Calculating Servo Draw or How Can I Tell I Need a UBEC? - Page 2 - RC Groups
Jul 01, 2012, 01:46 PM
Got shenpa?
Quote:
 Originally Posted by srnet All was fine, BEC current well within the capability of the linear regulator (3A).
This is the part that concerns me. I have never encountered a "3 A" linear BEC that can actually cope with 3 A of current draw for any length of time.

Since you have some data logging equipment, try this: Ohm's law tells us that a 1.66 ohm resistor would draw 3 amps from a 5 volt BEC. Hook up a resistor of about that value (capable of handling 15 W of heat) to a servo extension cable, between the black and red wires, plug it into the ESC, power up, and monitor BEC voltage with your Eagle Tree logger.

The easiest way to get a suitable 1.66 ohm resistor would probably be to parallel six ten-ohm, five-watt resistors.

The "finger on the BEC chip, wiggle the servos" test is much less sophisticated, but much simpler, and also well worth trying.

-Flieslikeabeagle
Jul 01, 2012, 02:32 PM
Stuart
Quote:
 Originally Posted by flieslikeabeagle This is the part that concerns me. I have never encountered a "3 A" linear BEC that can actually cope with 3 A of current draw for any length of time
I did not mean to imply that the BEC current was 3A, the average current was in fact substantially lower than that.

As for "The "finger on the BEC chip, wiggle the servos" test", what would be informative is to measure (in flight) the average surface temperature of the regulator.
Jul 01, 2012, 11:17 PM
Got shenpa?
Quote:
 Originally Posted by srnet ...what would be informative is to measure (in flight) the average surface temperature of the regulator.
Agreed! If you ever get data on that, I'd love to see it.

One thing to keep in mind is that throttle setting affects ESC dissipation. There is an intermediate throttle setting (somewhere between zero and full) that causes the ESC to dissipate maximum heat, and this is the condition under which the BEC is most likely to overheat and enter thermal shut-down, as the entire ESC circuit board will be warmest at this throttle setting.

The type of flying is also a big factor, the servos in a sailplane gently circling in a thermal are going to draw very little current compared to the servos in an aerobatic model zipping through a succession of manoeuvres.

-Flieslikeabeagle
Jul 02, 2012, 01:22 AM
Stuart
Quote:
 Originally Posted by flieslikeabeagle One thing to keep in mind is that throttle setting affects ESC dissipation. There is an intermediate throttle setting (somewhere between zero and full) that causes the ESC to dissipate maximum heat, and this is the condition under which the BEC is most likely to overheat and enter thermal shut-down, as the entire ESC circuit board will be warmest at this throttle setting.
I was using an 45A ESC at a continuous 25A, which was full throttle for the model. I was also exercising the big servo, the HS645 at lot, far more than normal.

No signs of stress from the linear BEC or ESC at all.

I kept the ESC with its linear BEC, with no evidence that the linear BEC was under stress, adding a UBEC (with a great deal many more components and extra wires\connectors) seemed to be an unnecessary additional risk.
Jul 02, 2012, 01:46 AM
Promoting Model Aviation...
Quote:
 Originally Posted by srnet I was using an 45A ESC at a continuous 25A, which was full throttle for the model. I was also exercising the big servo, the HS645 at lot, far more than normal. No signs of stress from the linear BEC or ESC at all. I kept the ESC with its linear BEC, with no evidence that the linear BEC was under stress, adding a UBEC (with a great deal many more components and extra wires\connectors) seemed to be an unnecessary additional risk.
What battery and how many servos are you running on that setup? How about your ESC?

I'm really surprised we do not see more data on this especially in an electrics forum. I guess we take it for granted that we should be using a BEC on 3S or more LiPo?

Frank
Jul 02, 2012, 03:02 AM
Got shenpa?
Quote:
 Originally Posted by WAA-08 I'm really surprised we do not see more data on this especially in an electrics forum.
I haven't seen a lot of formal data, but there are quite a few reports of electric models lost due to sudden loss of radio control. It's impossible to be completely sure how many of these are due to BEC problems - but it's a safe guess that many of them are.

Just try the "finger on the BEC chip, wiggle the servos" test - it may change your willingness to rely on a linear BEC. Remember, if you can't hold your finger on a hot surface for 3 seconds, it's probably over 60 degrees Celsius, and that's too hot for a semiconductor to be considered reliable.

Srnet, where was your loggers temperature sensor mounted during your test? On the BEC chip?

-Flieslikeabeagle
Jul 02, 2012, 03:14 AM
Stuart
Quote:
 Originally Posted by WAA-08 What battery and how many servos are you running on that setup
The servos were listed earlier, the battery was 3S.

I agree with you on the lack of real information.

There does seem to have developed a conventional wisdom along the lines of linear regulator bad switching regulator good.

This may be true in some circumstances, so should the advice be that we never be using linear regulators at all ?
 Jul 02, 2012, 09:48 AM Dave the Rave No offence guys, but I think this thread has gone way past the information the OP was looking for. He's got a small electric with 4 mini-servos in it, and was just looking for help in deciding whether or not to use the on-board BEC or an external UBEC. While all the talk and info on this thread is useful, I personally think we've gone way overboard with it. If the plane in question were a large 3D plane with high-torque digital coreless servos, the concern over current draw exceeding the BEC's capacity would certainly be warranted. But on a plane of the size and type the OP was asking about, don't you think it would be OK to suggest that the BEC built into most decent-quality ESCs should be sufficient? I mean, if he isn't safe using the BEC on that configuration, we might as well just say that no on-board BEC on ANY ESC can be trusted not to fail, and that's overstating the danger. If I were someone with no previous experience reading this thread, that's the conclusion to which I would come. And as I stated before, I've built a truckload of planes of that size and type, and some even larger, for myself and for other clubmates, and have yet to have one crash due to a BEC that failed. No need to "over-engineer" a simple 3s-size electric model. Just my \$.02 worth. (as he places anti-flame suit tightly around head and body)
Jul 02, 2012, 01:07 PM
Got shenpa?
Quote:
 Originally Posted by dmccormick001 Just my \$.02 worth. (as he places anti-flame suit tightly around head and body)
There's plenty of room for more than one opinion. No need for a flame-retardant suit.

Personally, if it were my model, I would most certainly use an inexpensive switching BEC in the OP's model. Four HS-81 sized servos and 3S battery? Yup, instant decision to use a UBEC if it were up to me.

Take a look at these oscilloscope traces - as you can see, just one single HS-81 servo drew 700 mA peak current: http://homepages.paradise.net.nz/bhabbott/Servo.html

The average current from that HS-81 is low because of the low duty cycle (short pulses of current with long gaps between them). Still, four times 700 mA is nearly 3 amps of peak current draw - high enough to suggest that using a nominally 2 A or 3 A linear BEC is a bad idea.

-Flieslikeabeagle
Jul 02, 2012, 01:14 PM
Got shenpa?
Quote:
 Originally Posted by srnet This may be true in some circumstances, so should the advice be that we never be using linear regulators at all ?
My take: if the model is big enough to carry an extra ten grams without feeling it, I'll put in a switching BEC.

The BEC is a single point of failure for the entire model aircraft. If the BEC fails in flight, the pilot loses all control of the model. As such, in my opinion, anything that can be done to improve the reliability of the BEC is a Good Thing (TM). Having seen how close to disaster most linear BEC's run, I couldn't get away from them fast enough.

And the flip side is that before inexpensive UBEC's were available, thousands of models were built and flown using linear BEC's, and most of them didn't crash. Clearly you can often get away with a linear BEC, even if it feels a bit like Russian Roulette sometimes.

Happy and safe flying to everyone,

-Flieslikeabeagle
Jul 02, 2012, 02:04 PM
Stuart
Quote:
 Originally Posted by flieslikeabeagle The average current from that HS-81 is low because of the low duty cycle (short pulses of current with long gaps between them). Still, four times 700 mA is nearly 3 amps of peak current draw - high enough to suggest that using a nominally 2 A or 3 A linear BEC is a bad idea.
I suppose its possible that all 4 servos moving continuously could get close to 3A peak, but a 3A regulator is obviously capable of that.

But I thought it was heat build up that was the problem, not short term peak currents.

The graph for the HS81 shows that the average current is around 70ma, so 4 servos have an average current of 280mA, that should not cause any problem at all. assuming there is a heatsink of sorts.
Jul 02, 2012, 10:22 PM
Got shenpa?
Quote:
 Originally Posted by srnet I suppose its possible that all 4 servos moving continuously could get close to 3A peak, but a 3A regulator is obviously capable of that.
We have different philosophies on this. I try never to use any electronic device at 100% of its rated capacity. In case of a component as vital as a BEC, I'd prefer a 100% safety margin - I wouldn't want to draw more than 1.5 amps from any device rated at 3 amps maximum.
Quote:
 Originally Posted by srnet But I thought it was heat build up that was the problem, not short term peak currents.
So did I. I didn't think peak currents were that high until I saw those 'scope traces. After seeing those, now I think both average power dissipation in the BEC and peak servo currents are worth considering.
Quote:
 Originally Posted by srnet The graph for the HS81 shows that the average current is around 70ma, so 4 servos have an average current of 280mA, that should not cause any problem at all. assuming there is a heatsink of sorts.
If you connect an 18 ohm 5 watt resistor to a spare servo lead and plug it into your ESC, you can find out if there is enough heatsink or not. That resistor will draw a nominal 278 mA from a 5 volt BEC.

If you try the experiment, please let us know what you find - it could be very useful data.

-Flieslikeabeagle
Last edited by flieslikeabeagle; Jul 02, 2012 at 10:49 PM.
 Jul 04, 2012, 08:44 PM Promoting Model Aviation... Excellent discussion and I'll have to say it again after reading this thread again. I am so surprised that more folks do not share their servo draw data. I mean look at all the motor and prop data that is being published by so many people. With that said, besides the Eagle Tree, what other "device" would one need to monitor their servo draw to determine if they really need a BEC or not? Perhaps the cost of the device is prohibitive and we just take it for granted we need one or don't. Frank
Jul 06, 2012, 11:43 AM
Stuart
Quote:
 Originally Posted by WAA-08 With that said, besides the Eagle Tree, what other "device" would one need to monitor their servo draw to determine if they really need a BEC or not?
Well you can do simple tests as in the graphs linked to above, and they do (in the case of the HS81s) show an average current of 70ma, which I suspect is a great deal lower (for an average) that most would have expected.

But then there is the issue of how the average current is affected by control source loads, sticky control rods and the like. So the only realistic way of monitoring this, and to obtain real world figures, is to measure in flight while your giving a plane a thrashing.

Problem is that if you do this, the real world figures may show a truth that is not acceptable to some; that the average linear BEC should have no problem coping with 3 or 4 average servos.
 Jul 06, 2012, 03:43 PM Registered User If you are really concerned and wanted to test this, then you might try this. The absolute worst case is going to be if you were to take one of your servos and measure current while stalled. This will set an upper threshold for the device. Multiply this times the number of servos that you have, and then you will have the max power required. If you are using the servos in a way in which you will most likely not ever get all of them close to that point, then set this as your absolute (transient) rating. Most likely in average use you will only consume ~50% of this number (3D flight maneuvers or servos with little operating margin may cause this assumption to not be valid). Once you have established usage, then set up something that can statically draw that much current (resistor bank would work sized for the power consumption). Then you can check to see if your bec is getting too hot. Adam