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5 cell NimH and 6V servo


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A few years ago I got a good tip from the bods at motion rc. Before you take to the skies, hook up all the servos in the model to a servo tester / cylcer and let them run for at least ten minutes. If any of the servos get hot....bin them. I know its a bit basic but works for me, having found a couple of duffers.

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50 minutes ago, Ron Gray said:

In my experience, no, it happened to me mid flight! JR591 servos, 2S LiFe.

The JR591 was only rated at 4.8v and it's pretty well known they didn't like more than a 4 cell Nimh/Nicd, I've got a few models that have the ubiquitous futaba 3003 servos in them (and some 148s) and all have been fine on either 5 cell Nimhs/2s LiFe. 

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8 minutes ago, Frank Skilbeck said:

The JR591 was only rated at 4.8v and it's pretty well known they didn't like more than a 4 cell Nimh/Nicd...

I think that applies to pretty much all JR servos! I think they did make some dedicated HV servos, but all the common ones were 4-cell only.

 

Frankly, I've never understood this obsession with 5-cell packs. I've never used anything but 4-cells NiXX sets since the late 60s, and never experienced a problem with them. Even my early Spektrum sets never showed any inclination to "brown out". It is my belief that most of the issues that 5-cell packs were meant to overcome were down to poor quality switches and/or connectors. Over the years, I have witnessed some real horror installations...!

 

I have ONE model that uses a 2-cell LiFe pack and high voltage servos. One of the (quite expensive!) servos failed within days of purchase. I can only recall one servo ever failing on 4.8V, and that was a retract servo. I have no idea how many servos I've had down the years (decades?!) but it must be a lot! I've got servos that are nearly 50 years old, still functioning perfectly on 4.8V.

 

Run them on what they were designed for, not what "old wives tales" tell you you should!

 

--

Pete

 

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Mine was a silly mistake as I wanted to fly the model and as the NiMh Rx pack was low I swapped it for the LiFe pack. That mistake cost me my first ESM Hurricane!

With the exception of 2 models, all of my models use LiFe or UBECs and LiPo packs.

Edited by Ron Gray
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7 hours ago, SIMON CRAGG said:

A few years ago I got a good tip from the bods at motion rc. Before you take to the skies, hook up all the servos in the model to a servo tester / cylcer and let them run for at least ten minutes. If any of the servos get hot....bin them. I know its a bit basic but works for me, having found a couple of duffers.

 

That's a good tip. I sat and wiggled the aileron stick on one of my models a while ago, and one aileron noticeably slowed down after about 20 cycles. It had a one way trip to the bin - the servo that is. 

 

Following the demise of my Valiant, I removed the voltage regulators from my new Valiant and Ultrastick. Repeated the above 'wiggle test' and felt/smelled the servos. All seemed fine so I flew them both - carefully at first then with more exuberance - no issues, no warm servos. When I refurb the Ultrastick, I may give it a 2S LiFe like the new Valiant, or stick with the 5 cell NiMH.  Will also bin any 4.8v only servos just in case I forget!

 

Edited by GrumpyGnome
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On 30/06/2022 at 06:57, GrumpyGnome said:

Stupid question time..... if you were to use a 2S LiFe with servos that were not necessarily rated HV, would they fry immediately?

 

"It depends"

 

Very much on what the "pinch point" is.

 

Some types of component are sensitive to voltage. (mentioned in thread, tantalum bead caps)

These would smoke almost straight away.

Your servo may or may not have some of these components.

 

Past that one?

 

Other types of component are sensitive to current. (mentioned in thread, old style servo control ICs with internal transistors)

Other types will be sensitive to power (normally heat, really). (mentioned in thread, power transistors, which get hot during switching)

These might be ok - until the extra available power starts being used, at which point all bets are off.

 

I'm with Pete C on this one. The ultimate best answer is really to run bog standard 4.8V servos on bog standard 4.8V packs, you're on firm ground... unless you happen to know for certain why it is ok to run a particular setup with higher voltage and current.

 

Now folks are using 2 cell LiFe... 2 cell LiFe is a different kettle of shrimp. Which needs different servos. You pays yer money...

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