Hi. I need to modulate an LED using a current source. The LED is part of a solid-state relay that is to switch at 100Hz for the ultrasonic ranger. It's purpose is to disconnect the receiver from the transmitter during transmission otherwise the receiver basically goes deaf (just like a bat). But in order to get the minimum distance I want the switch must connect the receiver to the transducer within 0.5ms. As a result, I need to drive the SSR's LED at 20mA out of a maximum of 25mA. The LED forward voltage also varies by too much so I can't just use a precision resistor value blindly without potentially burning out the relay.
SO I want to use a current source. Basically this one (except it's actually all inside an adjustable voltage regulator)
https://en.wikipedia.org/wiki/Image:Op-amp_current_source_with_pass_transistor.png
and switch the current source on and off. Except that it seems to me that the regulator cannot respond fast enough. If I modulate the enable PIN on the regulator it takes 1ms to turn on (not including the delay of the relay).
So I either have the option to place a MOSFET in series with the LEDand modulate it that way, or play a shunt MOSFET in parallel with LED and modulate it like that. But the series case requires both the amplifier current and voltage output to adjust during switching while the shunt method requires the amplifier voltage output to change (and not so much the current output). Either way, I don't know if any of this will help.
Any ideas?
SO I want to use a current source. Basically this one (except it's actually all inside an adjustable voltage regulator)
https://en.wikipedia.org/wiki/Image:Op-amp_current_source_with_pass_transistor.png
and switch the current source on and off. Except that it seems to me that the regulator cannot respond fast enough. If I modulate the enable PIN on the regulator it takes 1ms to turn on (not including the delay of the relay).
So I either have the option to place a MOSFET in series with the LEDand modulate it that way, or play a shunt MOSFET in parallel with LED and modulate it like that. But the series case requires both the amplifier current and voltage output to adjust during switching while the shunt method requires the amplifier voltage output to change (and not so much the current output). Either way, I don't know if any of this will help.
Any ideas?
Last edited: