High Power LED running off an 18650 in a very tight container.

Thread Starter

Tribalbob

Joined Feb 5, 2020
8
Hey everyone, this seems like a great community and I'm hoping to get a bit of help with something.

Full disclosure, I'm getting back into electronics after not really doing anything major for almost 10 years.

So I'm building a very basic prop for a friend - a lightsaber. It's nothing fancy - just needs to basically light up when you switch it on. The tricky part is all the electronics need to fit into tube that's about 1-1/4" diameter and 12" long.

I'm using a high power LED for the light source - red 2.6V @ 700mA. To power this, I settled on an 18650 battery due to the higher voltage, decent capacity and it -just- fits in a clip inside the hilt. Bonus: it can be re-charged so it saves money on that front. I would have used a 9v, but it was a little too big.

Initially I planned to just hook up a couple resistors in parallel as an interim solution as well as a heatsink on the LED and then place an order for a micro driver. The saber won't be on for long periods of time - likely turned on for photos then off (maybe 30-60 seconds at a time). However, I've been wondering about using a regulator. My local store has a few adjustable step-up/down ones that are small enough they would fit. I've just never used one before, so I'm not certain how to hook them up. They have a Ground, Shutdown, Input and Output. I get Input goes to +Battery, output goes to +Led, then Ground goes to -Battery along with -Led?

I need to get this done in about a week and I live in Canada so a lot of US supply stores (LED Supply, etc) are going to take too long to ship or be ludicrously expensive. I have a really well stocked local store.

Thanks in advance for any help!
 

Thread Starter

Tribalbob

Joined Feb 5, 2020
8
Ah, so you suggest even with stepping down the voltage to 2.6, I should still use a resistor? I guess I would calculate that based on the new output voltage?
 

ericgibbs

Joined Jan 29, 2010
9,574
hi,
It is advisable to use a current limiting resistor, (Vout-Vled)/Iled.
say, (3.0Vout-2.6led)/0.5A = 0.8R say 1.0R

E
 

Thread Starter

Tribalbob

Joined Feb 5, 2020
8
Ok, I think I see what you're getting at. Regulator stepping down to ~2.6V and a 1ohm resistor to help. Thanks for your help, Eric!
 

ericgibbs

Joined Jan 29, 2010
9,574
hi,
Once you have it running, look into the use of the Shut Down pin to control the sabre light.
Some Shut downs require a pull or pull down, it should tell you on the module printed text.

Start with the Vout lower than 3V and gradually increase it to say 3v.

E
 

Thread Starter

Tribalbob

Joined Feb 5, 2020
8
I'm still not super clear on what the shut down pin does. Is it sort of a short-protection in case the voltage starts getting too high?
 

ericgibbs

Joined Jan 29, 2010
9,574
hi,
Its basically a On/Off for the Vout.
If you can post a photo of the module when you get it, we can maybe identify the module.
E
 

ericgibbs

Joined Jan 29, 2010
9,574
Hi,
This clip form the d/s explains the SHDN
Connecting SHDN pin to 0V will put the module in the 'sleep' mode, leaving it disconnected will allow the module to work.
E
Clip:
The SHDN pin can be driven low (under 0.4 V) to power down the regulator and put it in a low-power state.
The quiescent current in this sleep mode is dominated by the current in the 100k pull-up resistor from SHDN to VIN. With SHDN held low, this resistor will draw 10 µA per volt on VIN (for example, the sleep current with a 5 V input will be 50 µA).

The SHDN pin can be driven high (above 1.2 V) to enable the board, or it can be connected to VIN or left disconnected if you want to leave the board permanently enabled.



The four connections are labeled on the back side of the PCB
 

ericgibbs

Joined Jan 29, 2010
9,574
Yes, a simple push button for the sabre light On/Off and perhaps a slide switch to turn off the module when its not being used.
 

ronsimpson

Joined Oct 7, 2019
451
Here is what I would do. No resistor with the LED. Rs is to measure the LED current. Input voltage fron 2.5 to6V. Out put current set by Rs. 0.1 volts across Rs. Output current needs to be 1A or below.
1580936733679.png
The circuit above is inside this flash light bulb. (torch) You wanted it small.
1580943917411.png
Edited---------
The IC was built for 4.5V batteries and may not work well at low voltage. I will look for a batter part.
 
Last edited:

ronsimpson

Joined Oct 7, 2019
451
When making a LED constant current PWM it is good to have the battery voltage well above the LED voltage (buck) or well below (boost). When the LED voltage is too close to the battery voltage you can change to SEPIC pwm. I can now have the LED voltage close to the battery voltage. In this example the battery voltage is 2.7 to 4.5 and the LED voltage can be almost any voltage. (2.7 or 3.0 or 6.0 are just fine) So now I am saying a boost pwm in SEPIC mode is probably best but the parts count is higher.
1580951883498.png
 

Thread Starter

Tribalbob

Joined Feb 5, 2020
8
Thanks for the examples, I made a trip to the store so I'm going to give a few of these a go tonight and see which one works out best for my needs.
 

Audioguru again

Joined Oct 21, 2019
919
Please post the datasheet or part number of the "2.6V" red LED so we can see its range of forward voltage. Since it sets its own voltage then you must feed it a current, not a voltage.
Some might be 2.2V so if you feed it 2.6V it will burn out immediately.
Some might be 3.2V so if you feed it 2.6V then it will produce no light.

If you run it at its maximum allowed current then how will you cool it?
 

ericgibbs

Joined Jan 29, 2010
9,574
I need to get this done in about a week and I live in Canada so a lot of US supply stores (LED Supply, etc) are going to take too long to ship or be ludicrously expensive. I have a really well stocked local store.
Hi,
This what the TS said in his opening post,
This is the one I'm looking at getting https://leeselectronic.com/en/product/15117.html

He is trying to use what is readily/quickly available in his area.

E
 
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