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Motor Control

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Don Case

New Member
Hello. I have limited knowlege of electronics, I worked for the telephone company for 37 years but mostly used the waterpipe system to follow circuits. I am hoping to build a motor control for a model boat. The motor will draw 20 to 30 amps. I was hoping to use the control board from a servo and basically remove the existing motor and run the new motor through an H-bridge from the old motor leads. How does the basic idea sound? If ok, what model number Mosfets would work. I saw some 125 amp 24volt Mosfets on E-Bay - would they work or is there overkill in electronics? I will probably be using between 7.2 and 12 volts. An additional question. How do you tell from a Mosfet spec sheet what the operate voltage is? I am concerned that I won't have enough voltage from the servo to operate the Mosfets. Any additional ideas or insights would be very appreciated.
 

Roff

Well-Known Member
Don Case said:
Hello. I have limited knowlege of electronics, I worked for the telephone company for 37 years but mostly used the waterpipe system to follow circuits. I am hoping to build a motor control for a model boat. The motor will draw 20 to 30 amps. I was hoping to use the control board from a servo and basically remove the existing motor and run the new motor through an H-bridge from the old motor leads. How does the basic idea sound? If ok, what model number Mosfets would work. I saw some 125 amp 24volt Mosfets on E-Bay - would they work or is there overkill in electronics? I will probably be using between 7.2 and 12 volts. An additional question. How do you tell from a Mosfet spec sheet what the operate voltage is? I am concerned that I won't have enough voltage from the servo to operate the Mosfets. Any additional ideas or insights would be very appreciated.
Thanks
Don
I take it your servo runs on 7.2 to 12 volts also. Most MOSFETs will work at that voltage range. Does you servo support reverse (I'm not an RC aficionado)? You don't need an H bridge if you don't have reverse but that seems like it would be a necessity for a boat.
125 amp MOSFETs would probably be overkill. You may even have trouble driving 40-50 amp MOSFETs with the servo, if it was meant to control a small motor. Also - do the servo outputs swing rail-to-rail? For example, if you run it off 12 volts, do the outputs swing from 0 to 12 volts? How much current can they supply? Big MOSFETs don't draw DC gate current, but the gate capacitance is high, so your risetimes may be too slow if the servo outputs can't supply high transient current. If you can post some info on the servo, it would be helpful.
 

Don Case

New Member
I take it your servo runs on 7.2 to 12 volts also.

____No, The servo runs on 4.8 volts

Does you servo support reverse (I'm not an RC aficionado)?

____The servo motor turns both ways. There are 3 wires going to the motor in the servo. One goes to the motor case (noise reduction?)

You don't need an H bridge if you don't have reverse but that seems like it would be a necessity for a boat.

____I will need reverse.


125 amp MOSFETs would probably be overkill. You may even have trouble driving 40-50 amp MOSFETs with the servo, if it was meant to control a small motor. Also - do the servo outputs swing rail-to-rail? For example, if you run it off 12 volts, do the outputs swing from 0 to 12 volts?

____The voltage goes from 0 to 4.8 and reverses for the other direction.

How much current can they supply?

____I don't know. If I disconnect one side of the motor and put my meter(on the ammeter scale) in series and then stall the motor will this give me the information you need?

Big MOSFETs don't draw DC gate current, but the gate capacitance is high, so your risetimes may be too slow if the servo outputs can't supply high transient current. If you can post some info on the servo, it would be helpful.[/quote]


____Maybe this will be of some use. I will try to get some current readings on the servo tomorrow. Thank you very much for your help.
 

Don Case

New Member
I did the current test and the output to the motor is about 750 milli-amps with the motor stalled. The transistors that feed the motor are labelled GTE 4952. I searched on the internet and can't find it. One correction the voltage is 6.0 volts not 4.8. I forgot I was using alkalines not nicads. Hope this helps you help me.
 

ivancho

New Member
Let me see if I get this right.... you want to use a servo's control circuitry with the servo's motor removed and instead drive a MOSFET H-bridge to be able to handle 20-30 amps of your new motor Right?.

Servos circuitry work on a "timing" base. There are 3 wires.... 1 is power (I have been able to put upto 7.2V), one is GND, and the other one is a square signal. The location of the servo will be according to the width of that signal. The width of the signal is between 1ms and 2ms being 1.5ms where the servo positions itselfinthe middle.

The bad thing about using that control is that you will not be able to use PWM.... the good thing is that you only use 1 line for controlling forward and reverse motion. You are also going to need to adjust the board and replace the pot in there for a voltage divider to make the board think it is always at the middle, so that is outputs the motor signal constanly.

Just some info....

Good Luck

Ivancho
 

Don Case

New Member
ivancho said:
Let me see if I get this right.... you want to use a servo's control circuitry with the servo's motor removed and instead drive a MOSFET H-bridge to be able to handle 20-30 amps of your new motor Right?.

-----Right, but the new motor will be acting as a drive motor not a servo motor so I don't need the feedback that the pot supplies. If you disconnect the pot from the motor(physically not electrically) the motor will turn continously in one direction or the other at different speeds depending on stick position. I have done this part and it works well. I think all I need now is the smarts to connect this to an H-bridge that will handle the amperage. If you go to this site http://www.cpg1.freeserve.co.uk/servos/servos.htm there is drawings for what I want to do(I think) but I don't think this circuit will handle the load. I think what I need is for someone to tell me what components I need to upgrade this circuit to 20 or 30 amps. My guess would be that the 2N3053's and the BC214's and the 1K diodes would need to be replaced but that is what I need your help with.
 

Don Case

New Member
I'm sorry but I didn't understand much on those sites. It didn't look like they had reverse and the reverse has to be included with the speed because I only have two channels. I don't know what PWM is.
 

pike

Member
just a curious question:

What is a H-Bridge ??? I guessing it uses only diodes like a bridge rectifier. can anyone show me a schematic or configuration of the diodes??
 

ivancho

New Member
Bridge Rectifire and a H-bridge are two different things......

A Bridge rectigier does exactly that.... rectifies.... so if you have a AC wave, the Bridge rectifer makes the negative part of the wave a positive one.

Pretty much is the first step towards making a DC voltage.

And H-Bridge inverts polarity..... mostly used with motors..... and H-bridge is equivalent to change the leads going to a motor and invert them to reverse the motion of the motor. It is called H-bridge because the schematic looks like an H...... with the motor as the middle stick.



Ivancho
 

Don Case

New Member
Would using logic level mosfets make this any easier? Thanks for the help so far. I'm starting to get it straight in my head but it's going to be more difficult than I thought.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Don Case said:
Would using logic level mosfets make this any easier? Thanks for the help so far. I'm starting to get it straight in my head but it's going to be more difficult than I thought.
You're really looking at quite a complicated subject, which has great scope for destroying components.

Am I right in assuming that the purpose of this is to control the drive motor in a model boat?, forwards and backwards from one joystick, with rudder control via a servo from the second joystick.

This is a very common requirement, and I'm sure exactly what you want is available through the model boat trade - probably at less than you could buy the components. Presumably a model car ESC (Electronic Speed Control) would do what you require, they are available in various sizes depending on your motor.
 

TallPaul

New Member
From the limited experience I have in model electric vehicles, the cheapest and quickest way would be to by a standard servo to PWM motor speed controller from a model shop.

The size that you are after shouldn't be more than about £40ish here in the UK so they are probably about $60ish in the US.

It will do exactly what you want, and will have all the adjustments for center (Stop), Min (Full Reverse) and Max (Full Forward), which would be tricky if you were to modify a servo.

The speed controller will plug directly into your radio reciever, battery packs and the motor. So you won't need to worry about additional components either!

All of this is in a packages about the size of a small matchbox, which makes it easier to waterproof (as it is going in a boat!)

Good luck with whichever idea you choose!

Paul.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Don Case said:
Motor controls are about $100 Can. and I thought I could save a buck or two doing it myself.
It's doubtful you could make one for less than you could buy one, the ones I've seen usually have a bank of paralleled MOSFET's - the cost of buying the MOSFET's is usually more than the complete ESC.

Certainly, unless you know really well what you are doing, you are going to end up with a big pile of dead expensive parts!.
 
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