Model Boat Mayhem

Technical, Techniques, Hints, and Tips => The "Black Arts!" ( Electrics & Electronics ) => Topic started by: McGherkin on March 10, 2015, 08:58:54 PM

Title: UBEC woes. 4S to 12v.
Post by: McGherkin on March 10, 2015, 08:58:54 PM
Hi folks.

For the Tamar, I'm using 4S Lipos to power the model, but obviously as this battery setup goes up to 16.8v, the likelihood of the LEDs, Marks Model Bits Foggy and a small water pump agreeing with such a voltage is slim to none.

For that reason, I bought two 4.5 amp UBECs, with the intention of putting them in parallel to give a maximum of 9A which should be more than sufficient.

However, when connecting the system together, the MMB foggy does not work. The power light on the 12 to 24 volt converter flashes. If you connect both of the UBECs up, it flashes slightly quicker, but still no smoke. Replacing the foggy with a 3 amp fan, which just pulses, suggests that the UBEC is just making 'blips' of power rather than a continuous voltage. Using the pump (the Hunter Systems water cooling one) is fine, it runs no problem at 80mA. I don't have an LED on me to test with at present.

MMB rates the foggy at less than 1 amp at 24v, and the voltage converter which goes with the unit (no longer on sale) steps 12v up to that 24v. So assuming that the amperage doubles when the voltage halves, and accounting for some losses in the converter, I suspect a draw of 2.5a at 12v isn't far off.

So either I'm missing something or the UBECs are maxing out way below 2.5 amps, or theoretically less than 1.25 amps because even two of them together isn't enough.


Are they really maxing out so low (9a rated, actual <2.5-3a)? Is there a better solution for converting 4S to 12v, with a varying load (as these things will be switched on and off)?

Help!

Title: Re: UBEC woes. 4S to 12v.
Post by: Time Bandit on March 10, 2015, 09:29:10 PM
Are those UBEC 12V output versions? What exactly did you buy?

I guess itīs a problem of several switching voltage converters.
They often do not work if put in parallel (2 UBEC) or in series (2UBEC + foggy converter) due to "overswinging". Depends on how they work, generally like an ESC with pulse width modulation.
Generally the possible amps of voltage converters are exaggerated. You are lucky if you can draw 2A out of your "4,5A" system.
There is also a possibility that you already destroyed the UBEC and the foggy converter by putting them together.
Can you measure whatīs coming out of each UBEC?

So you can try different things:
1) Put more capacity on in- and output of the converters. Several capacitors with different values (100nF to at least 220ĩF) placed in a ring should help to reduce swingings. Do not use the 2 UBEC in parallel! No never!!!


2) In general use the LED on the 4S, just change the resistor, no need for an UBEC.
The voltage converter is probably a fixed output voltage converter, which you can also supply directly from 4S. But you should clarify that with the seller.

3) if not possible try linear regulators like L78S12 or alike. They do not switch the power (but waste the energy in heat  :D )
Title: Re: UBEC woes. 4S to 12v.
Post by: McGherkin on March 10, 2015, 10:12:32 PM
Yes, they are the hobbyking ones. They output 12.20v, even now.

If it's the voltage conversion causing it, how come the 3a 12v fan doesn't work either? It just pulses.

The voltage converter takes in 12-13.8v, and has selectable output voltages.

Is there a 'finished' solution available using the linear regulators you linked? Most regulators seem to be anywhere from 5 to 9 volts, but that's no good for the foggy or the pump.
Title: Re: UBEC woes. 4S to 12v.
Post by: Calimero on March 10, 2015, 11:15:34 PM
Why not go the other way around: use 3S batteries instead of 4S ? Wouldn't it be close enough to ditch all the UBECs ?
You'd have to swap the motors for higher KVs but it might even be cheaper than all this try-and-error.
Title: Re: UBEC woes. 4S to 12v.
Post by: malcolmfrary on March 11, 2015, 09:45:56 AM
If they are these - http://www.hobbyking.com/hobbyking/store/__18788__12V_4_5A_UBEC_2_5S_Lipoly_7_2_21V_.html - scroll down to the comments.  There are many mentions of a high level of noise on the output, this could upset a lot of electronics that doesn't have built in smoothing (i.e. a great big capacitor on the power input) and would almost certainly annoy any other Ubec trying to regulate against the output of others supplying the same feed.
There are lots of instances of "Chinese Amps" in ESCs.  It isn't a big leap of guesswork to the same standards being applied to other devices.  To check the output capability of a Ubec, try powering a 12 volt 24 watt bulb.  If it shines, you are getting 2 Amps.  If it does, add extra lower wattage bulbs to see how far it goes.
With the wide variety of brushless motors available, lower voltage ones pulling more amps to get the original power might well be the way to go.
Title: Re: UBEC woes. 4S to 12v.
Post by: inertia on March 11, 2015, 10:18:13 AM
I am currently (ouch!) testing a very neat little DC-DC converter for Component Shop. This will take any input voltage from 3.5v to 30v and give any output voltage in the same range i.e. it can step up or step down. The output voltage is adjustable with a multi-turn trimmer on the circuit board. Input is a maximum 6A and output follows the usual power rule i.e. if input is 12v x 6A (=72W) then output will also be a maximum of 72W, so when Vout is 6v the current will be 12A, or where Vout is 24v then current is 3A. There is a digital readout and small switch via which you can toggle between Input and Output voltages, so you don't need a DVM to set it up. Size is approx 30 x 22 x 17 (uncased).
Iain says it will be about a tenner.
There is a smaller version with a maximum current input of 2A (3A peak) but no readout for about a fiver. Pictures attached.
All enquiries to Component Shop, please - I know nothing else!
Dave M
Title: Re: UBEC woes. 4S to 12v.
Post by: McGherkin on March 11, 2015, 11:02:17 AM
Is that available yet? Any links?

Many thanks :)
Title: Re: UBEC woes. 4S to 12v.
Post by: Time Bandit on March 11, 2015, 07:00:32 PM
I buy those at ebay.
Left one is less than 2$ the bigger ones with 5A are like 4$.
There are also version with 15A available.

@ Inertia
The red step down module is also availabe with 2 big capacitors on the output.
Get this version! Itīs much better.
The one you are using can even corrupt non volatile memory of mikrocontrollers. Was a big troublemaker in 1 of my boats and induced a lot of jamming in all of its electronics.
Title: Re: UBEC woes. 4S to 12v.
Post by: McGherkin on March 11, 2015, 07:07:08 PM
Is this one OK then?

http://www.ebay.co.uk/itm/DC-4-5-30V-step-down-DC-0-8-30V-Max-12A-laptop-car-BUCK-converter-WT-/161223409993?tfrom=151311737680&tpos=top&ttype=price&talgo=undefined
Title: Re: UBEC woes. 4S to 12v.
Post by: Time Bandit on March 11, 2015, 07:34:29 PM
No matter what you buy,
that doesnīt solve your problem with the step up module for the foggy.
2 modules in a row are just not a good idea, especially if both are switching and not linear regulators. But even with linear regulators you can get in trouble and need a oscilloscope to fix it.
Try to find a way to get rid of it  :}
Title: Re: UBEC woes. 4S to 12v.
Post by: McGherkin on March 12, 2015, 10:46:42 AM
Okay, so it would be better to use a seperate step-up converter for the foggy and the rest on a step down like above?

Would a converter like this interfere with a small ESC if I wanted to control the flow rate of the pump?