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Variable speed control on a 12v DC motor that draws 40 amps

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Blank Stare

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12v variable speed motor

Hi there.

I am almost a complete newb to electronics, and I hope you will be patient with me, if I ask dumb questions, or offer irrelevant information...

I am building a control box for a salt spreader. The old controller is gone, and I don't want a new factory one, I want one that I can eventually take out of the project box, and mount in the dashboard.

Here are my parameters:

12 VDC up to 40 amp draw when starting the motor.
on/off switch
variable speed dial
variable speed over-ride, for a full speed surge.
50 amp relays (one is controlled by the main power switch, the other is controlled by the surge by-pass switch.
push button breakers, 10amp on the switch side, and 40 on the motor side.
Must be very simple, and cost under 100.00 US
project box is a double-gang electrical box. I will get dimensions when I can find my ruler - LOL!!!

In a nutshell, I have everything designed, and ready to install, when I found out that a rheostat would get too hot. It was suggested that I use a PWM MOSFET, and I think I understand the MOSFET is nothing more than a fancy power relay, but I do not understand how a PWM fits into the schematic?
 

dc variable speed controller

Think about PWM the same way as of average (power): 10% duty cycle =>10% power, 50% duty cycle => 50% power, and so on ..
Take a look at this example:
**broken link removed**
In your case, I would use 2 or 3 power MOSFETS connected in parallel (with a small gate resistor for each one, 10-100Ω) ..
This is for sinking current to 0V ..
If you prefer to source current from +12V rail, use P-channel MOSFETs ..

Rgds,
IanP
 

12v speed control

OK, here is where I get to show my ignorance...

Could you help me to understand what 2 or 3 MOSFETs look like in a circuit schematic?

Also, do I inderstand that PWM is not a piece of circuitry, but rather, a concept?

Please forgive me, I feel like I am swimming in the deep end, here...
 

12v dc motor controller

look at the last schematic on the link given by IanP
Q1 is the N mosfet
you need 2 or 3 in parallel with a resistor on the G pin
 

variable dc power 12vdc

Kripton - G pin = Ground pin, number 1 on the chip? Feeling dumb here, as I don't see a letter G on the entire schematic

**broken link removed**

Externet - that box is rated for 24VDC, is it likely to work as expected on only 12VDC? The price with shipping is friendly, but I am not experienced enough to know what happens when you halve the voltage input...

Thank you everyone for being patient with me, I'm an old dog, and I am slow to learn new tricks.

~~~~~~~~~~~~~
Blank Stare
 

schematic for scooter speed control

Yes, that controller is rated up to 24 volt, should be able to work with 12V.
 

12v dc motor basic circuit

G pin is the pin 1 yes, but G means gate, not ground ...
 

12v electric motor variable speed

Wow, I just took a second look at that unit. 11 wires? Ummm...WTH?

Ultimately, I need 3 wires into my project box...one ground, one 12vdc from the battery, and one after the mosfet, to control the motor speed. What in tarnation do they do with the other 8?

Lemme guess...

:idea:

a pair to the pot, a pair to power, a pair to the motor...lessee...that's 5 left over...

a pair for the toaster, and a pair for the radio, and one for an extra ground? LOL!!!

I really like the price, it beats building one hands down. I am gonna have to chew on them extra wires for a bit, though...

I'd also have to step down the voltage for the switch sides of the relays...this one only handles 1-4vdc...minor issue.

As always, thanks again for your patient assistance.

Now, if I build this from scratch, how does my new schematic look? (I blatantly copied the circuitry discussed in earlier posts - I hope that doesn't hurt anyone's feelings?)
 

12v 50 amp motor controller

Kripton2035 said:
G pin is the pin 1 yes, but G means gate, not ground ...

heheh - thanks
 

12v+dc+motor+speed+control

Just a little FYI.
Most controllers that will automatically shut down if the voltage falls below a certain level. This is to keep from destroying gell cells and AGM batteries. For instance, a 24 volt controller will probably shut off when the voltage falls below 21 volts. The truth is that repeatedly discharging a set of batteries to even 21 volts will kill them in short order.
 

12v variable speed switch

deloiter said:
Just a little FYI.
Most controllers that will automatically shut down if the voltage falls below a certain level. This is to keep from destroying gell cells and AGM batteries. For instance, a 24 volt controller will probably shut off when the voltage falls below 21 volts. The truth is that repeatedly discharging a set of batteries to even 21 volts will kill them in short order.

Thanks. Something like that was why I asked if it would work. I was afraid that there might be a wrench in the works somewhere.

I have priced out all the parts that I need for the PWN/MOSFET circuit, for about 75 dollars, and barring any brilliant ideas, (Mine or from others...) I plan to order the parts, and attempt to build this tiny goliath.

So, if there are any great words of wisdom, encouragement, or suggestion, now would be a dandy time to hear them. :D
 

12v electric speed controller

Too many wires ? It was not built for your project but for an electric scooter, with the potentiometer using 3 wires, and others for leds to show the remaining charge amount on the twist grip.
 

control motor 12vdc

Externet said:
Too many wires ? It was not built for your project but for an electric scooter, with the potentiometer using 3 wires, and others for leds to show the remaining charge amount on the twist grip.

LOL!!! That will teach me to cra ck wise on a message board! (edit: cra.ck is filtered? Wisecr ack?...wow, I had no idea that was a bad word...moderator, if I have broken a rule, by modifying the word to fit here, please feel free to change it back to "fix" - thanks.)

Sorry, I wasn't trying to be a PITA - I was trying to make a joke, at the expense of my own ignorance. Thanks for explaining what the extra wires are for. I think I am determined to do this from a handful of parts, rather than gamble that the unit might work at lower voltage. One more day of chewing on my design, to make sure I am satisfied, I think.

Any suggestions?
 

variable dc motor

OK, parts are ordered. wish me lucks, and thanks for all your helpful discussions.

:D
 

Re: Variable speed control on a 12v DC motor that draws 40 a

you ever get this thing built my module on my poly caster just shit the bed and i dont wanna pay $400 for a new one
 

Re: Variable speed control on a 12v DC motor that draws 40 a

Yep, works, too. Good luck with yours.
 

Re: Variable speed control on a 12v DC motor that draws 40 a

i assume you also had a poly-caster. why fisher used such cheap parts in the module is beyond me seems like everyone i talk to has had to replace it 2 or 3 times. anyway mind providing me with links to where you got your relays
 

Re: Variable speed control on a 12v DC motor that draws 40 a

I have no idea what a poly caster is.

As it says in my initial post - I'm a complete newb, but with the help I received at this forum, I did build a working controller.

I bought EVERYTHING at mouser.com, except the project box, which I found on eBay, and the wiring harnes, which I made from heavy duty extension cords.

I don't have links for individual parts, because I did the transaction by phone, and they walked me through each and every component, helping me to select the right ones.

Good luck with your project!
 

Re: Variable speed control on a 12v DC motor that draws 40 a

Blank Stare said:
I have no idea what a poly caster is.

As it says in my initial post - I'm a complete newb, but with the help I received at this forum, I did build a working controller.

I bought EVERYTHING at mouser.com, except the project box, which I found on eBay, and the wiring harnes, which I made from heavy duty extension cords.

I don't have links for individual parts, because I did the transaction by phone, and they walked me through each and every component, helping me to select the right ones.

Good luck with your project!

Hi,

I would like to build a PWM controller for a drill im making and wonder if it would be too much to ask if you sent me a copy of the schematics you came up with when you built yours.:D

Regards
 

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