High Power LED Driver Circuits controlled by relay

[themoog]

Im not sure if its because i'm on nights or i'm a pleb but I want to control this circuit with a relay dry contact not a MCU.... what am i not getting today




http://www.instructables.com/id/Circuits-for-using-High-Power-LED-s/step8/a-little-micro-makes-all-the-difference/

Thanks chaps....feeling odd today

[bookaboo]

You want to switch LEDs on/off using the relay or you want to PWM them? relays will be to slow for PWM.

[themoog]

Hi Bookaboo, yes sorry i was a bit vague there!

Well i was hoping for the option...

So I might want to use a Mcu to PWM...... or in a very basic case I just want to turn a high brightness led on with a dry relay closure.....set brightness...

Depends if I fit a MCU in the light or not....

Thanks so much for your help

[danadak]

Relays switch very slowly, unsuitable for most PWM applications. Thats reason
MOSFETs are used. You can use relay for simple on off switching. Note there
are specialized relays that switch fairly fast, but no low cost, still PWM application
not suitable.

When you switch a power MOSFET you are looking into a large C load, so either
you run PWM at low rates, frequency, or you use a MOSFET gate driver circuit/
IC.

Regards, Dana.

[themoog]

No i understand the difference between the relay and a mcu PWM signal... I want to ensure im driving the LED at constant current thats my main concern

[themoog]

More detail

Ive 3d printed some tally lights for Panasonic ptz heads. I'll send you a photo once version 2 is off the printer.

I want to use high brightness rgb LEDs. Now I want to make a little general purpose PCB where you can either use a pwm from a mcu to drive each channel (3 mosfets for each channel) or don't use the micro and just control from a relay 'dry'

Here's a nice constant current driver for pwm

http://cdn.instructables.com/FV0/2PEY/J4VEWIFNTO8/FV02PEYJ4VEWIFNTO8.MEDIUM.jpg

[Kilrah]

Just wire the relay to shunt Z1, which is what the micro would do in such an open collector scenario. Relay closed = LEDs off.

[themoog]

simples of course! How though could I invert the relay? Normal closed = on?

[senso]

You can use the Normally open or the normally closed relay pins so that it behaves like you want.

[highvoltpower]

Designing and types of relay circuit is vast, but in many electrical projects uses MOSFET as their main switching device, that’s why use are using these. We are only use relay for simply on/off switch. Ya justrelay closed and LED off.

[danadak]

Note in your circuit if you used an open drain output on UP you could use
that as switch to ground for the MOSFET base, providing R1's current (when
bottom end grounded) did not exceed ratings of output, typically 20 mA or
so.

Not Z1's high valued tolerance must meet the limitation most pins have on
UP that pin not exceed Vdd + .3V, otherwise you will be supplying current
into the pin, hence change Vdd rail of UP inside part. Cause is parasitic pro-
tection diode inherent in all CMOS I/O structures that will turn on. So do
the DC error analysis on Vdd and Z1's tolerance to make sure you do not
inject any current into pin. Use .3V as the CMOS parasitic diode turn on V.

https://www.fairchildsemi.com/application-notes/AN/AN-339.pdf

Regards, Dana.