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Extension Leads Problem

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Chris Bott - Moderator15/08/2020 12:45:06
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Some radios (maybe most) provide a servo pulse voltage of only 3.3v. some servos are right on the edge of being able to detect this and read it correctly as they are expecting a pulse as big as the battery volts.

I wonder if, in this case, the extensions are attenuating the pulse just that little bit?

If you Google "servo signal booster", you'll see that there are solutions, therefore it is known as sometimes an issue.

Having said that, it's probably as cheap to buy servos that are less fussy.

Martin_K15/08/2020 13:41:44
191 forum posts
Posted by Chris Bott - Moderator on 15/08/2020 12:45:06:

Some radios (maybe most) provide a servo pulse voltage of only 3.3v .....

I do not understand what you mean by that? Are you referring to the logic levels used in the RX? (In consumer electronics typically 3.3 or 5V).

Servos are actuated by the pulse width (duration in milliseconds) of the signal, not by the signal's amplitude.

Andy Stephenson15/08/2020 14:10:51
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Martyn,

What is being said here is that the control pulse amplitude from the receiver is quite often less than the supply voltage and if the servo needs a pulse of a voltage greater than the receiver is supplying it would cause it not to be recognised by the servo. This being independent on the pulse width.

A.

Chris Bott - Moderator15/08/2020 14:26:31
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Martin you're right, these days most Rx will have a CPU that works at 3.3v so it's output pulses will be 3.3v "tall".

Yes the Servo measures the width of these pulses. It likely does so by having a threshold (slice) voltage, effectively measuring the time that the pulse is above that voltage. In a perfect world the pulses have vertical sides and a flat top so the servo will work if the pulse height is greater than the slice voltage. (And won't work at all if pulse is lower)

In the real world the pulses can have quite rounded corners, probably more pronounced with long extensions, so if the slice voltage is up in the rounded area things can start to get quite jittery.

Martin_K15/08/2020 14:44:10
191 forum posts

Unlike other electronics components, no servo I have bought came with a datasheet describing the requirements for connecting to the device. It would be nice to know what to expect in advance of testing.

Martin McIntosh15/08/2020 21:10:30
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Never had that problem with servos but electric retracts yes. I got another set of HK small MCR series ones but they would not respond to Orange Rx`s, OK on others and a servo tester, also some replacement amps I got for them were the same. After complaining that the manufacturer had probably made a change they sent me a free set, just the same so I checked various Rx outputs to find that some were only providing a 3V pulse, others which worked were 5V. Fat lot of good if they will not work with HK gear.

If those extentions are made from flat wire, not twisted then that could be where the problem lies.

Cuban816/08/2020 11:08:09
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This type of problem - although on the face of it, one that should be easy to sort out given the few components involved - can be a pig to nail down to an exact cause. Some years ago back in the 35 Meg days, I had a pair of servos of the same make but not one of the mainstream brands, that would always chatter and hunt around centre at switch on. The other Futaba servos in the model were all working as they should. Give the suspect servos a nudge on the sticks or by pushing the output arms and they'd be fine until switch off and on again and the same thing would happen - not always, but probably three times out of five to add to the mystery.

After faffing and substitution, swapping this with that etc etc it turned out that they'd be fine on their own directly from the Futaba RX but use another particular servo as well with them (a small and expensive futaba that I had on the rudder) and that would set them dancing at switch on, until they were nudged. All the servos would work perfectly in isolation.

An odd problem that I have a theory as to why it occurred, but again something that does happen when mixing and matching parts from various brands.

IIRC, I bought another couple of the 'dancing servos' and fitted all four of that type on their own in another small model without an issue until I sold the model on years after.

Edited By Cuban8 on 16/08/2020 11:09:52

Doc Marten16/08/2020 11:42:56
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I had a similar problem with a pair of noisy 9g Coronas that I was using on 35meg, they were fine before they were extended but would twitch like mad any distance greater than 20ft, they behaved themselves when closer. I tried them on different combinations of servo, battery, layout, plug slot, Tx, Rx but nothing worked until I tested them in a 2.4 setup where they where fine.

Edited By Doc Marten on 16/08/2020 11:45:00

J D 816/08/2020 12:21:46
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Funny thing but this morning I fully assembled for the first time a bitsa motor glider I have been putting together and found the airbrake servo was messing things up not going the correct travel [ the only non Futaba ] and with chatter on ailerons and even setting the motor off into little bursts. On its own it worked fine. Everything normal with it disconnected .

Now to dig it out of the awkward position in the wing. teeth

Tosh McCaber16/08/2020 14:40:19
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"Then I remembered that I have some unused TowerPro SG90 (plastic geared) servos. They work fine on the extensions, with no juddering. However, I don't particularly want to use them on this 2 metre wingspan motor glider."

After positive prompts from my clubmates about them, I'm going with the SG90s. Keeping my fingers crossed!

Many thanks for all the comments and suggestions.

Martin McIntosh16/08/2020 16:02:00
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Tosh, I hope that they are genuine Tower Pros and not ebay ones which will give you huge unforseen problems.

On another thread I posted what was happening with my TN electric Hurricane. It had Hitec 82MGs on ailerons and flaps and a JR 591 on rudder. Although it had been flown many times a Hitec started to oscillate to full travel. Replaced twice to no avail then a forumite suggested that another servo was affecting it. Turned out that the JR did not like the 5.5V BEC output. Replaced and problem solved.

Tosh McCaber16/08/2020 18:26:26
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Right- before we finish the thread- can any of you give me a heads up on a good reliable (reasonably priced) servo (or different brands of servos) that would be able to fit into my TowerPro MG/SG 90 slots?  I'm quite happy to pay for peace of mind!

Thanks in advance!

Edited By Tosh McCaber on 16/08/2020 18:28:46

Denis Watkins16/08/2020 18:46:16
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Hitec good reliable servos available with metal gears sizes shown in details

**LINK**

Doc Marten16/08/2020 21:46:28
710 forum posts
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Posted by Tosh McCaber on 16/08/2020 18:26:26:

Right- before we finish the thread- can any of you give me a heads up on a good reliable (reasonably priced) servo (or different brands of servos) that would be able to fit into my TowerPro MG/SG 90 slots? I'm quite happy to pay for peace of mind!

Thanks in advance!

Edited By Tosh McCaber on 16/08/2020 18:28:46

Get some genuine TowerPro's from the servo shop Tosh, they're good budget servos.

Genuine Tower Pro

Ben B16/08/2020 23:35:30
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Hmmm. Interesting thought. My first thought was "ferrite bead" but agree a counterfeit servo could explain a lot.

FlyinBrian17/08/2020 00:24:11
664 forum posts
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I noticed a query earlier asking if the ext leads were twisted or flat, if flat then there are two possible issues.

1/ The leads are picking up interference from each other due to the signal wires laying parallel to each other over a long(ish) distance. This is why telephone cables are not only twisted but when cable joints are made pairs get cross connected to minimise pickup of signals from adjacent wires.

2/ the cables are picking up other signals ie R/C transmissions or if you are on a workbench rather than out at the field it could even be mains interference from Fluorescent lights.

Dependent on the electronics in the servos some servos will be affected more than others by either form of pickup.

Have you tried twisting the extension leads?

Also very valid points mentioned about knock off servos, Fake Tower Pro and Futaba servos are rife on ebay.

Get MG90s from Steve Webb (Servo Shop) they are genuine. I use quite a few.

Keith Berriman18/08/2020 10:13:00
780 forum posts
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Hi Tosh. PM sent

Keith Miles 218/08/2020 14:03:57
464 forum posts
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Posted by Tosh McCaber on 15/08/2020 06:45:39:

I naively thought that extension leads were extension leads- all being equal. Not a question of money! Can anyone give me a lead (pardon the pun!) to somwhere who sells "heavy duty" leads?

Thx!

Belated input!

Despite a comment to the contrary, leads are available in at least two different guages (e.g. Futaba STD and HD).

They are also available flat or twisted and I think (though I could be wrong) that the latter are usually HD.

A web search will provide you with a number of stockists.

Tosh McCaber18/08/2020 16:27:18
192 forum posts
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Can you tell me what guage I should be lookin for with long extension leads? I've seen 22 AWG twisted leads advertised- is that standard- or thicker?

Keith Miles 218/08/2020 17:21:54
464 forum posts
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Posted by Tosh McCaber on 18/08/2020 16:27:18:

Can you tell me what guage I should be lookin for with long extension leads? I've seen 22 AWG twisted leads advertised- is that standard- or thicker?

RC World is showing the Twisted leads as HD 22.

Not looked at Ripmax site but you will probably be able to cross reference that on there.

Of course, you don’t have to buy Futaba but at least you can probably trust the specs!

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