How to dim a dimmable trafo with a touch dimmer

Thread Starter

Linbaato

Joined Oct 12, 2021
8
Hello forum!

I am a beginner in electronics and for a lighting project I bought a dimmable trafo and a touch dimmer with the hope to be able to combine these.

The trafo is a MeanWell LDB-600L, that is a 600mA constant current trafo with a "DIM" input that takes either a PWM signal (100-1kHz) or a 1-10V signal (Reference: https://www.meanwell.com/Upload/PDF/LDB-L/LDB-L-SPEC.PDF).

The touch dimmer has a 12V input and some undefined output which I assumed would transform the input voltage to 1-12V (possibly via PWM).
(Reference: https://a.aliexpress.com/_uiitwY)

I have an input of 12V available to me and just tried to string this together, but to no avail. The touch dimmer works differently than I expected. It works great with 12V LED strip but without a consumer the output voltage only dims from around 12.3V to 12.7V. I think that is why the trafo does not react at all to the touch dimmer input and just works at seemingly a 100%. With the LED strip attached tto the touch dimmer the voltage ranges from 6.9V to 10.9V which produces a nice lighting result but the lower voltage is still much higher than I anticipated.

I would love to keep using these touch dimmers because I also want to dim two sets of LED strip and place the dimmers next to each other. It would be nice if they have a consistent look.

Is there a way that is not too complicated to achieve this?

Thank you all in advance :)
 

ElectricSpidey

Joined Dec 2, 2017
1,962
Two things

1. The touch dimmer does indeed need a load to operate and the "trafo" does not present any.
2. The touch dimmer will not go below a certain voltage because LEDs stop working below a certain voltage.

I would suggest a commercial 0-10 volt dimmer, or build a simple 555 PWM circuit.

Getting the touch dimmers to work with the trafo will probably be "complicated".
 
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Thread Starter

Linbaato

Joined Oct 12, 2021
8
Thank you for your reply ElectricSpidey,

1. I think I understand why (physically). It makes sense to me for a PWM signal.

2. That also makes sense.

Thank you for your suggestions. I researched and further tested a bit and I can detect the PWM frequency of the touch dimmer with my multimeter (yeah I did not know it can do that). It seems to be about 32kHz. Now I also found PWM to 0-10V signal converters like the one here:

https://www.amazon.de/dp/B07SZ8JYLT/ref=cm_sw_r_apan_glt_i_V59BKAB693S5Q0FRR8GF?_encoding=UTF8&psc=1

Until now I could not find one that supports above 30kHz and 12V input, but do you think this could work? Or am I missing something here? I do not know how that voltage is produced, that is if it still "contains" a PWM signal and the driver (I guess trafo was the wrong word?) will make problems due to the DIM input being a hybrid sort of input.

I also searched for methods that would bring the frequency of the PWM signal down, but most are too complicated for me. Except for converting to a voltage and then converting the voltage to PWM again.
 

ElectricSpidey

Joined Dec 2, 2017
1,962
So your concept is to convert the output of the touch dimmer to 0-10 volts with that converter you listed?

I have no idea if that would work or not.

Can you explain exactly how you hooked up the touch dimmer to your trafo when you tried to set it up.

I wish there was more info on the dimming aspect of that driver in that data sheet. And more info on the output from the touch dimmer would help a lot as well.

I'm kinda shooting in the dark here.
 

Thread Starter

Linbaato

Joined Oct 12, 2021
8
Exactly, that would be the plan.

Dang, but totally understandable considering the lack of information. That is a bit frustrating.

I connected the output+ of the touch dimmer directly to the DIM pin of the driver. So I left the output- unconnected since I assumed the ground is connected anyways. After that did not work I also tested to connect the output- to the DIM pin. But the driver just keeps driving at 100%.

Interesting is to me that if I leave the DIM pin unconnected (i.e. to an about 30cm cable since this might be relevant) the light intensity fluctuates noticably. But I guess this is to be expected.
 

ElectricSpidey

Joined Dec 2, 2017
1,962
Can you do a continuity test to determine if the grounds are in fact connected? (unpowered)

Yea, your 30cm cable is acting as an antenna on a high impedance input, when you have the circuit working this shouldn't happen.

EDIT:

That 32k sounds a lot more like a DC/DC converter ripple than PWM for a LED, which would most likely be somewhat less than 10k.

If in fact the touch dimmer is PWM it's most likely switching on the low side, therefore you would need to attach the neg output from the dimmer to the DIM input of the trafo and you may also need a pullup resistor to positive.
 
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Thread Starter

Linbaato

Joined Oct 12, 2021
8
I did a continuity test as you asked and to my surprise (and seemingly not yours) the + sides of the touch dimmer are connected, but the - sides are not.

From your text it seems this would be a hint that it is indeed a PWM. But now I wonder why the frequency would be so high since that seemed unusual to you. Since I could not find any converters from that high frequency (only lower ones) I wonder if there is something strange about this.

But aside from that I will test your suggestion and report back!

Thank you so much for your help until now BTW!
 

Thread Starter

Linbaato

Joined Oct 12, 2021
8
Soo I am confused now.
As I hoped: It does something now! As I expected: the voltage range seems to not fit anymore, but way differently than I expected (according to my True RMS multimeter).

I used a 67k Ohm resistor between both outputs of the touch dimmer since the positive output should just be the 12V as established before.

I will call the voltage between the 'pulled up negative touch dimmer pin' and the 'negative input voltage' "output" from now on. If the touch dimmer is "off" then the output is 11.86V (the input voltage is 12V) and the LEDs shine with what I would say is 100%. If it is on and dimmed as bright as possible it is 2.37V. The light is quite bright, way brighter than I would expect it to be for 2.37V/10V. I would say 70% power but I just noticed I could measure the power output instead of guessing it. If it is dimmed as dim as the LEDs permit (in other words they still shine without fluctuations) the output is around 0.3V.

I cannot see higher values than 2.37V. I assume this is because the resistor would have to be lower. But it confuses me already that the 2.37 volts I see (if my multimeter is even correct) make the driver produce such a bright light. Also there is the problem that the driver produces the max brightness when the touch dimmer is turned off.

I could test again with a lower resistance but I assume the general problem stays.

Update: With the 2.37V the driver outputs 467mA to my LEDs. I am now quite convinced that my reading is not correct. And I guess this has something to do with reading PWM voltages with my multimeter.
 
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ElectricSpidey

Joined Dec 2, 2017
1,962
Yea, the driver will be full on when the touch dimmer is off because the DIM line must be pulled low to turn off the output. (see datasheet)

That assumes of course that the touch dimmer is PWM and switching the low side.

And you are probably right about reading PWM voltages with a DMM.
 

Thread Starter

Linbaato

Joined Oct 12, 2021
8
Screenshot_2021-10-15-21-05-18-000_com.miui.videoplayer.jpg

Hey again,

So I bought a simple handheld oscilloscope and would say it is indeed a 32kHz PWM signal. Sadly it is not a "clean" signal. I assume some electronic component like a capacitor produces the pattern shown in the attached picture. I also assume it might be problematic to try to further work with this signal.

What really riddles me is why the min-voltage is that high.

Well I am a bit lost again.. If you have any tips I'd be thankful. Otherwise thanks for your help until now.
 

Thread Starter

Linbaato

Joined Oct 12, 2021
8
Ohhh I think I got it. I was so sure it must have been an error of some sort but it might be this:

The minimum voltage is the minimum voltage with which the LED strip is assumed to still work. And a PWM signal just controls the voltage in the range to the max voltage. I already wondered why I can get the duty cycle extremely low but that would explain it.

ALSO:
It just came to me that the signal looks like it because of the huge LED strip strapped to it... I will analyze it again with just a resistor this time
 
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ElectricSpidey

Joined Dec 2, 2017
1,962
Is that the minimum Duty Cycle you can adjust the output to?

If it is, that wont work very well with the trafo. (never get low enough)

The signal should be more "square" but it should still work, try reading it with it hooked to the trafo and the pullup. the pullup should be between 10 & 1 k.

(pullup resistor connects between supply+ and the DIM input)

Also get a reading with the touch dimmer "off".

I think if you do get it working, you will never get the total range you are wanting, and the "on" when touch dimmer is "off" will have to be solved with a SPDT switch.
 
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