Hi, everyone I'd like you to answer a question for me. I'm less than an amateur in electronics, but can solder some simple parts for what I need occasionally. So I have a question, I have opened this account as I couldn't find a conclusive answer anywhere.
It is a simple question.

If I use a 18V 3A power supply adapter (from an old laptop)
and after that put a step down converter and reduce the voltage to 12V
My question is how powerful can my LED at the end of the circuit if 12V?
36W (12x3) or is it 54W (18x3A)?

Thanks for helping me...
.

 

Good question.
The immutable rule of ANY power conversion scheme is that:

The power out will always be less than the power in. Sometimes it will be much less.

Let us begin with your step-down converter going from 18V to 12V
Power in = 18V * 3A = 54 watts. Assume for a moment that this process is 85% efficient, which is typical for a buck converter. This means the output power will be 54 watts * 0.85 = 45.9 watts. Now 45.9 watts from 12 V means that there are 49.5/12 = 3.825 Amperes available on the output. When you drop the voltage, you can actually get more current at the lower voltage, so the power relationship is preserved.

We can do the same calculations for a better buck converter that is 90% efficient, but as you should know reaching 100% efficiency is quite simply impossible. At some point you compromise due to the increasing difficulty of squeezing out more efficiency. This problem is similar to accelerating to light speed, the closer you get the more difficult it becomes to go even a tiny bit faster.

You might wonder about the 85% estimate. It is a compromise value that is useful for setting your expectations. When you actually put one to use you can measure the efficiency of the conversion by reducing the load impedance until the output voltage begins to drop. This is the point of maximum current that the converter can produce. Then you will know what the efficiency is at least for a sample of one unit.

In the case of an excessively cheap or bad design the efficiency will be something a bit less than say 75%. A reasonable expectation for DC-DC converters would be 85%± 10%. In designing a converter, I usually start with a goal of 80% with a good deal of confidence that the end result will end up better than that.

 

Welcome to AAC!

My question is how powerful can my LED at the end of the circuit if 12V?
36W (12x3) or is it 54W (18x3A)?

If you use a regulator to reduce the voltage, you're never going to get 100% efficiency.

It would be helpful if you told us what type of LED you were using. A datasheet with specs would be best.

 

Thanks for your quick replies, my logic told me it should be something like this, but didn't think of the efficiency. This shouldn't be a problem as I probably won't need to max it out anyway. I'm using a simple converter from aliexpress it is rated 5A, I have another one at 10A but it was unnecessarily big for my housing so I put the smaller one . I'm actually making DIY microscope illumination...
Currently I'm using a 18V cree led from mouser 700mA (https://tinyurl.com/y8dzhkhc) so I'm not even remotely close to the limit. But I will need another LED as this one has too big of a peak in the blue spectre, I have another 18V similar one at home with a higher CRI index which I will try and see if has better light. If not, I'll probably go to 9V or even 3V products, so I was wondering if I needed an another adapter for this... 10-12W should be enough for LED. more would be ok also, but then they would heat too much. So a completely different approach would be needed.

Also I would like to be able to switch to halogen occasionally with the same power supply, I'd use 12V and 20-30W so I wasn't sure if it would work with the same adapter.

The reason I'm using a 19V power adapter is that they are easily and cheaply (or freely) available from old laptops and are rated at 3-4A which is quite a lot for my application...

 

I actually did think of buying a better quality step down converter from Mouser, but I was overwhelmed by the numerous products, so I dropped it. That store is really built for professionals and I got lost there instantly...

 

I am a little confused.
You have a 18V supply, which you want to reduce to 12V?
You have 18V LEDs?

 

Currently I'm using 18V LEDs, but would like to use 12V halogen occasionally with the same power supply and might switch to 3V, 9V or 12V LEDs for some reasons like the size, CRI, or something... I can use my step down converter to lower the max voltage an to dim the leds with 2 parallel pots...

 

If not, I'll probably go to 9V or even 3V products, so I was wondering if I needed an another adapter for this...

Since you have a 18V adapter, you should put strings in series if you can. 2 9V or 6 3V.

The reason I'm using a 19V power adapter is that they are easily and cheaply (or freely) available from old laptops

I always look for bargains when I'm at secondhand stores.

Also I would like to be able to switch to halogen occasionally with the same power supply, I'd use 12V and 20-30W so I wasn't sure if it would work with the same adapter.

As long as the regulator can handle the current and power dissipation.

 

You could try PWM to set an average voltage. By using a microcontroller you unlock many possibilities and you can expand if you want. A good deal of calculation will become necessary so you don't stress your power supply. As long as you observe the available power you should be fine.

 

By using a micro controller you also have the option of a software glitch applying the full voltage for several minutes to your 12 volt device. That might be a problem.
I suggest experimenting with using a 12 volt light bulb, as they are cheaper and more forgiving.

 

You could try PWM to set an average voltage. By using a microcontroller you unlock many possibilities and you can expand if you want. A good deal of calculation will become necessary so you don't stress your power supply. As long as you observe the available power you should be fine.

One calculation would be, how quickly does a 12v LED strip let out the magic smoke when powered by 18v so you can get the maximum pulse width for the PWM.

 

One calculation would be, how quickly does a 12v LED strip let out the magic smoke when powered by 18v so you can get the maximum pulse width for the PWM.

Are you saying PWM is incompatible with an LED strip? Wouldn't the maximum duty cycle in this case be 66% ignoring losses?

By using a micro controller you also have the option of a software glitch applying the full voltage for several minutes to your 12 volt device. That might be a problem.
I suggest experimenting with using a 12 volt light bulb, as they are cheaper and more forgiving.

Agreed, but that hasn't happened yet in my projects.

 

I can't say I'm able to follow the discussion with full understanding, but that is ok... I'm currently using this:

https://a.aliexpress.com/_mroV8NY

I have added pot parallel to one of the trim pots, so I can dim the led with it and use the trim pot to adjust the maximum voltage. It works good.
It would be great to add a 3way switch for switching max voltages e.g. 18V, 12V and 9V
For that I suppose I'd need 3 trim pots and a switch, but not sure how would I wire this and what kind of a switch I would need...
I probably won't be an active member here ever, as my knowledge and ambition in the matter are limited. But I do appreciate your help, so thanks...

 

I have added pot parallel to one of the trim pots, so I can dim the led with it and use the trim pot to adjust the maximum voltage. It works good.

Unless you're using the pot to control some LED controller, a pot isn't an effective way to dim LEDs.

 

Are you saying PWM is incompatible with an LED strip? Wouldn't the maximum duty cycle in this case be 66% ignoring losses?

I'm saying that in pwm, you have an on-period and an off-period. And even if the average is 12v a full on-off cycle, you still risk applying 18v for too long and charring a 12v strip. Your PWM scheme may (MAY!) work at 10kHz where the on cycle is 67 microseconds and off cycle is 33 microseconds but very unlikely to work at 75Hz where the on period is 10 milliseconds.

Most LEDs can be overdriven with 50 to 100% higher current but for only double-digit microseconds. Assuming the 12v LED strip is 3 LEDs with Vf=3v, and a current limiting resistor, the 18v pulse will deliver a current 300% of the current delivered with a 12v supply. 10milliseconds is a long time at 300% of specified current.