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Author Topic: Resistor for red led in TCSS crystal  (Read 406 times)

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Offline scott

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Resistor for red led in TCSS crystal
« on: January 05, 2020, 06:45:54 PM »
Hi all -  looking for guidance in the optimal size and spec resistor for the red in the TCSS crystals. I calculated the proper resistance and it suggests 1.8ohm/1.8W, and TCSS says the closest they sell is 2ohm/5w. I have over resistored the red because the specified closest is the fat white block that doesn’t fit in any of my builds. I have done this 10+ times and they work and last, just finally got annoyed with how dim the red crystal is in the Kylo I just built.

I am thinking of switching over to the newer pixel crystal that just popped up on TCSS, which looks promising so also interested in any discussion about how they work!  Thank you in advance for the tips.

Offline Darth Chasm

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Re: Resistor for red led in TCSS crystal
« Reply #1 on: January 06, 2020, 05:33:05 AM »
Responded on your TCSS post.

Offline scott

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Re: Resistor for red led in TCSS crystal
« Reply #2 on: January 06, 2020, 07:20:10 AM »
Noted and solved...150/.25 works as expected...thanks again.

Offline Ridire Fíréan

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Re: Resistor for red led in TCSS crystal
« Reply #3 on: January 13, 2020, 06:00:53 PM »
Hey @scott  !  What soundboard are you using?  And what pad are you using to supply your LED?  And which of the TCSS Crystals are you using?

I've found being specific about what you have often leads to a more correct and timely response.  ;)



If you're using one of the TCSS RGB LED Crystal Style 3 Common Annode - https://www.thecustomsabershop.com/RGB-LED-Crystal-Style-3-Common-Anode--P1266.aspx

And if you're coming off a pad that supplies the full battery power, and you were using a single 3.7V(4.2V fully charged) battery...

Using 4.2V for the max battery Voltage and 1.9V for the low-end of the 1.9-2.0V requirement of the Red die to be as safe as possible...


R (Ohms) = 4.2V - 1.9V / 0.02A = 115 Ohms

P (Watts) = 115 Ohms x 0.02A(squared)(so, 0.0004) = 0.046 Watts


You'd be looking for a 115 Ohm / 0.046 Watt Resistor.

There's a bunch of resistors that would be close to your requirement at Digikey...

Request Rejected

The link should sort them by smallest size, but if not just click on the UP Arrow on the "Size/Dimension" column.



If you're coming off a pad that supplies 3.3V for accessory LEDs or whatever...

Using 3.3V for the supply Voltage and 1.9V for the low-end of the 1.9-2.0V requirement of the Red to be as safe as possible...


R (Ohms) = 3.3V - 1.9V / 0.02A = 70 Ohms

P (Watts) = 70 Ohms x 0.02A(squared)(so, 0.0004) = 0.028 Watts


You'd be looking for a 70 Ohm / 0.028 Watt Resistor.

There's a bunch of resistors that would be close to this requirement as well, again at Digikey...

Request Rejected



Once you pick the resistor you think will best fit within your hilt, head over to Mouser.com, sometimes they have the same resistor for a lower price than Digikey, just remember to factor in shipping costs!  I try to put together a list of all the parts I need and order them all at once to offset the cost of shipping.  If I only need one resistor, I just head over to the local electronics shop and buy one for a few dollars.  Ordering one $0.10 resistor from Digikey or Mouser ends up being double or triple the local price once you add in the shipping.


I see you already picked a 150 Ohm / 0.25 Watt, and you're probably thinking, "Dude, I said 'solved'", but I've been on Digikey all afternoon and evening picking resistors, and having just finished, and wanting a break came here, and saw your question and was like, What The Force, one more resistor won't kill me. Since I had all my notes out in front of me and hadn't closed the Digikey page yet, I thought I'd chime in show you what I found.

I hope this helps you some, if not for this build, then for your next one!



p.s. I just previewed this and I'm not sure why, but the Digikey links are showing up "Request Rejected"(probably too long), however, the links seem to be working.

If they don't work later on for whatever reason, go to digikey.com select...

Product Index > Resistors > Through Hole Resistors

...then select your "Resistance", "Power (Watts)", and check the "In Stock" box under "Stock Status", click on "Apply Filters", then click on the UP Arrow on the "Size/Dimension" column and you'll be able to see them arranged from smallest to largest dimensions.

Alrighty now, I can tell it's dinner time because I'm typing too long.  Off to it then!


Offline scott

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Re: Resistor for red led in TCSS crystal
« Reply #4 on: January 13, 2020, 09:01:52 PM »
Hey - thanks for all of that detail!  I wish I had a local electronics store, but don’t thing I have one in my neighborhood...the “closest sold” in TCSS has worked out pretty well so far. For sure, the exact resistor would be ideal. Unfortunately, my OP was based on being a goof.  The 150/.25 looks great on the standard TCSS crystals, and I only had to fix my current build as I used the correct calcs on all of my prior ones.

FWIW, I am now on to quartz crystals, so grabbing a bunch of rgb leds and experimenting in lighting them appropriately!  Thanks again. Scott