Voltage boost circuit to constant current (solved:User error) - Page 1
Forum section: electronics / beginners

author: Raj
author_rank: Frequent Contributor
timestamp: March 29, 2024, 04:07:55 pm
content: So I tried to turn a voltage boost circuit to a constant current one and ended up charring the current sense resistorI intended to mod an mt3608 by replacing the R1 with a LED that needs 600mA and had 1ohms for R2 as in diagram

author: Raj
author_rank: Frequent Contributor
timestamp: March 29, 2024, 04:10:55 pm
content: When I put it to test, the resistor started to glow brighter than the led (cause the led was an IR one)The voltage across it was 2 volts and current through it was 1.5A. Luckily the LED survived somehow.

author: Raj
author_rank: Frequent Contributor
timestamp: March 29, 2024, 04:15:32 pm
content: Stupid me realized that input voltage was higher than output and it's a boost circuit. Please delete the thread. case closedRealized while I was typing my input voltage. I intended to use a 15V LED but tested the circuit with a 1.5v led as a stand in.

author: CaptDon
author_rank: Super Contributor
timestamp: March 29, 2024, 07:47:44 pm
content: They don't make 15 volt LED's. And you don't put 600ma through a LED.

author: Zero999
author_rank: Super Contributor
timestamp: March 29, 2024, 10:02:26 pm
content: Quote from: CaptDon on March 29, 2024, 07:47:44 pmWhy be so pedantic?One can buy power LEDs which are really multiple LEDs connected in series and parallel. A forward voltage of 15V would be five LEDs in series and there are power LEDs which can easily handle 600mA, even single dies.

author: Raj
author_rank: Frequent Contributor
timestamp: March 30, 2024, 03:58:22 am
content: Quote from: CaptDon on March 29, 2024, 07:47:44 pmCob cluster of LEDs indeed, but the seller didn't use an 's' after led(s) either.anyway, tested it with the intended led and it works...just need to stress test it now.need to make a water cooled mount for it first.

author: Zero999
author_rank: Super Contributor
timestamp: March 30, 2024, 12:36:03 pm
content: Quote from: Raj on March 30, 2024, 03:58:22 amI wouldn't recommend running it at the maximum rating current continuously. LEDs are less efficient at higher currents, so run at 75% or 80% of the maximum for long life and you'll barely notice the reduction in brightness.

author: CaptDon
author_rank: Super Contributor
timestamp: March 30, 2024, 02:08:21 pm
content: With 600ma through the resistor it would radiate .36 watts of heat so you need a 1/2W or even a 1W resistor for good measure. I missed the fact that the LED was the load your were driving. I though you were using it as a regulation element for an external load connected to VOUT. Now it all makes more sense. When I saw the node called VOUT it looked like you were driving something not shown in the schematic. If that LED really does drop 15 volts at 600ma you have 9 watts of heat in a tiny area. How many lumens does that LED make?

author: Raj
author_rank: Frequent Contributor
timestamp: March 30, 2024, 05:40:13 pm
content: Quote from: CaptDon on March 30, 2024, 02:08:21 pmoh, another omission of facts, the led I used to test it was-https://www.aliexpress.us/item/3256803465818683.htmlWhile final wil have -https://quartzcomponents.com/products/5w-led-chip-high-power-cool-white-cob-light-300ma?_pos=2&_sid=79840cc95&_ss=randhttps://quartzcomponents.com/products/10w-led-chip-high-power-cool-white-cob-light-300maSince I doubted that modifying a constant voltage circuit into a constant current circuit will work, I wanted to test out with LEDs that I no longer need.so final will be 0.6v across and 300mA through the resistor, (180mW) which will work out fine for 250mW resistor, though I won't mind using 4 resistor to get same resistance just to make cooling them easier.