Powering lights with dmx decoder

lizg

Member
Joined
Sep 18, 2019
Location
Chicago
Hi! This is my first post on this forum. I'm wondering if someone can help me out with figuring out how to control some led string lights with a dmx decoder and power supply.

There are four groups of them, so I hoped that I could control them with a four channel decoder and connect all of the neutral legs together.

I'm a little confused about the power requirement and possible resistors. Each LED takes 3v and 30mA, and are in three groups of 50 and one group of 60.
The led strands don't have any resistance built in without their battery packs.

Hopefully someone has some insight
Thanks!
 

RonHebbard

Well-Known Member
Premium Member
Joined
Jun 12, 2004
Location
Waterdown, ON, CA
Hi! This is my first post on this forum. I'm wondering if someone can help me out with figuring out how to control some led string lights with a dmx decoder and power supply.

There are four groups of them, so I hoped that I could control them with a four channel decoder and connect all of the neutral legs together.

I'm a little confused about the power requirement and possible resistors. Each LED takes 3v and 30mA, and are in three groups of 50 and one group of 60.
The led strands don't have any resistance built in without their battery packs.

Hopefully someone has some insight
Thanks!
@j
Hi! This is my first post on this forum. I'm wondering if someone can help me out with figuring out how to control some led string lights with a dmx decoder and power supply.

There are four groups of them, so I hoped that I could control them with a four channel decoder and connect all of the neutral legs together.

I'm a little confused about the power requirement and possible resistors. Each LED takes 3v and 30mA, and are in three groups of 50 and one group of 60.
The led strands don't have any resistance built in without their battery packs.

Hopefully someone has some insight
Thanks!
@theatrewireless Here's another post with your name all over it.
Toodleoo!
Ron Hebbard
 

RC4Wireless

Jim @RC4Wireless #RC4DoesThat
Joined
Aug 21, 2009
Location
Raleigh, NC
Hi! This is my first post on this forum. I'm wondering if someone can help me out with figuring out how to control some led string lights with a dmx decoder and power supply.

There are four groups of them, so I hoped that I could control them with a four channel decoder and connect all of the neutral legs together.

I'm a little confused about the power requirement and possible resistors. Each LED takes 3v and 30mA, and are in three groups of 50 and one group of 60.
The led strands don't have any resistance built in without their battery packs.

Hopefully someone has some insight
Thanks!
Hey lizg — My day job is wireless control of exactly the kind of things you’re asking about. Of course, before you can get it to work at all, the questions you’re asking always come first.

Can you tell us more about what these LED strings are? Are you making them yourself? I’ve never seen a commercial string that did not somehow accommodate the need for current limiting.

Most LED strings do use a current limiting resistor beside each individual LED, but it’s not uncommon to run a few LEDs in series and then use one smaller resistor for each of these groups. There are Christmas-tree LED strings that have no resistor at all, it’s just groups of LEDs with a total voltage drop (Vf, or V-forward) that adds up to nearly what the line voltage is.

Give us more info, and I’m happy to help you and/or point you to websites that provide all the detail you need.

Jim
RC4 Wireless
 

lizg

Member
Joined
Sep 18, 2019
Location
Chicago
Thanks Jim!
This is a link to the lights my LD ordered to use on the show. They're some very cheap fairy lights, and we're using them in groups of 50 leds (cutting chunks of ten leds from the 3m strands that they come in, and wiring those in parallel).
https://www.amazon.com/gp/product/B01EWBC55A/?tag=controlbooth-20

The website didn't have a data sheet or very specific info about them, but I found some similar SMD strands - which is where I got the 3v 30mA from.
 

RC4Wireless

Jim @RC4Wireless #RC4DoesThat
Joined
Aug 21, 2009
Location
Raleigh, NC
These are very inexpensive, so I’ve ordered 3 of them to play with. Once I have them in-hand, I’ll be able to give you more specific information. That said, what it looks like to me is:

The current limiting is the AA batteries themselves. When 30 LEDs in parallel are connected to that battery pack, they divide and share the maximum current the small batteries are able to deliver. If you were to connect a high-current 4.5V supply, you would instantly destroy all the LEDs, because the power supply would not be inherently limited like the batteries are.

It works because of the often overlooked internal resistance of an AA battery. The same trick is at play when you see a red LED across a coin cell, like a 2032. It’s a direct short from battery to LED, but the LED isn’t destroyed because the battery can’t deliver enough power to do so — the current limiting is “automatic.”

The bigger the battery and the bigger the load, the more sensitive this approach becomes. For example, Alkaline batteries deliver higher current than cheaper carbon batteries. They’ve probably presumed you’ll use Alkaline, and the life you’ll get from carbon will be considerably shorter. Rechargeable cells are typically 1.2V per battery, so the total voltage from 3 of them is only 3.6V, almost a full volt less than 3 Alkaline cells that are 1.5V each. But when you measure the voltage with the LEDs connected, it’s likely you’ll see around 3.0 or 3.1V, regardless of which batteries you start with. With the rechargeable batteries, the drop from 3.6 to 3.1 is much less than the drop from 4.5 to 3.1, and the result is that the LEDs will look dimmer (or might not light at all) with rechargeable batteries.

You can add a resistor in series with a higher current power supply, then the current limiting happens in the resistor. This is the “normal” and “correct” way to handle the problem. But the resistor will generate some heat, so you have to be careful with sizing and safety, to avoid something that could burn someone or something.

Safety first, always!

Jim
RC4
 

RC4Wireless

Jim @RC4Wireless #RC4DoesThat
Joined
Aug 21, 2009
Location
Raleigh, NC
These are very inexpensive, so I’ve ordered 3 of them to play with. Once I have them in-hand, I’ll be able to give you more specific information.

RC4
Hmmm... I didn’t realize I have to choose which length to order. Please tell me *exactly* which one you have or will be purchasing, and I will get that same specific string.

Jim
 

lizg

Member
Joined
Sep 18, 2019
Location
Chicago
That makes sense. We got the 3m length strands. And it would've been easiest to leave them on their battery packs, but they'd like to control them from the board during the show.
We don't have much budget left so I'm looking to borrow a decoder and power supply and run them from that.

I'm trying to figure out the correct resistors to prevent them from burning up, but that's where I got stuck.
 

JimmyM

Active Member
Joined
Nov 22, 2015
Location
USA
Are you wanting to have "dimmable" control or strictly on/off? I would imagine they have resistors already, it is possible to get LEDs with built in ones.

If you just need on/off I would get a DMX relay pack and a beefy 3v PSU, then run LV 3v to each light and use the relay pack to get control. You could use multiple PSU and relay channels if you wanted more control. I think a good advantage of this system would be that you could reuse the relay packs if you don't have any already.
 

RC4Wireless

Jim @RC4Wireless #RC4DoesThat
Joined
Aug 21, 2009
Location
Raleigh, NC
Are you wanting to have "dimmable" control or strictly on/off? I would imagine they have resistors already, it is possible to get LEDs with built in ones.

If you just need on/off I would get a DMX relay pack and a beefy 3v PSU, then run LV 3v to each light and use the relay pack to get control. You could use multiple PSU and relay channels if you wanted more control. I think a good advantage of this system would be that you could reuse the relay packs if you don't have any already.
If I’m right about how they work (see previous post), there are no resistors and the current limiting is the internal resistance of the batteries themselves. Once I have samples, I can quickly measure current draw and come up with a recommendation for a resistor that provides the same current limiting regardless of the power source. This can be done for any voltage, it doesn’t have to be 4.5V — it comes down to R = V/I where V is the operating voltage and I is the current you need to deliver. It’s basic Ohms Law. As mentioned previously, the resistor has the potential to get hot and may need to be quite large with a high power rating. The higher the voltage, the more this becomes an issue.

I can’t do the math until I know what the value for I is, and we need a string to measure that with ammeter.

Jim
RC4
 

RC4Wireless

Jim @RC4Wireless #RC4DoesThat
Joined
Aug 21, 2009
Location
Raleigh, NC
That makes sense. We got the 3m length strands. And it would've been easiest to leave them on their battery packs, but they'd like to control them from the board during the show.
We don't have much budget left so I'm looking to borrow a decoder and power supply and run them from that.

I'm trying to figure out the correct resistors to prevent them from burning up, but that's where I got stuck.
Hey lizg — it’ll be a couple of days before my samples arrive. In the meantime, I’ve come up with a way to use an RC4 Wireless dimmer while still using the battery pack that comes with the string. You power the dimmer with separate battery. This is a sort of “advanced configuration” where you end up with the same “automatic” current limiting for the LEDs plus they’re fully and smoothly dimmable.

I understand your budget is limited. We have a rental program, and if you’re working in education we have additional assistance as well. Let’s chat privately if you’re interested in considering any of that.

Regardless of whether or not that’s of interest, I’ll be back in a couple of days with more info about the strings.

Jim
RC4
919-229-9953
[email protected]
 

lizg

Member
Joined
Sep 18, 2019
Location
Chicago
Jim - that sounds like a great option - and is a great idea for another project I'm working on. I may end up contacting you again when I get to ordering equipment for it. Unfortunately, I know we can't afford that for this show (although I'm sure that would've been a much cleaner solution).
Thanks anyway!
 

Amiers

Renting to Corporate One Fixture at a Time.
Joined
May 28, 2009
Location
Phoenix, Az
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RC4Wireless

Jim @RC4Wireless #RC4DoesThat
Joined
Aug 21, 2009
Location
Raleigh, NC
Hmmm... what's the white thing inside the battery case?
What_is_that_white_thing.jpg


It looks like...
It_is_a_resistor.jpg


RED_BLK_BLK_20-ohm.jpg


A red-black-black resistor!!
20_OHM_resistor_color_code.png


It's 20 ohms. That's the current limiting for the LEDs.

The batteries are delivering 4.5V, the resistor provides a 1.5V drop, resulting in 3V on the string of LEDs:

4.5V_battery_V.jpg


3V_after_the_resistor.jpg


1.5V_voltage_drop_across_resistor.jpg


And, of course, the current limiting produces heat in the resistor. I can measure this with a very handy tool:

Temperature_Measuring_Tool.jpg


Ambient temperature on my desk is 24.5C:

Ambient_Temperature_24C.jpg


The resistor, after running for only a minute or so, has already risen to 70C and is still rising as I type this:

Resistor_Temperature_70C.jpg


Reminder: the boiling point of water is 100C. Power resistors can handle considerably higher temperatures than that. This is why the resistor is packaged inside a thermally protective white braid sheath (see first pic above).

Now, the math!!! Woo hoo!!

Ohms law to find the current draw:
I = V/R
I = 1.5V / 20 ohms = 0.075A = 75mA

And we also want to know how much power, in Watts, is being dissipated in the resistor:
W = VA
W = 1.5V x 0.075A = 0.1125W

That's just over 1/10th of a Watt in heat. Judging by the size of the resistor, it's most likely rated for 1/2W (0.500W), so there's plenty of headroom there. It's hot, but it's inside a sheath inside a container. It's relatively safe.

Want to run this string at 12V instead? The LED voltage will always be 3V, that's the Vf, or Forward Voltage specification of the LEDs. The V we use in our ohm's law calculation is the drop from supply down to 3V. In this case, we solve for R:
R = V/I
R = (12V - 3V) / 0.075A
R = 9V / 0.075A
R = 120 ohms

W = VA
W = 9V x 0.075 = 0.675W

Using a 12V power supply or battery, you need a 120 ohm 1W resistor, protected to avoid fire and/or injury. You'll be dumping more than 2/3 of a Watt in heat.

I hope this helps. :)

Jim
RC4

* I have written all of this very quickly, and I cannot be responsible for errors or omissions. Every attempt has been made to provide accurate information. Math should be double-checked and real-world tests should be run with caution to ensure all is well. *
 

jm_in_tx

Member
Joined
May 16, 2017
Location
DFW, TX
When I saw the part about the resistance in the batteries limiting the current, I knew I needed to jump in. FORTUNATELY Mr. RC4 ( @theatrewireless ) saved me from having to do that. This is exactly the kind of educational post I would have loved to have done because I've been tinkering with LEDs just like that for a long time. But I would never have been able to do it the same justice that Jim did. A doff of the old chapeau to you! I've been an engineer for 40+ years and this advice is spot on.
 

lizg

Member
Joined
Sep 18, 2019
Location
Chicago
Hmmm... what's the white thing inside the battery case?
View attachment 18496

It looks like...
View attachment 18497

View attachment 18498

A red-black-black resistor!!
View attachment 18499

It's 20 ohms. That's the current limiting for the LEDs.

The batteries are delivering 4.5V, the resistor provides a 1.5V drop, resulting in 3V on the string of LEDs:

View attachment 18502

View attachment 18503

View attachment 18504

And, of course, the current limiting produces heat in the resistor. I can measure this with a very handy tool:

View attachment 18505

Ambient temperature on my desk is 24.5C:

View attachment 18506

The resistor, after running for only a minute or so, has already risen to 70C and is still rising as I type this:

View attachment 18507

Reminder: the boiling point of water is 100C. Power resistors can handle considerably higher temperatures than that. This is why the resistor is packaged inside a thermally protective white braid sheath (see first pic above).

Now, the math!!! Woo hoo!!

Ohms law to find the current draw:
I = V/R
I = 1.5V / 20 ohms = 0.075A = 75mA

And we also want to know how much power, in Watts, is being dissipated in the resistor:
W = VA
W = 1.5V x 0.075A = 0.1125W

That's just over 1/10th of a Watt in heat. Judging by the size of the resistor, it's most likely rated for 1/2W (0.500W), so there's plenty of headroom there. It's hot, but it's inside a sheath inside a container. It's relatively safe.

Want to run this string at 12V instead? The LED voltage will always be 3V, that's the Vf, or Forward Voltage specification of the LEDs. The V we use in our ohm's law calculation is the drop from supply down to 3V. In this case, we solve for R:
R = V/I
R = (12V - 3V) / 0.075A
R = 9V / 0.075A
R = 120 ohms

W = VA
W = 9V x 0.075 = 0.675W

Using a 12V power supply or battery, you need a 120 ohm 1W resistor, protected to avoid fire and/or injury. You'll be dumping more than 2/3 of a Watt in heat.

I hope this helps. :)

Jim
RC4

* I have written all of this very quickly, and I cannot be responsible for errors or omissions. Every attempt has been made to provide accurate information. Math should be double-checked and real-world tests should be run with caution to ensure all is well. *
Wow. Thank you so much for testing and explaining that so thoroughly. This is super helpful! I' have to come back and post a picture of them all working once I'm finished with the install.
Thanks again

Liz
 

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