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Topic: MOSFET N channel transistor curiosity (Read 1 time) previous topic - next topic

17Nico17

Thanks for your help PerryBebbington! Ohm's law yes ;) Kirchoff's law too, but not studying that for 10 years  :smiley-roll:
All I remmember is that tensions are the same potential at the same nodes and in the different "loops" and that we have current additions at these same nodes.

Regarding my motors, stall current is 20A per motor.

I try again for the circuit picture (this time with the link from my PC). Picture is also attached in case :




PerryBebbington



The usual convention is positive at the top, negative at the bottom of the schematic, the opposite to what you have.

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Regarding my motors, stall current is 20A per motor.
Then the diodes need to be rated for at least 25A. I don't have a specific diode in mind, time for you to do some research, unless someone here can suggest one.

17Nico17

#17
Feb 20, 2020, 10:15 am Last Edit: Feb 20, 2020, 10:15 am by 17Nico17
Ok, I have so many things to learn  :o
I have found some diods, do not know if it should to the job...?

1st : DIOTEC SEMICONDUCTOR P2500K
2nd : MBRB2545CT (seems to be two diods inside?

I guess that I do not have the good key words in Google to find what I need because I do not know much about this domain unfortunately... If some guys know about diodes, I take  :D

Thank you again (+ for the picture)!

PerryBebbington

Something I didn't pick up on in my previous post:
You have 2 motors in parallel that between them might draw 40A. Is the power supply and the device switching the power capable of supplying 40A?

Also, now I am thinking about it, I am not sure about using 2 diodes that will be in parallel as the motors are in parallel. I can see a risk that one diode will end up carrying the kick back from both motors, which is potentially 40A. I think one 50A capable diode is needed, not two 25A diodes.

17Nico17

#19
Feb 20, 2020, 10:48 am Last Edit: Feb 20, 2020, 10:51 am by 17Nico17
One point for you PerryBebbington! My general switch is 20A...  :smiley-confuse:

Ok, for the diode, I also thougth about something similar for the parallel/serie stuff. As I do not know how it has to be cabled in the case of 2 motors in parallel, I supposed it was a good idea two have 2 diodes.

My motors will turn freely in a normal use. The only duty they have is to send a ball that will be sent by friction with 2 wheels. Using a instrumented alimentation, current is lower than 1A even when the ball is launched (max normal current).

It could be a good idea to add a fusible (20A) before the motors maybe? A fusible that can be reingaged (I have some like that, they do a "click" when above the indicated current and you can re-engage them). The only way the current is 40A is if a ball is stuck or something like that.

Edit : I forgot to say that my alimentation will be a LI-PO battery, 3S 5000 mAh 20C (11.1V).
So I search for a 50A diode now?  :smiley-lol:   Do you have an idea of the scheme with only one diode?


PerryBebbington

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My general switch is 20A...
It's not going to last long... Maybe you need one for each motor.

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My motors will turn freely in a normal use. The only duty they have is to send a ball that will be sent by friction with 2 wheels. Using a instrumented alimentation, current is lower than 1A even when the ball is launched (max normal current).
When the motor is stationary and you apply power it will draw its stall current, even if only for a fraction of a second. This probably won't show on a multimeter. You have to account for this. Also, you cannot ever guarantee that 'normal use' will always apply.

If you put the motors in parallel and the diode across each motor then the diodes are in parallel. The electricity won't care which diode is where, it'll just follow the rules of Ohm's law. If that means more current in one diode than the other and the diode is overloaded then it goes pop, quickly followed by the other diode and the driver. I think probably you should have separate drivers for each motor and a diode for each motor. If the motors have a 20A stall current then I suggest drivers rated for at least 25A and diodes the same.

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It could be a good idea to add a fusible (20A) before the motors maybe? A fusible that can be reingaged
It won't protect your diodes or drivers. Silicon blows a lot faster than any fuse. Fuses are mainly for protecting against fires caused by overloaded cables. The battery you suggest is easily capable of starting a fire if shorted out; you should use fuses for that reason, but don't expect them to protect your silicon devices, they won't.

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alimentation
That's a new word to me, what does it mean?

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So I search for a 50A diode now? Do you have an idea of the scheme with only one diode?
The more I think about it the more I think you need 1 driver and 1 diode per motor, not 1 driver and 1 diode shared between them.

If there is someone reading this with a different perspective please will you comment and help?


 


17Nico17

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It won't protect your diodes or drivers. Silicon blows a lot faster than any fuse. Fuses are mainly for protecting against fires caused by overloaded cables. The battery you suggest is easily capable of starting a fire if shorted out; you should use fuses for that reason, but don't expect them to protect your silicon devices, they won't.
What I meant is to have a general fusible to protect the main switch. The idea is to keep the diodes above (stronger) to be sure I will not have any issue with them. Regarding the motor size, I am quite sure the fusible will not pop in 99.9% of the situations. If it pops, it is acceptable for me to re-engage it and maybe better to have it rather than having a strong global circuit for quite nothing finally.

Alimentation is a french word. I thought it was transparent. This means feeder (electric feeder, power supply is maybe better).

For the diode, I do not hide that it would be easier for me to control everything only with one transistior and one diode as I do not have much space and I want to keep the whole package as light as possible. If I have to, I will do. But if a solution exists with only one diode, I'll take it. ;)

PerryBebbington

#22
Feb 20, 2020, 12:39 pm Last Edit: Feb 20, 2020, 12:40 pm by PerryBebbington
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For the diode, I do not hide that it would be easier for me to control everything only with one transistor and one diode
Then the transistor and the diode need to be able to comfortably handle the total stall current of both motors, plus a margin of maybe 20% - 25%.

Something I've not pointed put is that you can't put 2 diodes in parallel and expect them to
carry twice the current of 1 diode. Diodes conduct when the applied voltage reaches their Vf. This will vary between different diodes due to manufacturing differences and temperature. Whichever diode has the lowest Vf will carry most or all the current, the other one will get very little. The one carrying the current will warm up, lowering its Vf and thus making sure it gets still more of he current, until it dies.

17Nico17

#23
Feb 20, 2020, 01:03 pm Last Edit: Feb 20, 2020, 01:20 pm by 17Nico17
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Something I've not pointed put is that you can't put 2 diodes in parallel and expect them to
carry twice the current of 1 diode. Diodes conduct when the applied voltage reaches their Vf. This will vary between different diodes due to manufacturing differences and temperature. Whichever diode has the lowest Vf will carry most or all the current, the other one will get very little. The one carrying the current will warm up, lowering its Vf and thus making sure it gets still more of he current, until it dies.
Ok, I understand, it is worse and worse. We call it "Vicious circle" in french.

Ok, so if I find a 40A diode I put between general positive of my 2 motors and drain of the transistor I am ok normally if I take a 15A fusible to protect the whole system? I have 30% margin for my diode and a comparable margin for my switch.

I can begin like that and see in the future. If I have to re-activate the fusible too often, I will make an upgrade then ;)

What do you think about this plan? Just need to find a 40A diode, will be huge no?...  :o 
I just see that I must have a 60A mini because of the fact the fusible would probably take more time to pop even if it is less strong than the diods on the paper.....

Something like that is ok?

[url/]https://fr.rs-online.com/web/p/diodes-de-redressement-et-schottky/7003734/[/url]

PerryBebbington

I think that's reasonable to try.

Do your motors really take 20A stalled? Have you measured it?

17Nico17

It is mentionned on the datasheet the 20A. I did not measured it. What I measured is that my consuption during normal use was around 10W for 2 motors together (motors are not always at 12v).

I have found this kind of diode too. There are so many types i am lost to be honest....  :o

https://www.conrad.fr/p/diode-tvs-littelfuse-15ke39ca-do-201-371-v-15-kw-140802

PerryBebbington

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It is mentioned on the datasheet the 20A. I did not measured it. What I measured is that my consumption during normal use was around 10W for 2 motors together (motors are not always at 12v).
It seems too high to me. I suggest you measure it. You might be trying to solve a problem that doesn't exist.

17Nico17

Ok, sound quite high to me too. Arff I do not personnaly have the tools, I will have to ask a friend. Anyway, maybe this is a good idea to have a strong diode?

Did you see my links? There are many types, cylindrical, square, with different names & prices, I really do not know what is good or wrong.  :smiley-confuse:

PerryBebbington

Find out what the real stall current is then look again.

I looked at some of them, but as I suspect the current we are looking for is too high I think you should wait until you know what you really need.

17Nico17

Ok, I should have this info tomorrow.

What did you find anyway? Mine are not suitable I guess  :smiley-yell:  (they were 60A mini)?

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