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Part of a circuit I'm designing needs to control servo motors rated for 5V. I've measured that they can draw over 1A when active.

The issue is that my circuit is powered by a 2-cell LiPo battery, which I've gathered has a 6V minimum, 8.4V maximum output.

Another part of the circuit uses 3.3V, so I used a switching regulator from 6-to-28V down to 3.3V, and it works perfectly. I thought that a similar component to regulate down to 5V would be about as expensive ($2-$3 ish), but I'm finding on DigiKey that I'm looking at at least $16 per regulator for even a 1A-output 5V regulator, and significantly more for a 2A output.

This is really way too expensive for me, especially since I will have 4+ servos to power. Is there a better way to power servos from a 2-cell LiPo? I would really like to power everything off of one battery, but it seems like I may just need to power the servos with a separate 5V battery (NiCd or other, not sure).

Edit: My search parameters earlier somehow caused me to miss the LM2596, for $2.5. Will this work how I want, if I adjust it to have a 5V output? It seems a little too cheap to be true. :)

Datasheet: http://www.onsemi.com/pub/Collateral/LM2596-D.PDF

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  • \$\begingroup\$ You can find cheap regulators on ebay, aliexpress, dx, etc. \$\endgroup\$ – kva Jun 15 '17 at 21:03
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    \$\begingroup\$ @kva Can I trust components from those sources? I've only ever ordered from DigiKey before. (When I see titles on AliExpress with "100% Genuine" it makes me very suspicious, to say the least...) \$\endgroup\$ – Jashaszun Jun 15 '17 at 21:06
  • \$\begingroup\$ Depends on what your application is. Definitely not suited for medical applications, but usually good enough for hobby projects. High sellers' reputability score and good user feedback are good indicators of a sellers trustworthiness. Edit: Do not buy things that are too good to be true. They generally arent. \$\endgroup\$ – kva Jun 15 '17 at 21:14
  • \$\begingroup\$ "I've measured that they can draw over 1A when active." - each? Note that peak current draw can be much higher than average bhabbott.net.nz/Servo.html \$\endgroup\$ – Bruce Abbott Jun 16 '17 at 1:07
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The cost of the regulator seems about right, and is definitely not too good to be true. But what makes you think that the regulator is all you need? Look at the data sheet you linked to, and look closely at the applications section. You will need several more components, particularly including an inductor which cannot be just any old inductor. Even worse, you'll need a way to mount all your components, and you cannot just solder them together. Switching supplies are susceptible to layout errors, so a printed circuit board is a really, really good idea.

Between the components and the pcb, for small quantities of finished product (like, let's say 100) you'll almost certainly end up spending more than 16 bucks a unit.

So eBay is probably your best bet.

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  • \$\begingroup\$ Don't worry, this is all going on an existing PCB design, so PCB cost is not an issue. (In fact, I just finished soldering and testing v1 of the board, and it works perfectly!) I did look at the applications section, and I suppose I will need to see how much all the components are worth. The only thing I'm concerned about now is Figure 4, on page 5 of the datasheet. I'm trying to get the full 3A output, so what I'm seeing is that at 25 degrees celcius, the dropout voltage is around 1.6V, which means that I need a minimum of 6.6V battery voltage. (continued...) \$\endgroup\$ – Jashaszun Jun 15 '17 at 22:38
  • \$\begingroup\$ However, it could be important that I have a lower dropout so that I can use more of the battery life. I'll have to see if I can even find low-dropout regulators that work at these voltages... \$\endgroup\$ – Jashaszun Jun 15 '17 at 22:39
  • \$\begingroup\$ Driving cheap asian electronics to their maximum (e.g. 3A) most often requires a good heatsink. \$\endgroup\$ – DIYser Jun 16 '17 at 1:59
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The standard regulator used with 'hobby' servos is called a 'UBEC' (BEC = Battery Eliminator Circuit). These are usually based around an SO8 package switch-mode buck regulator IC, with a small inductor, Schottky diode, and a few capacitors and resistors completing the circuit.

You could get ready made units such as this and wire them onto your board, or buy the individual components and make you own. I used a Sanken SI8005Q, but several other chips are available which have similar specs. If you are not experienced at designing switch-mode supplies then follow the example layout in the datasheet.

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