[sdiy] get 5V from 12V - DC-DC converter or 7805?

Brian Willoughby brianw at audiobanshee.com
Mon Dec 28 02:43:21 CET 2020

On Dec 27, 2020, at 17:05, Chris McDowell <declareupdate at gmail.com> wrote:
> my bet would be that a switching regulator will likely not trash your supply rails if you have an electrolytic and ceramic cap on the input, and an inductor and cap suitably sized for your current draw. this isn't super complicated, but might be more worrying than you want to do / need to do.

In addition to the cap(s), I'd recommend something in series like a ferrite bead, inductor, or a really small resistance. This will create an RC (or LC) filter between the supply and regulator, and help isolate the two. Without something in series that would allow a voltage drop, there's a limit to what capacitance can do to remove noise. Of course, there will be some resistance in the traces and connectors, but it's better to just drop a purposeful component there.

> an LDO like the LM1117 works great for what you want. you will likely waste about 200mW, which in this application probably doesn't matter. it would be a questionable choice for something with batteries, but this aint that.

Agreed. I've used the AZ1117 from Diodes Inc. It seems to work fine, but I've not done any measurements.

> I've used cheap chinese dc-dc in these kinds of situations, it works and did not completely trash the rails. popular euro manufacturers use the LM1117 and folks seem to be happy!

I tend to choose switching chips that have a fixed output voltage, so I don't have to add a pair of resistors to set the voltage. It also helps me to stop stressing about the accuracy of the resistors that I might choose. Of course, if you need variable voltage then you'll want an adjustable regulator. But in my case I've never needed the option to adjust the voltage, and I'd rather have fewer components - especially high-precision components.

> if you are worried about using too much current, the arduino nano might be the wrong choice in the first place, but the simplicity trade off might be worth the high-ish current draw. I'm not immediately aware of arduino compatible tiny boards with super low draw.

The Texas Instruments MSP430 seems to be available in really low draw. There might be even lower wattage parts. The MSP430 is easy to develop for in C.

> cheers,
> Chris
> On Dec 27, 2020, at 6:41 PM, MTG <grant at musictechnologiesgroup.com> wrote:
>> What about an LDO if the current draw isn't much?  I used these a long time ago:
>> LP2950ACZ-5.0
>> https://www.ti.com/store/ti/en/p/product/?p=LP2950ACZ-5.0/NOPB
>> GB
>> On 12/27/2020 2:18 PM, Tony Allgood wrote:
>>> The OKI-78SR-5/1.5-W36-C is pin compatible with a traditional 7805 linear regulator. The disadvantage is that it is more expensive and creates a small amount of high frequency noise that is both radiated and into the power rails. The amount of noise depends on the current draw from the +5V line, with greater currents creating more noise.
>>> If the current draw is less than 100mA, I'd be tempted to use a 78M05.
>>> Tony

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