0-10V Dimming Circuit Not sinking as much current as I had simulated :(

Thread Starter

DanielLitwin

Joined Mar 23, 2017
51
So I'm running an ultra-low power design and a 0-10V dimming circuit is a part of it. All of the dimming circuits I've done using op-amps have all previously been sourcing current; however this time I'm trying to sink current (at a minimum of 58mA).

I'm currently using a simple op-amp circuit (TLV9XXXX Series) that runs rail to rail from 0-3.3V. As of right now, the circuit is taking a PWM signal straight from the micro. It passes through a high impedance path before it hits the inverting input. The output runs through just 270ohm resistor which passes to the gate of an NDS331 N-ch FET. The source of the FET is grounded and the Drain has a very low resistance pull-up; which branches to the feedback I have set up going back to the non-inverting input (just a parallel RC loop) with a mild pull down to ground (As shown in the photo I've attached).

I'm well aware that how much this 0-10V circuit can sink is up to the MOSFET; though I'm attempting to get the most linear relationship between the PWM input voltage (0-3.3V) and the output voltage (0-10V with min-dim at around 500-700mV) that I can while also also being able to sink a decent amount of current (in relation the rest of the project).

Right now, the value of the resistor at the drain of the MOSFET is 2.2ohms, along with the cross-line resistor that's in series with 10uF ceramic.

I know there are much easier ways to dim; I have another dimming circuit that runs off of an isolated low voltage transformer with a current mirror feeding an output FET but I'm trying to conserve energy on this one as well as run it from only one micro output (I'm running low).

Ultimately, I'm wondering if there is a better way to configure this set up (PS Sorry for the bad photo but LTSpice is no better lol).

Thanks :)
 

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Alec_t

Joined Sep 17, 2013
10,376
That's a somewhat unconventional circuit.
Are you aware that as the (smoothed) PWM voltage rises the load current falls (assuming 'out' is connected via a load to the +ve supply)?
 

Audioguru

Joined Dec 20, 2007
11,251
Why do you show a strange "dimming" circuit but you do not show what you are trying to dim?? Please show it and the voltage and current it uses.

You do not understand that PWM is not linear. Instead, PWM switches its output completely on then switches it completely off so that the switching uses very low power. It is the duty cycle of the pulses that produce brightening or dimming. Wide pulses produce the most average voltage and narrow pulses produce the least average voltage.

A PWM circuit usually does not need an opamp and certainly does not use the filters in your circuit.
 

Thread Starter

DanielLitwin

Joined Mar 23, 2017
51
lol I'm quite familiar with PWM'ing various types of signals, I also understand that the and perceived linearity will have zero effect on the luminosity of the lights given they have entirely separate curvatures. I show this image because my hands are tied. Live with it or don't comment. Perhaps my wording made things confusing, but no need to be so condescending.
 

ebeowulf17

Joined Aug 12, 2014
2,942
If your goal is ultra low power consumption, why are you trying to create a linear control system (which wastes power in the FET at anything other than 0 or 100% duty) instead of using traditional PWM dimming, which is about as efficient as anything can be?

You're using more components and achieving worse efficiency. Why do you want to do it this way?
 

Alec_t

Joined Sep 17, 2013
10,376
What is it that you want to dim? If it's a LED, then you should be PWM controlling the LED current rather than trying to generate a 0-10V output, especially if you're aiming for maximum efficiency.
 

BobTPH

Joined Jun 5, 2013
2,028
I think he is trying to take a PWM signal and convert it to a 0-10V analog signal to drive a dimming input on some device.

Bob
 

ebeowulf17

Joined Aug 12, 2014
2,942
I think he is trying to take a PWM signal and convert it to a 0-10V analog signal to drive a dimming input on some device.

Bob
I think you might be right. The discussion of how much current his circuit can sink made it sound to me like this circuit was directly handling the load, but the circuit makes a lot more sense to me as you've described it.

If that's the case, it might help to know what device is being controlled - what's the make and model of the dimming device?

Specifically, I'm wondering about the source impedance and open circuit voltage of the 10V source that needs to be controlled. I'm not entirely sure off the top of my head, but I'm wondering if that source impedance might effect the voltage divider in the feedback loop. Although, the more I think about it, the more I doubt that this is the problem.

Just to be clear, and to make sure I'm understanding the situation correctly now, the inability to sink enough current results in the output voltage staying too high, so the problem right now is that this circuit provides dimming, but that the lowest achievable setting is still too bright?
 

Bernard

Joined Aug 7, 2008
5,265
Sink 54 mA, sounds familiar like sinking 54 ma for a ring of LED's on a co. logo plaque ?
If your hands are tied, what are the restrictions.
What is the load ? What is the project ?
 
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