Ebay DC - DC boost converter

flippineck · Dec 13, 2013

I bought a dinky little DC to DC boost converter on ebay. It has a twin screw terminal input block, and a twin screw terminal output block.

The listing says it will accept a DC input of something like 10 to 60 VDC, and output anything from 12 to 80 VDC.

On the board, there are two trim pots. One is labelled 'voltage adjust', and one is labelled '(input) current adjust'.

The board is populated with a large 2-pin toroidal inductor, several large value electrolytics, two eight-pin SMT IC's, and 3 three-pin IC's which look like black, rectangular, bolt-to-a-heatsink via a tab type power transistors. Amongst a bunch of smaller assorted SMT components. It's a rectangular board say 2 inches by 3, and the whole thing is mounted on a decent stonking black aluminium heatsink.

What I can't figure out is, I'm thinking, the load is going to present a basically constant power requirement in stable operation? so, why do I need a trim pot for 'input current adjust'?

Here is the unit: http://www.ebay.co.uk/itm/310705571577

I was expecting that, once I set the output voltage with the load connected, with a constant input voltage, wouldn't the input current just look after itself? a la P=IV & V=IR? why would I want to try and alter the current away from what the load 'wanted' to take?

I must be missing something in my thinking..?

BTW although this boost converter is designed as a general purpose module (i think), my own specific application is to take the 12V output from a car battery, and run an LED rated at 32V from it. Maybe my own simple application is colouring my thinking.

davenn · Dec 14, 2013

its appears to really be an output current limit adjustment ( which is very handy)

with a max of 15A input, the output can be adjusted from 0A ( presumably) to 10A max ( stated)

Dave

PS.... its a real neat convertor, there's times where I would like a 28V supply for powering a portable RF amplifier that runs off 28V

flippineck · Dec 14, 2013

a follow-on question... I just rigged up this unit to a 12v battery (big lead acid leisure type) on the input, and a 3 amp, 100 watt 32-34 volt led chip on the output.

all worked absolutely fine until I began being silly and twiddling the voltage adjust on the regulator, blindly, with the led at a high state of brightness.

I was trying to stop the combo 'flickering' slightly at high brightness.

POP.. nasty smell from the regulator.. the smoke came out :-(

looking at the IV curve for a white led, does it look like I maybe produced a small voltage increase, say from 34 to 36 V.. and that resulted in a huge runaway increase in current?

and could the flickering I was getting at max brightness, be caused by some kind of limit - clipping in the regulator? i.e. a capacitor / inductor setup running out of puff & having to restart some kind of oscillator over and again at random?

trying to decide whether, the regulator is just not man enough for the job, or whether the failure really was just directly as a result of me twiddling knobs without watching the voltage and current carefully...

edit - thinking about it, it could easily have been the current adjuster I was twiddling when the smoke came out..

(*steve*) · Dec 14, 2013

flippineck said:
a follow-on question... I just rigged up this unit to a 12v battery (big lead acid leisure type) on the input, and a 3 amp, 100 watt 32-34 volt led chip on the output.

The important thing here is 3A -- the voltage is secondary.

all worked absolutely fine until I began being silly and twiddling the voltage adjust on the regulator, blindly, with the led at a high state of brightness.

I'm fearing the worst...

POP.. nasty smell from the regulator.. the smoke came out :-(

Yep, there you go. It was either that or the LED (which you may have damaged anyway)

looking at the IV curve for a white led, does it look like I maybe produced a small voltage increase, say from 34 to 36 V.. and that resulted in a huge runaway increase in current?

Yes, and that's one reason (there are other even more important ones) why you should regulate the current through a LED rather than the voltage to it.

and could the flickering I was getting at max brightness, be caused by some kind of limit - clipping in the regulator? i.e. a capacitor / inductor setup running out of puff & having to restart some kind of oscillator over and again at random?

I haven't had a look at the regulator, but I'm guessing that current regulation would be a secondary thing with it, especially since the current control is labelled as input current.

trying to decide whether, the regulator is just not man enough for the job, or whether the failure really was just directly as a result of me twiddling knobs without watching the voltage and current carefully...

Either could be the issue. I'd be leaning toward the latter, but the former needs to be tested (would it have gotten too hot and failed anyway?).

edit - thinking about it, it could easily have been the current adjuster I was twiddling when the smoke came out..

That could also have done it.

flippineck · Dec 14, 2013

Is there a simple, cheap 2 wire series component that could limit current? Like a fuse, except that it wouldn't 'blow' but simply hold the current and prevent it from rising above some predetermined limit?

I was just thinking, if I ordered some more LEDs maybe I could protect them and the regulator in this way, cheaply..

I could use say 2 amp fuses, but is there something that wouldn't have to be replaced if the current became excessive

Something like a modern, high-current version of a 'curristor' tube

(*steve*) · Dec 15, 2013

flippineck said:
Is there a simple, cheap 2 wire series component that could limit current? Like a fuse, except that it wouldn't 'blow' but simply hold the current and prevent it from rising above some predetermined limit?

Well, there are some current limiter circuits that can do this. However they can be highly dissipative (i.e they get HOT) if you short them out.

The best solution is a proper LED driver which can do this magic in a more efficient manner (you're looking at 3 or 4 wire devices though.

The power supply you have *may* be able to do this if you don't fiddle too much. You would need to find out what chips it's using and in what configuration to determine how the current limiting works. Your experience with the flickering suggests it's not great.

Perhaps get a LED driver for that LED to be placed after the boost converter (or find one which operates from 12V)

I was just thinking, if I ordered some more LEDs maybe I could protect them and the regulator in this way, cheaply..

The important thing is to protect them properly. For that you need a current regulated (possibly as opposed to limited) supply or a proper LED driver.

I could use say 2 amp fuses, but is there something that wouldn't have to be replaced if the current became excessive

Yeah fuses might even be too slow in some cases!

Something like a modern, high-current version of a 'curristor' tube

Well, such things exist, but see my first answer above. For these type of currents they're not practical.

flippineck · Dec 15, 2013

Thanks Steve. Had a chance now to dig around on ebay & turns out there are some similar devices to the first one I got, which specifically list 'LED driver' in the suggested applications and seem to have output current control - and happily some of these seem to be even cheaper than the one I destroyed.

Never mind the IV curve, this is a good *learning* curve this one lol