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Author Topic: Schottky Diodes Warning for those who don't know  (Read 3512 times)

BobF

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Schottky Diodes Warning for those who don't know
« on: December 06, 2006, 07:45:42 PM »

Hi all,

Sadly I have just burnt out my electronic speed controller, because I was unaware that an electric motor fitted with a Schottky Diode, must not be also used with a forwards and reverse speed controller  :'(

I know that the electronically minded will know, but it's not obvious to others out there, and there was no warning on the box. It says that a forwards only controller must have one fitted. But when the motor was used for a different model, I never gave it a thought.

Any donations towards the cost of a new controller will be gratefully accepted  ;D

Bob
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Colin Bishop

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Re: Schottky Diodes Warning for those who don't know
« Reply #1 on: December 06, 2006, 08:10:25 PM »

Sorry to hear that BobF. News to me as well. Perhaps one of our electronic experts can give an explanation we can all understand?
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wombat

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Re: Schottky Diodes Warning for those who don't know
« Reply #2 on: December 06, 2006, 09:21:34 PM »

Hi Bob,

I think the reason is this....

The shottky diode is placed across the motor terminals with the cathode to positive terminal. The motor controller controls the speed by feeding pulses to the motor - the narrower the pulses the slower the motor runs. However, because the motor is inductive, when the motor controller switches off, the motor tries to keep the same current flowing as was flowing while the ESC was switched on. As a result the motor tries to generate a large negative voltage spike which can wipe out the electronics. The shottky diode starts to conduct when this happens and damps out the voltage spike before it can affect the ESC.

However when you put on a forward-reverse esc, as soon as you put the controller in reverse, it is trying to force the full output voltage across the diode, which will only allow about 0.5V. As a result the current goes up as high as the battery will supply - net result is that the drive transistors on the ESC go foom.

Tim the Wombat
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OMK

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Re: Schottky Diodes Warning for those who don't know
« Reply #3 on: December 07, 2006, 12:15:31 AM »

Bob:

Sorry to hear of your ESC's early demise.
An old dodge to prevent that happening would be to connect two inverse-parallel diodes across the motor.

I'm pretty sure I got a spare ESC in the shed, if it's any use to you.
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Martin [Admin]

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Re: Schottky Diodes Warning for those who don't know
« Reply #4 on: December 07, 2006, 08:35:52 AM »


Does the same apply when fitting capcitors to the latest high frequency Electronic Speed Controllers?
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BobF

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Re: Schottky Diodes Warning for those who don't know
« Reply #5 on: December 07, 2006, 12:29:46 PM »

Hi PMK,

Thanks for your kind offer of a speed controller, but If I had a spare controller in my shed, I would be building another boat. So why aren't you?   ;D

Thanks again Bob
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Faraday's Cage

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Re: Schottky Diodes Warning for those who don't know
« Reply #6 on: December 07, 2006, 06:27:13 PM »

Quote
Does the same apply when fitting capcitors to the latest high frequency Electronic Speed Controllers?

Martin,

The capacitors used on the back of motors for suppression are not polarity sensitive.(Can be connected either way around).

On the other hand diodes are polarity sensitive. (Must be connected a certain way).

As soon as you reverse motor with a diode across the motor terminals it will act as a link and put a dead short across the motor and heaps of current will flow through speed controller with disasterous results.

Bottom line - non polarised capacitors ok,
Diodes wired across motor terminals on forward/reverse speed controllers wil result in a visit to your local model shop for a new controller.

Terry.
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OMK

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Re: Schottky Diodes Warning for those who don't know
« Reply #7 on: December 07, 2006, 11:24:11 PM »

Gentlemen, hold the front page!

The comment I made earlier regarding the inverse diodes dodge... please disregard.
While searching the shed for the aforementioned ESC, it suddenly occurred to me that what I'd suggested is nothing short of bad advice. For, in this instance, two diodes in inverse parallel would put a dead short across the output of the ESC - no matter which direction you wanted the motor to turn. So rather than enjoy the benefit of added protection, it would mean just the opposite would happen.
Sincerest apologies for stating what could otherwise be an expensive cock-up.
The advice that Terry has given in the above post is sound. And in fact, further to Bob F's initial post, after searching box after box, I eventually found not just the ESC, but the paperwork also. And guess what?... Being that it's a reversible job the bumph specifically states that you should not install a diode across the motor(s).
Terry's advice concerning the non-polarised caps is also sound. But am I thinking along the wrong lines, or did I really read something, somewhere, about electrolytic (polarised) caps involved with ESCs and motors? Can anyone throw some light on that?

Bob:
You ask heap good question. For which there are two answers...

1) With the exception of one store-bought ESC in particular, I normally use homemade speed controllers. The one stashed in the shed is an 80-amp LRP job. Used just once, then returned to its box. The only store-bought ESC that I do like is the one purchased from Dawnmist.org.

2) Moving to a new QTH shortly. All my shed'dy stuff is already boxed - hence no boat-building fun for a while, and nowhere for the spare ESC to go.


Apologies again for the previous misleading comment.
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BobF

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Re: Schottky Diodes Warning for those who don't know
« Reply #8 on: December 08, 2006, 12:55:40 AM »

Hi again PMK,

I thought that the ESP I was using had motor short circuit protection built in, so this should have stopped it burning out shouldn't it?

It's too late at night to bother looking for the instructions now, and it may be the newer version that I also use has this protection.

If you really want to get rid of your ESP, let me know what you want for it, and we may be able to sort something out.

Regards Bob
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OMK

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Re: Schottky Diodes Warning for those who don't know
« Reply #9 on: December 08, 2006, 07:41:22 PM »

"I thought that the ESP I was using had motor short circuit protection built in, so this should have stopped it burning out shouldn't it?"

If the label on the box says it had s/c protection, then surely that's what it should have done. It obviously didn't. And without taking a closer look it's difficult to pinpoint exactly why.

"If you really want to get rid of your ESP, let me know what you want for it, and we may be able to sort something out."

it's only fair to tell you that I chopped off the two bullet plugs on the ends of the yellow and blue wires. At the time I figured it would reside permanently in the cabin cruiser. And since I prefer to solder all my connections...
You get the picture.
What do I want for it? Well, I took pity on your plight and implied in the earlier post that it's yours if you want it... gratis. I'd sooner see it go to a good home, rather than sit idle in the shed. However, I'm always open to receiving a couple super-bright LEDs, <nudge, nudge!>.
One other thing...
In their instruction sheet, LRP suggested to glue the heatsinks to the FET tabs with a dab of cyano. I did - so now they're fixed there permanently. It means the added height makes it too big to fit back inside its original box.

If you can live without the bullet plugs and original packaging, it's yours.
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BobF

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Re: Schottky Diodes Warning for those who don't know
« Reply #10 on: December 08, 2006, 10:33:24 PM »

Hi PMK,

I've sent you an E mail.

Regards Bob
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malcolmfrary

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Re: Schottky Diodes Warning for those who don't know
« Reply #11 on: December 11, 2006, 03:14:04 PM »

If I remember right, Schottky doides have the property of very fast switching, and may very well have beaten the protection in this case.  Having said that, it looks like the short circuit protection should more accurately be called "over-current protection".
For forward-only motors the diode is a great idea, as any interference-generating signals never get beyond the motor terminals.  On my home-made circuits I arrange for a protection diode across the controller power input to ensure that the fuse gets blown when I connect the battery the wrong way round. (When, not if).  Then again, my models don't demand maximum performance.
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OMK

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Re: Schottky Diodes Warning for those who don't know
« Reply #12 on: December 14, 2006, 08:35:19 PM »

BobF, just in case you haven't received a mail from an unfamiliar address already, don't panic... 'tis just I.
Pse don't use the Hotmail account - it's gone belly-up. Instead, it's probably better to contact via here.

Can you let me know if you Rx'ed my question regarding the length of the wires?
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BobF

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Re: Schottky Diodes Warning for those who don't know
« Reply #13 on: December 15, 2006, 12:20:51 AM »

Hi PMK,
I did send you an e mail regarding what guitar I played plus potted life history.

I don't remember anything about wire length, though this will not be a problem.

Don.t worry till your all settled though.

Regards Bob
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BobF

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Re: Schottky Diodes Warning for those who don't know
« Reply #14 on: December 21, 2006, 12:55:54 PM »

Hi PMK,

The speed controller arrived this morning. THANK YOU very much. I was expecting some old looking clapped out thing. Not one looking brand new in a bag with instructions. Beyond the call of duty, was fitting new longer leads for me as well.

I've not yet had a chance to check out the other item you sent me, as I have to go out, and the post only just arrived. I'll be looking when I get back, and visiting the web addy you gave me.

Once again, thank you very much, and rest assured that it will only be a matter of days before the speed controller is installed and sailing on the pond at Bridlington.

Regards Bob.
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BobF

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Re: Schottky Diodes Warning for those who don't know
« Reply #15 on: March 23, 2007, 03:35:53 PM »

Hi all,

Back with another question re Schottky Diodes.

For testing reasons that I won't go into, I reverse polarity connected a small battery to my 700bb fitted with a S D and it ran backwards.

I thought that fitting one of these diodes stopped any voltage running in reverse.
Now I am baffled. Please can one of you tech lads explain why it does this.
I did notice that it ran slower when connected the wrong way.

Bob
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Tug-Kenny

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Re: Schottky Diodes Warning for those who don't know
« Reply #16 on: March 23, 2007, 04:31:44 PM »


Sounds like a leakage here.  Have you tried this with a new Diode ?

Cheers...Ken
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BobF

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Re: Schottky Diodes Warning for those who don't know
« Reply #17 on: March 23, 2007, 10:24:19 PM »

Hi Tug Kenny

I'll have to buy a couple tomorrow to try. I know my local model shop has them in stock, and it's only two hundred yards away. I don't want to destroy yet another controller.

Cheers Bob
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OMK

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Re: Schottky Diodes Warning for those who don't know
« Reply #18 on: March 23, 2007, 11:45:09 PM »

Nope, nothing to do with leakage. That's just the diode doing its thing. Any diode will drop whatever voltage there is going into it by a small amount. Most bog-standard silicon diodes will drop as much as approx. 0.5 to 0.7 volt. Schottky diodes, on the other hand, only drop approximately 0.3 volt. So sticking a diode in series with a battery (as you did) would have dropped the battery voltage by 0.3 volt. That's why the motor ran slower.
And being that it's a diode, it obviously means it will only conduct current in one direction. Depending on the polarity of the battery AND the diode, it is possible to make the motor reverse - as you seen.
Try the experiment again. Keep the battery polarity the same, but invert the polarity of the diode.... then see what happens.
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OMK

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Re: Schottky Diodes Warning for those who don't know
« Reply #19 on: March 23, 2007, 11:56:17 PM »

Oh, hang on!....

Have I read your post correctly?
Are you saying that the motor will still run - regardless of the polarity of the diode?
If that's the case, than what T-Kenny is saying is indeed plausible. Maybe you've mashed the diode. Maybe the P-N junction has taken a dive and allowing current to pass when it shouldn't.

But I've NEVER known of a diode to go short circuit. OPEN circuit, yes. But not t'other way around.

Maybe you've inadvertantly discovered some hitherto unknown characteristic of electron flow.
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BobF

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Re: Schottky Diodes Warning for those who don't know
« Reply #20 on: March 24, 2007, 11:35:00 AM »

Hi PMK,

Nice to hear from you again. I've not seen any postings from you for a while and thought you may have moved and been busy. Are you still at the same address as you were in December? I just had a quick look at your last postings, and think I may not have read those topics.

As I see it, and my explanation may not have been clear, the battery is connected in parallel with the diode. I had NOT gone in one end of the diode and out the other and then on to the motor. The Diode is connected across the motor terminals, as was the battery. Does that explanation make sense to you? or to put it another way, I fitted it as I would a suppressor cap across the motor terminals.
Does this mean that the S D should have been in the pos supply to the motor and not across the motor terminals. I have not blown the forwards only speed controller fitted to this boat (YET) so assumed I had done it correctly. Thinking about it now, the diode is to stop power running back up the pos lead into the speed controller init? But as you already know, I have very little technical knowledge regarding electronics.

I can sort most problems regarding how it should be done, if something doesn't work. I can normally sort the reason if something has been fitted incorrectly, but don't understand the technical working of the components.

Best regards Bob
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malcolmfrary

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Re: Schottky Diodes Warning for those who don't know
« Reply #21 on: March 28, 2007, 08:41:41 PM »

Hi BobF,
If I read it correctly, the diode is connected directly across the battery.  As long as the diode is non-conducting, nothing will happen.  If the diode conducts, it will only do so until its power rating is exceeded.  It will usualy wind up being a disconnection then, and will indicate the transition with a bang. 
I have never had cause to use schottky diodes, just read a bit of theory, which indicated that in most ways they are much like normal diodes.
I have always found that when mystified by a circuit, testing with a meter is a great help.  Quite respectable ones can be hadfor less than a tenner these days.  Testing this diode could show whether it is a diode, or two bits of wire, either joined together or otherwise.
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