Setups without SM Services units!

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Steve McDonald
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Setups without SM Services units!

Postby Steve McDonald » April 17th, 2016, 3:44 pm

Now that we no longer have SM Services, what are you all using in terms of toggle switches? Also what setups are being used for dual batteries (single Rx) without using a megabucks PowerBox type unit.

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Re: Setups without SM Services units!

Postby Steve Rickett 2333 » April 18th, 2016, 5:40 pm

I have had great success with the Jeti Max BEC Units. The Boston and Dragonfly use two each powered by A123's.

https://www.electricwingman.com/jeti/max-bec-2.aspx

https://www.electricwingman.com/a123-lifepo4-2500mah-2s-battery.aspx
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Steve McDonald
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Re: Setups without SM Services units!

Postby Steve McDonald » April 19th, 2016, 6:38 pm

Thanks for that Steve. I have never seen these before and was a bit puzzled when I saw the BEC! They look like a good option, I think I'll give one a try. Never tried the A123 batteries either but I have batteries for the plane I'm working on, I'll leave that for another day!

Thanks again.

Steve

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Re: Setups without SM Services units!

Postby Chris Hurst » April 20th, 2016, 9:32 am

Leaving aside the red / green battery monitors the SM services backer was simply two diodes which can be bought for under 50p each.

Providing you use two similar batteries many would argue that the diodes are not actually necessary. They simply prevent one battery from charging the other if the voltages are unequal. If you calculate the likely charge current it is actually very small. It would need a very unusual fault on one battery for it to draw sufficient charge current from the other to cause a problem.

Ultimately you balance that risk against the risk of the diode failing open circuit!

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Re: Setups without SM Services units!

Postby Steve McDonald » April 30th, 2016, 7:42 am

I have bought one of the Jeti Max BEC2 units to try. I have also bought a couple of Schottky Diodes and will try that option too. Chris, you are correct in saying that technically the diodes are not required as the internal resistance couple with a fairly low potential difference between a charged and a discharged battery will mean only a minimal current will flow between the two. So I intend to try that option as well. I have sufficient batteries at the moment but the next time I need batteries I will try the A123 option. Thanks for your input guys.

Steve

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Re: Setups without SM Services units!

Postby John McNamara » January 31st, 2017, 8:11 am

Hi Guys,
I have just come accross this Thread. The thing most people miss is that by using two Batteries we are trying to add redundancy, and that is there to compensate for the possibility of some failure occuring. Measuring the conditions when no failure is present is futile. Yes the cross drain on batteries in normal conditions is neglible. However, the diodes are NOT there for normal conditions, but for a failure condition. There are circumstances where one battery SYSTEM can fail that will apply a dead short to a fully charged battery with devastating results.
My friend took this bad advice about not needing diodes with his first jet, because this information was written down in a magazine he preferred to believe a guy in a magazine column rather than me. I did my education in the field of electronic, wheereas the magazine correspondent admited that electricity was a mystery to him (why he was going into print on a subject that he knew nothing of, baffles me).
My friends model burst into flames. This was because there was a dead short in one switch, the switch of the other battery. This meant that a fully charged 5 cell nicad wad discharged more or less instantaneously, through the receiver and the associated wiring loom. The tracks on the receiver vapourised, and the insulation on the wires melted. Had it not been a jet, we probably would not have had a fire extinguisher on hand....
For the sake aof a couple of quid and a little time soldering this risk can be eradicated.
John

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Re: Setups without SM Services units!

Postby Rob Buckley » January 31st, 2017, 10:02 am

John,

That’s some interesting experience there. For putting a diode in line, it needs to be in the right place, after all parts of the battery system just before the electricity goes into the receiver. If a diode had been between the battery and the switch in the case you’ve described it wouldn’t have helped at all. (I know you know, but it may well help anybody else reading this).

I found when I managed to try and short a 2 cell A123 battery through a receiver that the negative track on the PCB worked as a fusible link and vaporised before any more serious damage was done. The receiver still works, but not in anything flying!

As with all things model related where getting into slightly more complicated systems than a Wot4, it pays to have a think about what it is you’re trying to achieve, how it could all go wrong, and how those failures could be mitigated.
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Re: Setups without SM Services units!

Postby Tony Collins 1073 » January 31st, 2017, 10:12 am

Well said John. Perhaps after reading your post, the many people who think they know but don't will stop suggesting that is is
OK to run batteries in parallel without separating diodes. The advantages of using two batteries ie extra speed and power, are obvious but would you believe that there are still some idiots relying on a single 4.8 battery with a single pole switch just because their model is smaller.
Safety is paramount in our hobby and someones life could depend on the quality of our work.
Thanks for putting this old lemon to rest John.

PS there will probably still be some who think they know better unfortunately.

Tony.

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Re: Setups without SM Services units!

Postby Steve McDonald » April 4th, 2017, 6:41 pm

This is the first I have looked at this thread since I bought the Jet Max BEC. Sad to say I have not tried it out yet, very soon I hope. I am still using SM Services units. If I was using two batteries, I would use diodes personally. Interesting comments guys!

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Re: Setups without SM Services units!

Postby Dave Parry » April 5th, 2017, 5:49 am

What is even more interesting is no one has ever thought of doing a small article for the journal on just how you do this. we seem to be banging on about safety but how is someone like me who doesn't have a clue about diodes etc.. suppose to understand how to set up a plane safely when there is no simple guide to follow.
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Re: Setups without SM Services units!

Postby Bob Thompson1894 » April 5th, 2017, 9:08 am

For smaller models, I always use two switches. But honestly guys, have you ever had a battery go down in flight? On a 6v battery, if a cell goes down (most likely option) you still have 4.8 volts? Obviously with over size or very expensive models dual everything is the way to go, but keep adding more and more electronics may be adding failure points? I like the KISS theory.

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Re: Setups without SM Services units!

Postby Dave Hayfield » April 5th, 2017, 10:21 am

For additional safety, without compromising electronic fallibility, fit one of my Engine Failsafe units. This will, at least, stop your petrol engine in the event of complete battery failure and prevent the model hitting the ground at full throttle or watching it disappear into the blue yonder. The receiver has to be working for the standard failsafe systems to operate and it won't if it loses power. See the add in the journal or give me a call on 07831546671
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Re: Setups without SM Services units!

Postby Chris Hurst » April 12th, 2017, 6:20 pm

John McNamara wrote:Hi Guys,
I have just come accross this Thread. The thing most people miss is that by using two Batteries we are trying to add redundancy, and that is there to compensate for the possibility of some failure occuring. Measuring the conditions when no failure is present is futile. Yes the cross drain on batteries in normal conditions is neglible. However, the diodes are NOT there for normal conditions, but for a failure condition. There are circumstances where one battery SYSTEM can fail that will apply a dead short to a fully charged battery with devastating results.
My friend took this bad advice about not needing diodes with his first jet, because this information was written down in a magazine he preferred to believe a guy in a magazine column rather than me. I did my education in the field of electronic, wheereas the magazine correspondent admited that electricity was a mystery to him (why he was going into print on a subject that he knew nothing of, baffles me).
My friends model burst into flames. This was because there was a dead short in one switch, the switch of the other battery. This meant that a fully charged 5 cell nicad wad discharged more or less instantaneously, through the receiver and the associated wiring loom. The tracks on the receiver vapourised, and the insulation on the wires melted. Had it not been a jet, we probably would not have had a fire extinguisher on hand....
For the sake aof a couple of quid and a little time soldering this risk can be eradicated.
John


John

I realise it is a few months since your post which is largely a reply to mine from last year.

I don't agree with your last line. Yes, you may "eradicate" (well, lessen anyway) one risk but you do introduce another. Your friend's scenario was a full short in a switch - a fairly rare occurrence. OK, in that situation the diode would prevent the second battery from being discharged rapidly but it wouldn't prevent the first battery heating up the faulty switch and wiring and setting fire to the model. Yes, until the fire damages some other critical component you would likely retain control.

However, with the diode setup you are relying on the junction of a component costing a few pence to pass all of the current drawn by the model on every flight. With a stalled servo that can be considerable. In the event of a momentary short circuit the diode will fail like a fuse well before any wiring will burn out. You will have no indication that this has happened unless you have onboard voltage monitor and turn each switch off in turn.

So, as I said in my original post, you swap one risk for another. Which is more likely? A short circuit switch or burnt out diodes? I wasn't advocating one approach, just pointing out both sides of the story.

Surely a far better solution would be to fit two receivers, each with their own batteries, monitors and switches but no diodes. With each receiver controlling half of the servos you would at least have some control in the unlikely event of a whole number of failures.

"A little time soldering" you say. Fine if you can do it properly (as I realise you can!) but I have seen some real horrors with terrible joints and wire totally unsupported so that the vibration of the engine is encouraging the wire to break next to the joint. In fact it was S M Services who said to me that he fitted the little screw terminal blocks to most of his units because he never ceases to be amazed just how badly many modellers soldered!

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Re: Setups without SM Services units!

Postby John McNamara » April 19th, 2017, 7:20 pm

OK, I think you are misguided in your beliefs.
David, the diode is the single most reliable device EVER created by human kind. The original one is still operating 60+ years after being put into continous use. They have got a lot better than that since then. The diode will not burn out like a fuse because in my own case it would be correctly spec'ed to do the job in the failure mode. I have actually had a digital servo fail and burn out, but the diodes are still doing fine thank you. The fact that a diode costs a few pence in no way relates to its fitness to do the job.
Lets take you mechanical switch. You asked a question which is more likely to fail a switch or a diode. Is this a serious question David?
As an electronics engineer, in the early years of my career, when repairs meant getting your hands dirty, I replaced mechanical switches daily. I don't remember EVER replacing a diode. In fact I dont think we carried any in our spares chest. We carried every type of switch.
I have been fitting diodes in this situation with two batteries into every model that I can for 30 plus years. I have never had a failure.
Today we can fit electronic switches in our models and I for one use them. They are generically known as "Safety Switches" because compared to their mechanical cousins, they are safer by a factor of several THOUSAND times. They use transistors, a type of film junction at the heart of the thing that costs a few pence, but unlike its expensive mechanical counterpart should last for an almost infinite period of time if utilised in the correct manner. Many of them are set up to utilise two batteries and guess what? Yes they are separated electronically by diodes. You can buy powerboxes and the like and the same applies.
Finally, and this really is the point, there will be no short circuit in the scenario that I described due to the fact that there are diodes in the circuit. So the diode ERADICATE this rick. They absolutely do not swop one risk for another.
If I have failed to convince you of the efficacy of the humble diode then I suggest David that this hobby is not for you. All of our radio components are full of them!
John

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Re: Setups without SM Services units!

Postby Dave Hayfield » April 19th, 2017, 11:05 pm

Hi John, I'm not sure which David you are replying to. My profession for almost 50 years has been as an electronics engineer and believe me I have had to replace hundreds, if not thousands of failed silicon diodes in equipment ranging from high power stage amplifiers to transmitting equipment, television receivers and various power supply units. Yes they are generally very reliable and must be the correct diode for the job but are still prone to failure. Nothing is perfect. Just as a matter of interest I designed and built an intruder alarm for my house over 50 years ago, it is in operation every day and the main component to trigger the alarm is a mechanical relay, never had to replace that.
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Re: Setups without SM Services units!

Postby Chris Hurst » April 20th, 2017, 3:07 pm

John McNamara wrote:OK, I think you are misguided in your beliefs.

John


No, as I thought I had made clear, I am not advocating one approach over the other. I am simply pointing out that it is not a case of "fit a diode and all will be perfect my son". There is for and against. We could debate the percentage risk but I do not accept your apparent belief that it is almost totally one sided.

The diode gives useful protection against certain risks. How likely those situations are and whether there are better ways of mitigating the risk needs to be considered.

The simple act of adding the diode introduces at least three potential points of failure (the diode itself and the connections to either side of it). Two of those are in the control of the person fitting it. Do they have the knowledge and soldering ability to do it properly? Sadly many don't.

I agree with your point about electronic switches. Providing the greater reliability statistic holds up in a high vibration environment then it clearly makes sense to use them. I assume that is the case?

However my other point remains. A twin receiver setup (with each driving half of the servos) also addresses all of the risks that would be protected by the diode whilst giving a whole number of other safeguards. Is this not a better way to go? OK, you may argue it is more expensive but then consider if it would be better to use two relatively cheap receivers rather than one expensive one. We could debate the relative risks of those two options I'm sure.

Interestingly that is the minimum acceptable installation for over 20kg models. However, as far as I know, the "quality" of the receivers is not specified!


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