Servo not moving

Thread Starter

LowTech

Joined Feb 9, 2020
7
Hi

I would like to know how to check a 8 pin Mosfet with a multimeter i'm trying to fix my 9gram servo it doesn't rotate when power is applied to it i check the motor i have continuity.
 

jpanhalt

Joined Jan 18, 2008
8,781
Hi

I would like to know how to check a 8 pin Mosfet with a multimeter i'm trying to fix my 9gram servo it doesn't rotate when power is applied to it i check the motor i have continuity.
Here is how to test a mosfet: https://forum.digikey.com/t/how-to-test-a-mosfet/1985?utm_adgroup=General&utm_source=google&utm_medium=cpc&utm_campaign=Dynamic Search_RLSA_Cart&utm_term=&utm_content=General&gclid=EAIaIQobChMImImSyKrF5wIViLzACh0arwWHEAAYASAAEgIXdfD_BwE

Note, it refers only to 3 pins. Many small mosfets (including yours) will have multiple pins connected together of the Drain, multiople pins connected together for the Source, and one for the Gate. An example:

1581281383219.png

Since you have 8 pins, one MIGHT assume 4 on one side were the drain and 3 on the other were source. You only need to test one pin for each connection, e.g., 1 pin for drain. To be safe, you need the part number for your mosfet to determine its true pinout.
 
Last edited:

Thread Starter

LowTech

Joined Feb 9, 2020
7
Can't do much with just vague details!
Mosfet part?
Servo?
How is power applied?
RC servo?
Schematic?
Etc etc?
Max.
Its a TowerPro servo, the number on the Mosfet is 117L912 and by connecting a lipo battery is how the power is applied,
i do not have a schematic and there is another 8 pin Mosfet on the other side of the board.

 

Thread Starter

LowTech

Joined Feb 9, 2020
7
Here is how to test a mosfet: https://forum.digikey.com/t/how-to-test-a-mosfet/1985?utm_adgroup=General&utm_source=google&utm_medium=cpc&utm_campaign=Dynamic Search_RLSA_Cart&utm_term=&utm_content=General&gclid=EAIaIQobChMImImSyKrF5wIViLzACh0arwWHEAAYASAAEgIXdfD_BwE

Note, it refers only to 3 pins. Many small mosfets (including yours) will have multiple pins connected together of the Drain, multiople pins connected together for the Source, and one for the Gate. An example:

View attachment 198710

Since you have 8 pins, one MIGHT assume 4 on one side were the drain and 3 on the other were source. You only need to test one pin for each connection, e.g., 1 pin for drain. To be safe, you need the part number for your mosfet to determine its true pinout.
Thanks here is the picture for my servo...

 

Thread Starter

LowTech

Joined Feb 9, 2020
7
Here is how to test a mosfet: https://forum.digikey.com/t/how-to-test-a-mosfet/1985?utm_adgroup=General&utm_source=google&utm_medium=cpc&utm_campaign=Dynamic Search_RLSA_Cart&utm_term=&utm_content=General&gclid=EAIaIQobChMImImSyKrF5wIViLzACh0arwWHEAAYASAAEgIXdfD_BwE

Note, it refers only to 3 pins. Many small mosfets (including yours) will have multiple pins connected together of the Drain, multiople pins connected together for the Source, and one for the Gate. An example:

View attachment 198710

Since you have 8 pins, one MIGHT assume 4 on one side were the drain and 3 on the other were source. You only need to test one pin for each connection, e.g., 1 pin for drain. To be safe, you need the part number for your mosfet to determine its true pinout.
On the source side on the second pin after the dot i got 0.499 with the multimeter on diode mode, on the third pin i got 3.309 on the Gate pin i got 3.496
 

jpanhalt

Joined Jan 18, 2008
8,781
Gate to any other pin should be open circuit. (You may get a reading while the capacitance of the gate charges up, but after that , it is OC.) If you are sure that is the gate, you have blown the mosfet.

Have you checked for continuity between the 4 pins on the other side and pins 2 to 4 on the same side? I am assuming it is a single mosfet as I have seen in other servos. Sometimes, however, mosfets come two to a package.

Eventually you will need more information to replace it exactly; although, a good guess may be OK.
 

Thread Starter

LowTech

Joined Feb 9, 2020
7
Thanks i found out it's a N Channel Mosfet i got a reading of 538 while my Negative Probe was on pin 6 & 7 my Positive Probe was on 2 & 3 then i switch my Multi meter to beep mode and there was no beep, just silence.
 

djsfantasi

Joined Apr 11, 2010
6,196
Some questions you didn’t answer: is it a RC Servo (Tower Pro makes RC servos)? Where’s your schematic? You do realize that an RC Servo is NOT run by DC only? You need an RC Controller to provide a 750ms to 2250ms PWM signal at 50mHz. What are you using for a controller???
 

Thread Starter

LowTech

Joined Feb 9, 2020
7
Yes of course it's an RC servo and i reiterate i do not have a schematic if you have a schematic why don't you post the schematic and facilitate but i don't need a Transmitter to make it move i use a servo tester and when i use a servo tester it doesn't move, so therefore it's a bad servo. I'm not a electronics technician but i'm trying to fix it, there is a SMD Transistor on the board next to the motor and what i know is all Transistors have 3 legs some people call them Mosfet their not Mosfet, Mosfets
has 6 legs, 8 legs but you also have some dual Transistors with 6 legs and here is where it doesn't make sense to me.

I check the voltage between Base and emitter and i got 0.032
 

jpanhalt

Joined Jan 18, 2008
8,781
... i use a servo tester and when i use a servo tester it doesn't move, so therefore it's a bad servo.
Not necessarily. The servo tester could be bad. Did you run a control?

I'm not a electronics technician but i'm trying to fix it, there is a SMD Transistor on the board next to the motor and what i know is all Transistors have 3 legs some people call them Mosfet their not Mosfet, Mosfets
has 6 legs, 8 legs but you also have some dual Transistors with 6 legs and here is where it doesn't make sense to me.
Did you bother reading posts #3 and #7? It was YOU in your first post that called it a mosfet. I simply pointed out they can and often do have more the 3 pins.

You want a schematic, here's one:
1581815008097.png

Now, if you will get off your high horse and give some useful information about what chip(s) are in the servo, maybe we can help. There are all sort of servos and there have been huge changes over the years from components to almost single chip devices. Until you test a known good servo, maybe your servo tester is bad. There is also the possibility that you messed up the pinout and killed the servo.[/QUOTE]
 

djsfantasi

Joined Apr 11, 2010
6,196
Even further than identifying that it’s a servo, is it an analog RC Servo or a digital RC servo? The latter won’t work in an analog RC servo tester.

You seem to have no clue what you’re working with... and get upset when questions are asked. You may have a clue; but it doesn’t appear so.
 

Thread Starter

LowTech

Joined Feb 9, 2020
7
Thanks i found the problem to the servo after doing my test with my multi meter on 4 of the 8 pins the only ones that suppose to have continuity are pins 6 and 7 this is the reason why the servo is not moving it's because i am getting
continuity on pin 5,7 and 8 on the 8 pin motor controller

And also there is a small tiny capacitor at the end of the board next to the motor and i was getting continuity on both side of it which is not suppose to happened but as soon as i remove the 8 pin motor controller the continuity stop. I would like to replace the 8 pin motor controller with one that has a higher voltage output like 9 volts.
 

jpanhalt

Joined Jan 18, 2008
8,781
In circuit testing of components can be very misleading. I would not trust those results without knowing more about the circuit.

But apparently, you now know the answer to your problem. Hope you can get it fixed.
 
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