless to problematic. Technically driving them at a constant (variable) power instead of a PWM waveform is more efficient, but in most cases people don't care enough to pay for the extra parts. This also shifts the colour temps of white LEDs slightly.Of course there's also the problem of when the PWM freq is so low that people and/or cameras notice it

author: Red Squirrel
author_rank: Super Contributor
timestamp: October 31, 2019, 05:05:38 am
content: Thanks for the info. So I'm mostly on the right track then. Basically debating on buying a cheap driver off amazon or making my own with arduino as I want to power a couple cheap COBs to make a camera light but I really don't want flicker. The issue with the amazon route is I have no way of knowing if they will use PWM or proper smooth DC. Might just chance it and I can always deal with trying to mod them after with the right filter like you said.It seems PWM is the popular choice for some reason, I've even seen car headlights use it!

author: jhpadjustable
author_rank: Frequent Contributor
timestamp: October 31, 2019, 09:54:13 am
content: Quote from: Whales on October 31, 2019, 04:44:48 amMostly seen in laser diode drivers, those touchy little buggers.Quote from: Red Squirrel on October 31, 2019, 05:05:38 amYou might be confusing PWM with a switching converter. PWM cannot regulate instantaneous current. If you try to PWM a 1A led with a 20% duty cycle switching a 5A source, you will be very disappointed very quickly. If the board claims to regulate current, the chances are very good it's a switching converter with current regulation. Dimming is usually a separate matter. Some chips have an input for PWM in the 100-1000Hz range, some have an analog input, some accept both, some do neither.If you feel comfortable laying out a board, you could build your own driver. I'm working on a project with the Brightpower BP1808 right now. It's got a 3A maximum switch current and can be used in buck, boost, or buck-boost configurations. It accepts analogand/orPWM dimming input. They're under 20 cents each. Like most buck-type constant-current switchers, just add inductor, bypass cap, compensation cap, freewheel diode and current sense resistor according to the datasheet selection guides, and Robert's your mother's brother. I found in my buck design that the analog dimming dynamic range is about 40:1 by current. What that works out to in aperture depends on the LEDs you use and their luminous flux vs. current characteristic, but figure about 5 f-stops.

author: Whales
author_rank: Super Contributor
timestamp: October 31, 2019, 10:12:08 pm
content: Quote from: Red Squirrel on October 31, 2019, 05:05:38 amPWM is the cheapest & simplest option, especially if you already have a microcontroller in your design (eg a car).Proper smooth DC requires filters and most often uses feedback. That's a few dollars more cost and complexity per unit than raw PWM.

author: Red Squirrel
author_rank: Super Contributor
timestamp: November 01, 2019, 12:29:01 am
content: Quote from: jhpadjustable on October 31, 2019, 09:54:13 amYeah I know they're different, but we all know how it works when it comes to stuff from China, it might say it's one thing but turns out it's not. You only find out when you plug it in.This is the one I'm looking at:https://www.amazon.ca/gp/product/B07CWMKJHB/ref=ox_sc_act_title_1?smid=A24JGA3DO5B17D&psc=1I think I should be good to put 2 of the COBs I have in series. Though it's cutting it a tad close since the minimum voltage of that PSU is 24v and when I put 12v on one of the COBs it was pulling more than 300ma. So might put 3 in series to be safe.I do want to eventually just make my own stuff of that nature as a learning experiment but for now I might just buy one. Well two in this case, which works out as I might make two separate lights. I still will want to figure out the thermals as well as diffusion but I can experiment with that after.

author: Whales
author_rank: Super Contributor
timestamp: November 01, 2019, 01:01:20 am
content: N.B. don't run cheap COBs anywhere near their marketed power levels. The numbers tend to be (dangerously?) exaggerated, and are so far into the inefficient zone of driving the LED that halving the power leads to a similar brightness level.

author: Red Squirrel
author_rank: Super Contributor
timestamp: November 01, 2019, 03:22:44 am
content: Actually now that you mention it the difference in brightness between 10 volts and 12 is negligible. (I only have a ATX psu to play with at the moment so testing with CV) Maybe I should really just drive them lower after all. At 10 volts it pulls around 160ma while at 12v it pulls close to 400 which is over spec so maybe I should be looking into something more in that range. I found this 250ma one, maybe a tad higher than I want to go but it is under the 300ma spec.https://www.amazon.ca/MEAN-WELL-Switching-110V-220V-Current%EF%BC%8CTransformer/dp/B06XYWG75D/ref=sr_1_2?keywords=250ma+LED+driver&qid=1572578098&sr=8-2As a side note I got a little too cocky... and I can confirm it won't take 17 volts.Think I will look at powering the 3 I have left with that PSU.

author: jhpadjustable
author_rank: Frequent Contributor
timestamp: November 01, 2019, 04:45:03 am
content: If it's on Amazon, it's probably fake. Compare the genuine article, and the prices:https://www.digikey.ca/product-detail/en/mean-well-usa-inc/APC-8-250/1866-1130-ND/7702581In fact, if you'd rather have low voltage drivers, have a look at the whole subcategory:https://www.digikey.ca/products/en/power-supplies-external-internal-off-board/led-drivers/137

author: Red Squirrel
author_rank: Super Contributor
timestamp: November 01, 2019, 04:47:49 am
content: I already have an Amazon order pending so just needed an item to bring me to the free shipping tier.But it looks like it's actually CHEAPER at Digikey so might go that route anyway and at least I know it's the real thing. Need a heat sink anyway so can get that there too. While I'm there I may as well buy some proper COBs.... lol.

author: james_s
author_rank: Super Contributor
timestamp: November 01, 2019, 06:14:29 am
content: I dunno why people buy little Chinese widgets from Amazon, it's never anywhere near the best price, and it's the same stuff you get from ebay, dx, aliexpress, etc for a lot less.

author: Red Squirrel
author_rank: Super Contributor
timestamp: November 02, 2019, 01:47:37 am
content: For me it's mostly when I need filler items to pad an order to get free shipping and just want some cheap crap to mess with.If I have an actual serious electronics project then I tend to go to Digikey that way I know I'm getting the real thing.