Controlling high power LED with transistor and Arduino

Audioguru again

Joined Oct 21, 2019
6,671
Like most transistors, the BD941 is fully saturated when used as a switch when its base current is 1/10th its collector current.
Its hFE is when it is NOT a switch but is an amplifier with at least 2V Vce.

I do not know the output voltage of the Arduino when its produces a current of +10mA but it is surely less than +5V.
Then the BD941 is not saturated and gets hot, with dimmed LEDs.
 

MrSalts

Joined Apr 2, 2020
2,767
How can I measure output current? Can I put resistor in series and measure its voltage when leds are on?
Current will only ohms law until you exceed the capabilities of the power supply and it will either pop or drop the voltage (and still obey ohms law).
Either your meter is measuring some transient (because there is no load) or the power supply that is rated 90v, is broken and outputting 185v. I can't tell you how to deal with a broken power supply unless you want to attach a proper load and try again, or try another power supply.

Are you supplying the unit with 220 to 240V AC at 50 to 60Hz as the nameplate specifies?
 

wayneh

Joined Sep 9, 2010
17,496
That’s a constant current supply delivering 185mA at UP TO 90V. It’ll raise the voltage to overcome Vf of the LEDs and hold it wherever it needs to be to hit 185mA. The unloaded voltage is just an artifact.

As I said before, you cannot PWM a CC power source. It’ll see the varying load and lose its mind trying to adjust.
 

Irving

Joined Jan 30, 2016
3,843
you cannot PWM a CC power source. It’ll see the varying load and lose its mind trying to adjust.
Correct, you need to put the PWM switch on the input to the CC generator. Or you need to switch the output onto a passive load. It's only 18W so that's easily doable.
 

Irving

Joined Jan 30, 2016
3,843
Sorry I don't think I understand can u explain it a little bit more?
The PSU adjusts the voltage to maintain 185mA through the LED. So it is approx 90v at that point. When no current is drawn the voltage rises as the PSU tries to force current out. Similarly, if you try to draw too much current the voltage drops, eventually to zero.

So to fool the PSU into thinking the LEDs are there all the time, when we switch them off, we need to switch on another load that's close to the same voltage/current point. I.E. a constant current load. That'll stop the PSU panicking! The simplest solution is a second transistor driven off the first so that when the LEDs are off it turns on with a load resistor equivalent to 90v @185mA or about 470ohms. The only downside is that the dimmer the LEDs the hotter that resistor will get as its dissipating around 20W.

The right way to go this would be to change the PSU for a constant voltage one and add a switched CC generator on the output.
 

Irving

Joined Jan 30, 2016
3,843
How can I measure output current? Can I put resistor in series and measure its voltage when leds are on?
Connect up the LED as normal with multimeter on 1A range in the + lead to measure current. Then remove multi meter, change leads back and switch to DC volts and reconnect LEDs direct to supply, then measure volts across the LEDs.

This will give you nominal working point V & mA.
 

Irving

Joined Jan 30, 2016
3,843
I do not know the output voltage of the Arduino when its produces a current of +10mA but it is surely less than +5V.
Then the BD941 is not saturated and gets hot, with dimmed LEDs.
It's guaranteed >4.6v @10mA and >4.3v @ 20mA, across full temperature range, subject to total chip loading, but here only one output is in use.

My experience is it's rarely below 4.8v.
 
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