# Ensuring common ground in a circuit with several voltage requirements

I'm working on a project that has components requiring various voltages.

3.3v --- to power a set of laser diodes (qty 24 @ 50mA = 1.2A)

5v --- to power a set of infrared transceiver sensors (qty 24 @ currently unknown current)

9v --- to power an Arduino board and some peripherals (some typical Arduino-esque current draw)

I've never done a project like this (with various voltage requirements), and I'm wondering what the best practices are for powering these separate sub-systems.

Would one of the following scenarios be most appropriate?

Scenario A - Multiple power supplies --- Use a separate AC/DC power supply for each required voltage (Meanwell or somesuch) --- Somehow connect all the grounds together

Scenario B - One power supply and voltage regulators --- Use one AC/DC power supply rated at 9v --- Step down the voltage to 5v and 3.3v using appropriately rated voltage regulators

Scenario C - Something I could never have imagined!

• Do you need to control the laser diodes separately? Or could you run some in series? And why 3.3V for the laser diodes? (Are they running off a pre-built circuit?) The IR transceiver sensors are photodiodes or something more? I'm mostly wondering why three supplies? Dec 4, 2014 at 2:39
• I remember for a project a long time ago, we kind of mixed A and B. We had a single power supply with a multitapped transformer and multiple output voltages, driven by multiple regulators. Dec 4, 2014 at 10:55

I think it partly depends on whether this is a "home" project or for a commercial product.

The "easiest" answer would be to have a single 9 V input, and then use linear regulators to take that down to 5V and 3V separately. However, using a linear regulator to drop 5.7 V @ 1.2 A is going to dissipate nearly 7 W of heat, so you're going to need a serious heatsink, not to mention it being hideously inefficient.

For the "easy way out", use a transformer with a centre-tapped secondary (or two secondaries connected in series, but make sure you connect them up the right way around), so that you get (for example) 0-6-12 Vac outputs. You can then use the 12 Vac winding to produce your 9 Vdc output, and the 6 Vac output for the 5 Vdc and 3.3 Vdc outputs (with the usual bridge rectifiers, smoothing capacitors and linear regulators).

For the "right" (IMHO) solution, have a single 9 Vdc input, and use two switching regulators to produce the 5 Vdc and 3 Vdc supplies. Perhaps something like the ADP2302 from Analog Devices. If the current draw on your 5 Vdc rail is low enough (say < 250 mA), you could get away with a linear regulator here.

Either scenario A or B would be suitable. The main thing is that in all likelihood the GNDs for all three supplies need to be interconnected so that there is operational compatibility between the TX, RX and MCU sections of the design. The choice of the A or B really depends on factors that go beyond the realm of the electronics design stuff we like to talk about at this site. That said there are factors that you have to consider in making your decision of which configuration to select. Here are some of those considerations.

1. Cost of implementation is often a primary driving factor in a situation like this.
2. System level packaging is always a consideration. What do you have room for? Want it big? Small?
3. What are your skills toward implementation. Sometimes we take a path in a decision tree based on what we are familiar with or feel confident enough to weigh the risks involved.

What ever you choose to do please do consider energy conversion that is efficient as possible. The days where we can choose to freely use inefficient linear regulators operating at 10 to 50% efficiency should be well behind us. Use switch mode energy conversion where ever possible. (Note that in some localities there are laws requiring certain classes of products to meet certain efficiency standards in order for those products to be sold in that area. Battery chargers are one of these categories).

I have used Scenario B with success using LM78xx voltage regulators. It's a very straight forward approach and doesn't increase the component count very much. Just be sure that your wall wart (or whatever you are using) outputs a DC voltage that is at least several volts higher than the rated output voltage of the regulator. Also be sure that it can supply enough current with some room to spare.

Scenario A seems rather unwieldy.

Scenario B. Use pre built switching regulators based on the LM2575 step down converter. The prebuilt boards are normally designed to be drop in replacements for the LM78xx series of linear regulators. You can buy them in 5V and 3.3V and feed them the 9V you are supplying your Arduino. The LM2575 is very easy to use and requires only 5 external components: diode, inductor, two resistors, and a capacitor. Fixed voltage versions only requires the diode, inductor and capacitor.