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Topic: Using mosfets to switch low current from a high current gate voltate (Read 2232 times) previous topic - next topic


Dec 31, 2012, 05:11 am Last Edit: Dec 31, 2012, 05:15 am by Suaveman Reason: 1
I understand what your saying. The application is a rf remote that can dim the buckpucks, possibly with another rf switch to switch between multiple drivers on different pwm pins.

This is the pwm dimmer i have-


Low pass filters may be necessary, like you are saying, but i think the buckpuck can dim via pwm and not just analog, as at one time i had a voltage trimmer pot hooked up to its output, and it worked to dim the buckpuck without an arduino in between to invert the logic. I'd like to either just use this dimmer, and another rf remote switch to choose what pwm digital output pin, and driver/led to dim, or one remote that has can do both. The latter option, i've looked for but havent found anything that fits said application.

If theres an easier/better way to be able to dim these drivers via a rf remote with/without an arduino then i'm all ears. I'm just working with what i have.

Is there a better rf device compatible with the arduino's analog in to fit this application that doesn't require low pass filters and optioisolators to change the voltage range?

I'd power the dimmer from a 5v source and just use a low pass filter to make it analog 0-5v, but it diddn't work when i tried, and i think it needs a higher input voltage to operate.

Like i said, i'm working with what i have, if theres a better rf remote compatible with the arduino's analog inputs, then i'm all ears, so long as its not too expensive and greatly simplifys the application. I haven't been able to find such a device.


Dec 31, 2012, 05:37 am Last Edit: Dec 31, 2012, 06:58 am by Suaveman Reason: 1
Alright, i take back what i said about it not working with a 5v input (the pwm dimmer that is). Using a 800ma 5v ac/dc adapter, i can get a 0-5v output using a 3k trimpot. Without the trimpot the voltage range is 4.6-6-2v. I tuned it down to 5v max when there is no dimming taking place, and can dim down to a few ma using this method. It did not work with a similar dimmer (5v input), but they have very different pcb's, and are different models.

So, knowing the buckpuck can read that pwm signal (from the arduino digital out pin), all in need is a low pass to make that 0-5v i get from the dimmer analog and input that to the arduino, correct?

Or can it read a pwm input signal (2560 mega) and no filter is necessary?

But that leaves the problem of current draw for the dimmer, i'd like to power the dimmer/arduino from one dc source. Since the 5v+ on the arduino can only provide 40ma (came across specs that current consumption of the dimmer is less than 500ma), powering the dimmer from the arduino's 5v+ is not a good idea. Assuming i can use digital read, to take that pwm input, and use it as a linear dimmer for a pwm output, something akin to-


But with digital read instead of analog in, since its pwm, is it possible, using the right resistance, to power the dimmer from the 9v1a the arduino is powered from, use a pot voltage divider (might be more/less than 3k, will have to expirement), and get a 40ma max pwm input from the dimmer?

In any case, after reading about rf filters, i assume since the dimmer can't be powered from the 5v+ on the arduino, i'd have to dig out the old heathkit scope to find the frequency of the pwm, and then devise some way to power the dimmer and limit the current it provides, along with building a rf filter circuit to make it analog.

Or am i overthinking this?

TLDR- Assuming i can use digital read to do something like analoginoutserial, to provide dimming without dac conversion from the pwm dimmer, IE pwm input, is it necesssary to limit the current provided to the input pin of the arduino when the power source for the dimmer is the 9v1a the arduino uses? If so, can this be accomplished by using resistors/trimpots alone?

Or do i need to figure out the frequency of the dimmer with my scope, and build a rf filter that will limit said current to 40 ma using the appropriate uf and ohm value cap/resistor to get an analog signal, 0-5v, 40ma max, from the dimmer when the dimmer's input is 9v1a, using a trimpot to voltage divide its output to 0-5v?

I think you are over thinking this a lot.

First, let me ask you what do you find really difficult about a low-pass filter?  Just a resistor and a capacitor will do.

From reading the Buckpuck's data sheet, it requires an analog voltage on the adjust input to work correctly.  That voltage can be between 0 and 5V, but the effective range is between 1.5V and 4.5V.  I do not think it will work using a PWM input.

However, there is absolutely no information worth anything about the RF dimmer at the link you posted.  You say it that it uses PWM.  Do you have a scope?  I'd like to know what the output waveform looks like when it is supplied with 12V and run without a load.  Can you supply an accurate picture/drawing of that?  I'd need to know the frequency (or period), the P-P voltage swing, wave shape, duty cycle range and the DC offset, if any.  If you have access to a DSO, then a picture of the waveform showing 2 or 3 cycles at each extreme (minimum and maximum) would be perfect.  It would also be a good idea to look at the output with a small load, maybe 1K ohm.

If it really does rely on PWM, then it might just be a simple matter of filtering and scaling its output and supplying that directly to the adjust input of the Buckpucks.

One more question, why do you need to use the Buckpucks?  This remote can supply 8 amps, and at 12V that is one huge number of LEDs (96 watts!).  Is it because you need more than that?
Facts just don't care if you ignore them.

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