I have five devices that need to be powered. I would like to use one DC source to power the four devices. Should I use voltage dividers, voltage regulators or a mixture of both? Also, any recommendations/tips about the required parts and circuit design is greatly appreciated.

Here are the specs of the devices that need to be powered:

A) Temperature Probe, v: 5v, current: 6uA
Temperature Probe - Datasheet

B) PH Sensor Circuit, v: 5v, current: 1.6mA
PH Circuit - Datasheet

C) EC Sensor, v: 5v stable, current: 4.2mA
Conductivity Circuit - Datasheet

D) Application Board, v: 4.5v - 9v, current: 500mA
MBED Application Board - Datasheet

E) Wifi Module, v: 3.3 stable, current: 180mA
RN-XV WiFly Module - Wire Antenna - Datasheet

  • \$\begingroup\$ Tell us a bit about the type of devices that would need the voltages. Some devices require little noise, while others will work fine with more. This will affect the type or regulators that can be used. \$\endgroup\$
    – AndrejaKo
    Mar 27 '13 at 22:53
  • \$\begingroup\$ @AndrejaKo I edited my original post with what the devices are and a link to their datasheets. \$\endgroup\$
    – dottedquad
    Mar 28 '13 at 0:07

My advice here would be to try to get a 5 V power supply with at least 1.5 A output just to be sure. Then get a linear 3.3 V regulator. A heatsink may come in handy as well, but most likely won't be needed if my assumptions are correct.

Here's my reasoning: Mbed uses 3.3 V I/O, so it would be best to run digital circuits at same voltage as to not have any potential problems with voltage translation.

So run everything except the Mbed and the E.C. circuit from the linear 3.3 V regulator.

The temp probe needs to be sampled with ADC and you'll get a bit extra resolution if you can run it from 3.3 V, since each bit of the ADC will represent smaller value. Also if the Mbed is going to sample it, then it would make process a bit easier due to reference voltage being more or less same.

The pH sensor is rated to run fine with 3.3 V operation, so just to make sure that there are no problems related to serial port high/low levels, run it from 3.3 V.

The WiFi is 3.3 V already, so no need to comment much about that. I guess that you're going to try to power it from Mbed. If that's the case, then it's good since it shouldn't interfere much with operation of other components.

The Mbed itself has its own 3.3 V regulator, so it would go directly to 5 V power supply.

Finally we have the E.C. circuit, which to me looks the most problematic. It's 5 V and Mbed is 3.3 V, but has 5 V tolerant I/O. If the circuit will work fine with 3.3 logic levels for input, then that's good. Otherwise some level translation may be needed. Unfortunately, the datasheet doesn't say anything. Next we have the "as close to 5 volts as possible" and "ripple and noise free" requirements which feel a bit unprofessional to me, since it isn't really explained what happens when voltage isn't exactly 5 V and there's no such thing as ripple and noise free. For that reason, I'd add a low ESR capacitor as close to its power supply pins as possible and maybe even a ferrite bead on the 5 V line, just to make sure the noise from power supply is away.

  • \$\begingroup\$ thank you! I was under the impression that I would need a 20 something volt power-supply, then branch the power-supply wire leads to individual voltage regulators then to the devices. Just needing a 5v power-supply vs a 20v power-supply is good to know. \$\endgroup\$
    – dottedquad
    Mar 28 '13 at 1:06
  • \$\begingroup\$ The E.C. uses a PIC18F with a Input High Level of 0.7*VDD on most pins. 0.8*vcc on Schmitt inputs. A level translator would be needed. On the same note, the pic18F does have a very wide VCC range. It could run on 3.3v, but you don't know how much that will affect the rest of the circuit, adc readings, or processing speed on it. \$\endgroup\$
    – Passerby
    Mar 28 '13 at 3:10

5V will cover the requirements with 800 mA or 4W capacity.

3term reg for 3.3V if you want some Red LEDs for show.. Bypass 3.3v reg with 20mA parts and up to 100R in series. No additional drain.

  • 1
    \$\begingroup\$ Like me, sometimes too hasty... would the input DC voltage level being 40V make you change your opinion? I'm not saying it is of course. \$\endgroup\$
    – Andy aka
    Mar 27 '13 at 23:07

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