PDA

View Full Version : Tying together UBECs



wireframewolf
09-18-2012, 03:41 PM
So, following some advice from other questions I'd had about robot power sources, I eventually ended up buying a couple UBECs (http://www.hobbyking.com/hobbyking/store/__6233__TURNIGY_8_15A_UBEC_for_Lipoly.html) as well as two batteries (http://www.hobbyking.com/hobbyking/store/__15518__Turnigy_6000mAh_2S2P_7_4v_25C_hardcase_pa ck.html).

Now, each UBEC is rated for 8A continuous, 15A burst. I have 5 servos at 2 Amps stall and 4 motors at ~1.6 amps stall to power. That means I have a possible ~16.4 amps pulled at any time. Would it be a reasonable solution to tie the outputs of 3 of those UBECS into a single line to power all of my servos and motors? Also, I would be hooking up the two batteries in parallel and connecting the inputs of all three UBECS to this source. Basically, something like this:

4164

This is my plan, but since I'm not knowledgeable about power supplies, I'm wondering if there are any problems with this setup, and what my alternatives might be. It's a small robot but it uses a lot of amps. Any help is appreciated, thanks :)

jwatte
09-18-2012, 09:12 PM
An "UBEC" is really just a packaged switching mode DC/DC power converter (/supply.)

A power supply really should be designed to be "paralled" to be 100% safe. What may otherwise happen is that, if one is only 0.1V off from the other, that actually creates a 0.1V differential with almost 0 Ohm resistance, causing very high current draw, and overheating of the higher-voltage power.

You can avoid this by hooking a diode at the output of each UBEC (to the positive lead.) The problem, then, is that you have to find a diode that can take the amount of current, yet doesn't drop too much voltage. Schottkys are great because the voltage drop is low (0.3V or so) but they are not found in very high amperage ratings.

Perhaps a better way to solve this problem would be to get a DC/DC converter that is actually rated for the current you will draw. Often, you will find heavy, industrial power supplies for this: http://www.digikey.com/product-detail/en/VHK75W-Q24-S5/102-2265-ND/2162747 ($130)
B (http://www.digikey.com/product-detail/en/VHK75W-Q24-S5/102-2265-ND/2162747)ut you may be able to find something cheaper and lighter: http://www.digikey.com/product-detail/en/OKX-T%2F16-D12P-C/811-2012-5-ND/2038937 ($13.50)
If you need to go to 6V, here's the first one I found: http://www.digikey.com/product-detail/en/NSR020A0X43Z/555-1139-ND/1964566 ($21, 20A)
If you need 12V (as for AX-12 etc) then prices go up pretty quickly: http://www.digikey.com/product-detail/en/UWQ-12%2F17-Q48N-C/811-2370-ND/3046816 ($75, 17A, 12V)
(http://www.digikey.com/product-detail/en/UWQ-12%2F17-Q48N-C/811-2370-ND/3046816)
Note that some of the latter may need heat sinking of your own choice. In general, though, an "UBEC" is nothing magic, and typically is no cheaper or better than just buying the parts directly from electronics suppliers. And they are often "optimistically" rated like consumer goods, rather than "realistically" rated for their capacity.

Finally -- if you already have the parts, you may want to consider driving different servos from different UBECs. For example, drive two servos each for two and one servo and two motors for the third.