Splitting a Power Supply to Multiple Voltages

I'm making a robotic bartender that I need 5, 6, and 12 volt dc for. I could just run a surge protector into it and have 3 different power supplies, but I'm using this project as a way to learn a few things. I have an 18.5 vdc 3.5 amp power supply that I have that I'd like to divide up, in some way, to different components. I did some research and it seems the best way to do this is with Buck converters. I found some cheap, adjustable ones with good reviews online: (http://www.amazon.com/dp/B009HPB1OI/ref=wl_it_dp_o_pC_S_ttl?_encoding=UTF8&colid=3LKG06RAAIZO1&coliid=I3LB04M0JQAR6I&psc=1#productDetails).

I wanted to know if this is a good idea or if there's a better one, and also how to exactly to hook this up if it is a good idea. Do I simply hook 3 up in parallel? Is there more that has to be done, a voltage divider prior to the buck converters perhaps? Will the amperage be supplied where it needs to be or do I need to do some work to make sure it goes where it needs to?

Power Requirements: -1.5 amps at 12 volts -300mA at 6 votls -5V for arduino Thanks in advance

Do I simply hook 3 up in parallel?

That will work.

Or, you could also connect a 6V linear regulator chip to the output of the 12V regulator. Linear regulator chips are much less efficient, but they just require the chip/device and one or two capacitors (saving cost, complexity, and space). The power (and heat generated) for a linear regulator is calculated as current through the regulator X voltage dropped across it. So, running it from 12V instead of 18.5V will dissipate/waste less power.

The Arduino has a 5V regulator on-board, so you can run it off 12V also as long as you aren't also powering a bunch of other stuff from the Arduino's power supply.

Is there more that has to be done, a voltage divider prior to the buck converters perhaps?

No. The switching/buck regulator is it. Voltage dividers ONLY work in very-low current applications where you need to divide a reference or signal voltage. They are NOT used in power supplies except they might be used to create a reference voltage in a power supply circuit.

Using a buck pwm regulator from 18.5V to 6V is definitely a good idea, and gets less hot than the linear regulator would. I've been using the 1Amp type off ebay with a small screw by which the 6V is set. I got one to break at 0.4A 11V out from 26V in when it got hot sun on it, so the rating as advertised might be a bit over-optimistic with the cheapest ones. . 18.5V is inside the range which can be accepted by the built-in buck pwm on the arduino board from V_in or the barrel connector, but check very carefully whether your robot battery might ever go over 20 whilst on charge or in other abnormal circumstances. For the slight efficiency improvement and substantial risk reduction, you might decide to buck down from 18.5 to about 9 to 14V to provide a 'nice' input to the arduino built in buck downconverter and well away from its 20V limit.

For your 12V heavy power use, the cheap buck converters which I've been using would break for sure, but the approach seems correct. See if you can find something whose rating is generous, and have a careful look at 'surge current' when starting motors to turn etc. It might be worth adding some fairly substantial capacitors between your 12V and your load. If your arduino V_in is going to go to this same 12V, then give it some length of not-too-thick cable and its own large capacitor near to the arduino end.

Until the parts arrive, you can test many 12V things with a scrap desktop computer ATX power supply.

Yes, those should work fine. Yes, connect them in "parallel", 18.5V to the input of each, common grounds.

Rated at 3A maximum, 1.5A is a practical maximum, so they should work fine for 12V.

You supply the voltage with a current capacity more than the maximum needed, the circuits draw as much current as they are designed to draw. No need to "steer" it other than supplying the correct voltages.

DVDdoug: Awesome. Thanks a lot! I'll get two or three and hook them up in parallel. Its a prototype that I'm making currently, but I'm designing it to be expandable so I may have more current needed at 5 and 6 volts later on.

ad2049q: I have five 12 vdc motors hooked up to a relay board. Would you suggest putting a capacitor in parallel at each motor or just one between the buck converter and the relay supply voltage. I'll be using a separate buck converter to power the arduino and some other small loads. Also, around what value capacitor should I be looking for?

d_vee: I wanted to know if this is a good idea

Yes.

PS: If it was me I wouldn't run the converters near full load, eg. If you need 1.5A, get a 3A converter.

Running at full load will get very hot.

fungus: Yes.

PS: If it was me I wouldn't run the converters near full load, eg. If you need 1.5A, get a 3A converter.

Running at full load will get very hot.

Yup, that is exactly the power requirements I'll be working with. Thanks

Buck converters can be used no problem. But you might need to add a radiator to them if you need more than 1-2 amps from them. Now, I understand that this is just a prototype, but buck converters would be more efficient if the starting voltage was closer to the output voltage. So plan on getting 12V regulated supply for the final product if you need to be efficient with energy use. Then, you can convert from there to the other 2 lower voltages you need, and you don;t even need to worry about the convertors getting hot, since you draw so low current on 6 and 5 V rails. And you can search ebay for buck converters, they are very cheap, and some sellers will also have the needed radiator for them if you need them too.

Much cheaper directly for Asian sellers on E-bay. Buy a dozen..

http://www.ebay.com/itm/New-DC-DC-Buck-Converter-Step-Down-Module-LM2596-Power-Supply-Output-1-23V-30V-/111357136205?pt=LH_DefaultDomain_0&hash=item19ed670d4d

If you have sensitive analog electronics with small signals then you'll probably want a linear regulator to power it (even if thats just a LDO dropping down 1V from the buck-converter output. All switching regulators put noise on the rail that's perfectly acceptible for digital circuitry but which can be a problem with analog.

For your setup a buck reg down to 12v followed by a buck down to 6V followed by a LDO linear regulator to 5V might be one way.

At the moment the cheapest DC-DC converters seem to be the plug-in replacements for linear regulators, so they even have the same pin out (if not the same footprint).

"What value capacitor .." I'd do one 'huge' capacitor near to the relay supply. It is possible to get 10,000 microfarad 16V rated capacitors which might be quite good on your 12V motor supply. You can always fit several of them parallel, and with delivery charges, buying several at once might be obligatory. Since "as big as possible" is the right size, get what looks like a bargain best £ per 1000 microfards rated >15V (to be used at about 12). Chances are that they'll be electrolytic. You might also get somewhere by stringing together 3's of 5.5V rated "supercapacitors", but as the peak current out of those might be a bit less than the motor start surge current, those would be 'as well as' electrolytic capacitors.

And I have to say it; large capacitors at 12V are sufficient to start a fire if someone drops a bit of steel wool onto the terminals. You might want a few automotive fuses, if being safety concious.

ad2049q: "What value capacitor .." I'd do one 'huge' capacitor near to the relay supply. It is possible to get 10,000 microfarad 16V rated capacitors which might be quite good on your 12V motor supply.

If the power supply needs 10,000 microfarad of smoothing then you need a beefier power supply.

If there's not enough current to keep the capacitors topped up they're going to drain eventually no matter how big they are.

Plus ... 16V is far too close to 12V for comfort. I'd go with 25V or more.

I agree with fungus. In addition, all that capacitance must be charged when power is first applied. And, the larger the capacitor, the worse its high frequency characteristics are.

Bigger is not always better.