speed controller and fuses

[captainslogParticipant @captainslog]

Some of you may know by my previous threads I am useless when it comes to electrics.I have an mfa speed controller rated at 6 to 10 amps. reading the instructions the inline fuse in the battery line or motor line should be 15 amp. shouldnt the fuse be LESS than the rating of 10 amp max? The reason being that if the controller can only handle 10 amp before it blows up then you dont want to draw 15 amps before the fuse blows as the controller has probably blown before it reaches that? i know mfa say do it but why? or am i just a bit dull and cant see the obvious reason

cheers

[captainslogParticipant @captainslog]

[Ray Wood 3Participant @raywood3]

Hi Captain,

Granted I've only built 25 electric boats in the last 5 years so you may ignore the fact if you wish but I've never used a fuse in any of them , maybe I'm just lucky to run in weed free lakes 😀

Regards Ray

[Colin BishopModerator @colinbishop34627]

It's just insurance really. 99% of the time not needed but if you do…..

Fuses cost pennies.

Colin

[captainslogParticipant @captainslog]

cheers both, i was just confused by mfa rating it at 15 amp. i looked at the features section and dave milbournes article on electrics. he says to fit a fuse less than the rating,which was my way of thinking. so less it is. just dont understand mfa logic. depending on the weather the wiesel will be out on the pond for a test run,in line fuse fitted for the macgregor s/c. I think i have found its rating as well, under the macgregor logo on the casing in very small print is "MR15" so i am presuming its rated at 15 amp. works fine so far on bench test, no smoke means no fire

cheers keith

[fredParticipant @fred20856]

Fuse rating could be based on 10 amp max continuous current and about 15 amp peak current

fred

[TelstarParticipant @telstar]

Aye Aye Capt'n. The inclusion of a fuse in any circuit is not to protect the equipment being supplied, but to prevent the everything else from being destroyed, usually by fire, in the event of that equipment failing.

The 'rated' current on the controller is an advisory by the maker to stop users overheating the controller hence destroying it. The fuse is there to burn out 'blow' if anything untoward happens, and so prevent the battery dumping ALL its energy into the fault and in doing so it could overload the wiring etc. and cause a fire.

Normal rules for fuse size, the fuse should be less than the current carrying size of the wire being used. ie 10amp wire – 8amp fuse the wire should be chosen to be thick enough to carry the current needed by the load (motor)

Also the controller should be enough to carry the load.

I served my time as a works electrician nearly 60 years ago, but the basics are still the same.

Good luck with your modelling.

Tom

[ashley needhamParticipant @ashleyneedham69188]

Fitting a fuse is as easy as cutting an esc supply wire, fitting two female 1/4 inch flat blade crimp-on terminals and plugging a car type fuse in.

Unfortunately I am in Ray`s camp, very few boats have fuses ….

Ashley

[Malcolm FraryParticipant @malcolmfrary95515]

Posted by captainslog on 06/11/2019 19:28:16:

Some of you may know by my previous threads I am useless when it comes to electrics.I have an mfa speed controller rated at 6 to 10 amps. reading the instructions the inline fuse in the battery line or motor line should be 15 amp. shouldnt the fuse be LESS than the rating of 10 amp max? The reason being that if the controller can only handle 10 amp before it blows up then you dont want to draw 15 amps before the fuse blows as the controller has probably blown before it reaches that? i know mfa say do it but why? or am i just a bit dull and cant see the obvious reason

cheers

I can't think why MFA might say that a fuse rated higher than the ESC that it is associated with,either. A fuse that is too heavy is reliable, but no real use.

[captainslogParticipant @captainslog]

My thoughts too malcolm, unless its a misprint. gone for a 10 amp.

Right then more to clear up. new batteries from howes. motor running fine on 7.2v 2000mah. Connected all up,rudder and s/c. a dry run at full speed on the stand fwd and rvs. rudders fine.

after roughly 20/30 minutes rudders went into fail safe but motor kept running until I switched off when they obviously ran the drive battery down. rec. battery 1300 mah.

recharged both batteries( overlander rcv6 charger) tried again. same thing as bove. WHY is the rcv putting my boat into fail safe? batteries in tx seem fine. could it be the s/c is putting a high drain on the rcv battery but leaving enough to keep the motor running?

CONFUSED and at the moment i dont feel safe taking it on the water. btw the motor is drawing .98 amps no load on prop. not far off the johnson spec of .78mah.

any help appreciated.

keith

[captainslogParticipant @captainslog]

ref above post motor ran for approx 1 1/2 hrs maybe a little less

[Dave MilbournParticipant @davemilbourn48782]

Keith

If the motor is drawing approx 1A with no load then it's a reasonable estimate to say that will be doubled when the model is in the water – say 2A. You have a 2AH battery pack, so at the very most your motor will run for one hour. New batteries reqiure several charge/discharge cycles before they reach their full capacity. You should also not charge at over the C rate (in this case 2A) for the first few cycles, whereas the Overlander charger is claimed to charge at up to 7A. If you can lower that rate to say 1A and charge for a couple of hours then it might help things.

As regards the receiver going into failsafe, that normally only happens when there's either a lack of a valid signal from the Tx or the supply voltage to the receiver drops below a critical value. Undue drain on a receiver battery pack is most often caused by a faulty or overloaded servo. Electronic speed controllers take very small currents to function, even very old ones like your MFA one.

Dave M

[Malcolm FraryParticipant @malcolmfrary95515]

What is "fail safe" with a rudder? If the battery supply drops the voltage to the rudder servo, all that usually happens is that it stops working. As long as there is a valid signal coming out of the receiver, the servo, with enough power, will do as it is told. Normally, a servo will not pull much more than 1/2 Amp, so should not normally be the first thing to suffer with falling supply voltage. A good servo, that is.

The control part of an ESC will normally take a very few mA to run itself, but if it is one using a relay for reverse, it is important to ensure that the relay only operates to reverse. The extra 100mA places a considerable drain on the RX battery if in use continuously for forward running. It might also have a noticeable effect on battery life in a BEC system. No specific experience with MFA ESCs, so just general thoughts.

[Stephen Paterson 2Participant @stephenpaterson2]

Ohms law is the way to work it out. That way it eliminates guestimates. Dependant whether you use micro blade auto fuses or the older glass style ones you most likely will not get one that matches the rating use the closest up from that.

[ashley needhamParticipant @ashleyneedham69188]

Whats needed here is a wattmeter, cheap enough and easy to use.

Plugs in between the batt and esc. Stab the little switch to go on to "amps" and run the boat in the water. See how much current is being drawn. Decide whether the batt/esc is being strained…fit fuse to something above the shown drain, but less or equal to the esc.

This will also show the voltage, and show if the battery is dropping below minimum esc/receiver value.

No guesswork. Best result.

Ashley

[Stuart Munro 1Participant @stuartmunro1]

Hi,

A new question on the topic of fuses. I am trying to get a neater solution to fuses in the hull of a new build scale sailing model. My access is small through which the battery needs to go plus access to maintain and repair (if needed) servo drum winches and steering.

i guess some will say that without a motor, a fuse is not needed but I've had one burn-out in a sail boat (due to a sticking digital servo) and am now cautious.

The problem is this – blade fuse holders are a/ large and b/ unsightly. Does anyone know of a better alternative?

Thanks

Stuart

[Colin BishopModerator @colinbishop34627]

Don't use a blade fuse holder. Just use insulated spade connectors to attach the wiring directly to the fuse. See photo below.

Colin

[ashley needhamParticipant @ashleyneedham69188]

I did suggest the blade fuse/ spade connectors earlier..

Ashley

[Colin BishopModerator @colinbishop34627]

Ah! Missed that but a picture is worth a thousand words etc.

Colin

[Ray Wood 3Participant @raywood3]

Hi Stuart,

You may have had some bad luck in the past , but you really don't need a fuse on your sailing boat's radio battery

Regards Ray

[ashley needhamParticipant @ashleyneedham69188]

Colin, you were just reminding everyone of wot I wrote!

The trouble with threads like this that useful information gets lost in the mullings-over.

However an acknowledgement at the time could have been useful.

Ashley

[Colin BishopModerator @colinbishop34627]

Ashley,

I think that is the trouble with any forum really. Nuggets of general information get hidden in specific subject threads and inevitably get repeated time after time. And then all that valuable information can disappear in a flash if the forum owners pull the plug which doesn't encourage builing up any sort of comprehensive online database. I think I must have used the image above at least four times in the last year in one online place or another.

It does show the value of the printed word which tends to endure rather longer in the form of books, and for those of us who hang on to our magazines. (does anybody still write model boating books?)

My article in the forthcoming issue about motors and props is intended to sweep up many of these basic queries and follows on from the excellent articles by Dave Milbourn but of course you do have to get the magazine!

Like many people, I do have a lot of useful documents and images saved on my PC which I often use to answer queries but they will only be available as long as I carry on with the hobby.

Colin

[Stuart Munro 1Participant @stuartmunro1]

Well thank you one and all! My own club have a theory; ask how to do something and you will get 3 different suggestions. You can then ignore them all with a clear conscience!

But seriously thank you. I do understand that a fuse should not be needed but…..should I ignore this advice connecting the wires directly with clips is a simple, yet brilliant solution.

And yes, a picture tells a thousand stories.

Stuart

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