Hi all,

First post here, please be kind. I have spend some time working on a solution and I really could use some guidance. I'm planning to use mbed to drive 0-10V devices like light dimmers, led drivers and controllable pumps, common place today in the aquarium industry.

The current plan is to use a SPI DAC to multiply the ports without much hassle using 4 or 8 ports per DAC that can connected in a daisy chain. I believe I will have to use an opamp (or use the "built-in" opamp on DACs like MAX5250) to bring 3.3V to 10V. The problem is driving many devices will surely pull more than the 4-40mA recommended for mbed. Being a beginner in the eletronic field, the question is: How can I use an external power supply to drive all components (mbed included) but pulling the amps for the devices from the power supply and not "via" mbed.

Of course, I jumped into the solution myself, feel free to educate me if the solution above is plain wrong and even more to propose new solutions :). I saw some suggestions of smoothing PWM signals but I really want to scale as needed (without running out of pins) and I think the DACs also can give me more precise output.

Thanks!

Silvio

A DAC like the MAX5250 can only operate on 5V supply - for both the analog and digital portions. So the output from the op-amps cannot go above 5V. You would need to use external op-amps to do that. But whatever you do you need a 3.3V supply for your embed internals, and 5V for the DAC. and then at least 12V for the op-amps that will produce the 10V you need.

The MAX5250 requires a minimum 2.4V for a logic high on the digital inputs, so it will connect directly to the LPC1768 with no level translation needed.

John

For what you need "light dimmers, led drivers and controllable pumps" you may use PWM outputs as well even with more flexibility and also more simple. A mosfet at pwm output may do the trick ..

Thanks for the replies!

Yes, PWM seems to be a popular solution. My two concerns with PWM are: 1) how to scale over the 6 outputs provided by mbed? 2) How precise will the output be? Passing 0.5 to PwmOut will give me 5V on the output?

Thanks for the pointers, went back to the research and I guess I understood. OpAmps have high input impedance so the current will be drawn out of the power supply not the V+/V- input, mbed would provide just a very small current due to DAC itself and opamp input bias. I would have to supply a regulated 5V to mbed Vin (won't be using USB) and DAC's Vcc and Vref. Each output would be connected to an opamp supplied with 12V on Vs+, forming a non-inverting amplifier with 2 x 10k resistors to bring 5V to 10V. A 0.1uF cap per IC for decoupling.

If I got it right so far the big question is: How to get a 12V power supply to provide 5V for mbed and DAC? A bit too much for a voltage regulator? Would be easier to use a 5V power supply and step it up to 12V just to power the opamp(s)?

It was really easier on my mind when I thought that opamps could generate 10V from 3.3V out of thin air and a couple resistors.

Silvio

Hi Silvio,

I can only repeat this

MOSFET at every output. They should be LOGIC LEVEL MOSFETs.

How much precision do you need? Yes. It will give 5V, if you power the MOSFET with 10V.

Take a look at mbed's pinout picture. PIN2 (VIN) is just made for you: 4.5V-14V input. In case you don't need extra DAC.

Regards.