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5 cell NimH and 6V servo


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I've just put together an ARTF model using Savox 0351 servos with a Hitec HS75BB retract servo for the mechanical retracts. All of these are rated for 4.8V or 6V operation. I usually power model radio gear with a 2 cell LiFe with no problems but I needed a fair bit of weight up front so I decided to use a '6V' 5 cell sub-C NiMh pack.

 

I did the bench setup with the pack as delivered which showed about 6.25V on my battery tester. All worked fine, the mechanical retracts worked perfectly with no binding at all.

 

I charged the pack, noted it topped off at about 7.25V, and set off to the field for first flights. The model was transported wheels up, so when assembled I powered up and selected gear down - all OK. Range checks, engine setup, etc then first flight. A few circuits, trims sorted, so selected gear up. Flew by and saw the gear was still down. tried cycling to no effect so left selected down. When I came in to land the gear just collapsed, obviously not locked down. The retract servo was totally unresponsive.

 

Back on the bench, checked all connections, still dead. Tried it on a 5V regulated supply, still dead.

 

So - I guess I've cooked it. I have another HS75BB but I obviously need to do something about powering it differently before I fry that too. I know I can use a separate battery for the retracts (and I know it's good practice to do so) but would the trick of adding a diode in the retract power supply drop the 5 cell NiMh voltage enough to keep the servo happy? Maybe two diodes in series?

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If the servo fried itself i expect it would be dead obvious, likely with smoke, melted case and that wonderful smell of toasted electronics. It clearly tried to move a bit if the down locks came out of the gear, did something jam it up and cause the issue?  I have a number of 6v rated hitec servos running 6v nimh packs and none of them have given any trouble so i am not convinced the voltage was the problem, especially when you say it had been on for a while doing all the function tests. That peak voltage after charging would fall of quickly once the model was on and drawing some current. 

Edited by Jon - Laser Engines
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The servo is now out of the model, I’ve taken the bottom of the case off and had a good look around the wiring and circuit board and there’s no sign of burning anywhere.

 

It is now totally unresponsive using a servo tester and regulated supply, both 5v and 6V. The replacement servo works fine with the tester.

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Had the identical issue with the same servo, fortunately on the bench.   Now has a voltage step-down device to prevent a recurrence but don’t ask which battery, I think it’s a 2S LiFe, or which voltage regulator, probably a Yeti.   I concluded that what I can do to and with all Savox servos I’ve ever used and some Hitecs too does not apparently apply to all Hitecs.

 

luckily my ignorance took me back to the retailer who replaced it without question and there was no magic smoke, no molten casing etc so I assumed something small had blown and put it down to experience.

 

I reckon on average I could save between £1 and £2 per servo by buying online.   This single episode has covered say 15 servos bought from the Precious Shop instead which is why I continue to divert by a few miles whilst out and about for true retail therapy at Modelshop Leeds.

BTC

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Just had a similar problem with a Hitec HS-5625MG servo, it was a lightly used 2nd hand servo, fitted on the tow release on a 1/4 scale glider, 2s LiFe battery around 6.6v, all was working well on test, including a pull test. Arrive at the aerotow, fit the tow hitch do a test to make sure it's working, it released and then promptly stopped working. Tested when I got home and it's completely dead, took it apart and no burnt electronic smell or obvious burnt components, but one of the chips maybe slightly swollen.

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16 hours ago, David P Williams said:

would the trick of adding a diode in the retract power supply drop the 5 cell NiMh voltage

 

why not use a 4 cell pack?

 

are you absolutely sure there was no mechanical resistance on the retract causing stalling / motor burnout?

 

 

on another forum, the question of hitec and 6v has been asked before...

 

https://www.rcgroups.com/forums/showpost.php?p=7573900&postcount=7

 

I note one part of that post:

 

Quote

None of the Hitec servos rated for 6V would require a regulator if using a 5 Cell Ni Cd/Mh. The 225 servos for example pack a lot of punch in a small package but their size means a smaller motor which is more susceptible than a standard size servo with similar torque to overheating if pushed hard. Torque alone should not be the deciding factor when choosing a servo

 

 

Edited by Nigel R
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I guess I have been lucky with the Hitec retract servos. 

So far, there have been no failures. 

 

BUT, I put a diode in the feeds to retract servos.  This is just to slow them down a little, but the reduction in voltage might possibly have preserved them. 

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Maybe.

 

Retract servos see quite a load at the mid point of travel. I wonder if it is simply that some retract setups (large wheels, sprung oleos?) will exceed the servo capabilities.

 

I've never used servo driven mechanical retracts with anything other than simple ( lightweight?) wire legs and regular small wheels.

Edited by Nigel R
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We call microprocessors microchips or chips, and these have developed and become very tiny black boxes containing any number of micro components that are, due to the amplifiers therin, very voltage sensitive, as the transistors trigger from under 1v.

5v is hugely common in microchip manufacture, but more recently 6v is used to define and spec chips, but many become overvoltage with as little as 0.3v over, it just depends which manufacturer uses what in where. If Hi-tec state 6v, that is it, there is no secure leeway.

7.2v rated servos are available at a price. Voltage regulation is simple and low cost if you have to use 2 cell life.

 

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There's truth in there Denis - but what in the servo does the power control for the motor?

 

Hitec 322 guts:

 

FJ71GOQFU54ZSIR.jpg?auto=webp&frame=1&fi

 

 

Just to the left of the big chip are two power transistors. They're the key components that must cope with increased voltage/current and heat dissipation. Everything else is much easier to rate for 5 cell operation - no heat problems for those components.

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17 minutes ago, Nigel R said:

but what in the servo does the power control for the motor?

 

 

Same as your wristwatch Nigel. Ever wondered how your watch runs 2 years on a tiny 3v cell.

The drive is pulsed, at very high speed

In reality the power can, in a moment in time, be more Off, than On

This negates the direct power connection that creates the electric fire effect.

Remember the mark space ratio of the 555 Times chip in school?

If not, school kids were introduced to pulsed motor control around the 1990s?

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A component that hasnt been mentioned here and is particularly voltage sensitive is tantalum smoothing capacitors.  If a tant has a max rating of 6v (which is a common standard) and you run it at 7, it will most likely fail, often catching fire!  Unless you know the individual component specs within a servo amp, or the manufacturer says its ok at 7+ volts, I'd suggest using 4 cells ?

Edited by Phil Green
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53 minutes ago, Denis Watkins said:

The drive is pulsed, at very high speed

 

Yes indeed. Switching speed is not instant though. And there are losses in the transistors.*

 

* I know that some control ICs will integrate part of the power transistor circuit, which puts a hard limit on what current those chips can control.

Edited by Nigel R
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2 hours ago, Denis Watkins said:

 

..................................

Remember the mark space ratio of the 555 Times chip in school?

If not, school kids were introduced to pulsed motor control around the 1990s?

When I was at school the only chips available came from the canteen ?.

 

Dick

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23 hours ago, David P Williams said:

I've just put together an ARTF model using Savox 0351 servos with a Hitec HS75BB retract servo for the mechanical retracts. All of these are rated for 4.8V or 6V operation. I usually power model radio gear with a 2 cell LiFe with no problems but I needed a fair bit of weight up front so I decided to use a '6V' 5 cell sub-C NiMh pack.

 

I did the bench setup with the pack as delivered which showed about 6.25V on my battery tester. All worked fine, the mechanical retracts worked perfectly with no binding at all.

 

I charged the pack, noted it topped off at about 7.25V, and set off to the field for first flights. The model was transported wheels up, so when assembled I powered up and selected gear down - all OK. Range checks, engine setup, etc then first flight. A few circuits, trims sorted, so selected gear up. Flew by and saw the gear was still down. tried cycling to no effect so left selected down. When I came in to land the gear just collapsed, obviously not locked down. The retract servo was totally unresponsive.

 

Back on the bench, checked all connections, still dead. Tried it on a 5V regulated supply, still dead.

 

So - I guess I've cooked it. I have another HS75BB but I obviously need to do something about powering it differently before I fry that too. I know I can use a separate battery for the retracts (and I know it's good practice to do so) but would the trick of adding a diode in the retract power supply drop the 5 cell NiMh voltage enough to keep the servo happy? Maybe two diodes in series?

 

David, over 10 years ago it seems there were quite a few problems with these servos that were exhibiting exactly the characteristics you have seen...

 

https://www.rcuniverse.com/forum/hitec-multiplex-radios-ask-hitec-customer-service-53/7802446-hs-75bb-retract-servo-failures.html

 

For me I would jsut take it as a sign and replace with somethng else. Whilst historicaly I have been a fan of Hitec servos, in recent years I do feel their lower end servos have been cheapened to compete with low cost models from China, and they seem to have got less robust and reliable as a result. YMMV.

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18 hours ago, Bruce Collinson said:

...Now has a voltage step-down device to prevent a recurrence but don’t ask which battery, I think it’s a 2S LiFe, or which voltage regulator, probably a Yeti.

 

Is there a reason you continually type Yeti instead of Jeti? Newcomers to the hobby could get confused, it's not terribly helpful...

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Yes Dick, me too. Electronickery baffles me, I thought 555 were cigarettes years ago.

Thanks for the input everyone. I’m as sure as I can be that there is no binding, they are wire legs and the wheels are not too heavy. The battery is buried under the fuel tank and if I change it to a four cell I’ll need to go upon the church roof again. 
So - I think I’ll stick with the five cell and either a second smaller 4.8v pack for the retracts  (lose some lead!) or a diode in the retract feed.

It’s a VQ ARTF and heavy and a bit naff generally, but I picked it up cheaply to use the 91fs I had lying around, so if I get a few flights out of it I’ll be happy.

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I use 6.6v life exclusively now as I got bored charging nimh and my charger makes life simple. I only have standard servos in all my planes and as yet none have gone up in smoke and all function well. I am led to believe that as long as your servo provides specs for 4.8v and 6v they will be fine with 6.6v life and the voltage will be lower then a 5 cell nimh which is what the 6v relates to.

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