Hi,
New to these forums, but I was wondering if you guys could help me with a project. I'm not the most experienced with electronics, so bear with me, but I have an electric motor that usually would be powered by a 7-8v source, and I wish to use a readily available 12v source to power this motor. A straight 12v to this motor probably wouldn't be a good idea, so I would like to lower the 12v to around 8v. One thing to keep in mind is that this motor will see different loads and current will be varying from 1A up to 40+amps. So what kind of circuit can I use? Are there any inexpensive solutions? I appreciate any suggestions, thanks!

 

Hi,
New to these forums, but I was wondering if you guys could help me with a project. I'm not the most experienced with electronics, so bear with me, but I have an electric motor that usually would be powered by a 7-8v source, and I wish to use a readily available 12v source to power this motor. A straight 12v to this motor probably wouldn't be a good idea, so I would like to lower the 12v to around 8v. One thing to keep in mind is that this motor will see different loads and current will be varying from 1A up to 40+amps. So what kind of circuit can I use? Are there any inexpensive solutions? I appreciate any suggestions, thanks!

An easy solution is to build a variable voltage regulator and boost its current capability by using transistors. A more complicated solution but more efficient is to search for DC-DC converter (switched mode power supply).

 

You could use a PWM circuit for that. For a 12v input, if your PWM circuit's duty cycle was 2/3 on and 1/3 off, the average output would be 8v. If the frequency of the circuit were above audible range (say, above 22khz) you wouldn't hear it.

Is maintaining a constant rotational speed of the motor a consideration?

 

I'd be concerned about getting the PWM current on and off quickly enough with that much current (40A). During the transitions, a lot of heat will be dissipated in the switches. We're talking about a 320W max load here. Even a 10% efficiency loss (which is good) is 32W.

Going linear is even crazier. You are dropping (12-7)*40 = 200W across your power transistors. Hell of a heat-sink needed for that!

 

ok you guys are losing me, you have any schematics? the motor is at a constant rpm but of course a load will affect its rpm. thanks for the suggestions.

 

I'd be concerned about getting the PWM current on and off quickly enough with that much current (40A). During the transitions, a lot of heat will be dissipated in the switches. We're talking about a 320W max load here. Even a 10% efficiency loss (which is good) is 32W.

Going linear is even crazier. You are dropping (12-7)*40 = 200W across your power transistors. Hell of a heat-sink needed for that!

What switches are you talking about? (in PWM)

what is your suggestion?

 

Caveman is right about needing a sharp transition for the PWM current.

A bipolar 555 would likely switch fast enough at 25khz to give a reasonably square (fast rise/fall time) pulse, and being able to source/sink 200mA should switch a power MOSFET on/off pretty quickly.

Fasteddie, tell us a bit more about the motor. Are the electrical connections to it a pair of wires, or is the frame of the motor the ground, and you have a single positive lead going into it?

I'm asking because it does affect what might be designed. I'm thinking an N-channel enhanced power MOSFET would be the easiest to implement and most efficient, but to be really easy, the MOSFET would need to be controlling the ground (low) side of the motor. Something like an IRF1404 through IRF1407 or IRF3703 would easily handle 40A @ 12v.

Power MOSFETS have a very low Rds(ON) (Resistance from drain to source when they are turned on.) There are P-channel power MOSFETS that can be used from the positive (high) side, but they are inherently less efficient. It is possible to use N-ch MOSFETS on the high side, but that requires more complex drive circuitry.

 

the motor has 2 tabs, one for power and one for ground.

 

How did you derive 40 amps, that's quite a bit?

Forget about simply dropping the supply by 4V (12V-8V) at 40 amps (160W heater/toaster), and consider that creating a PWM supply that can supply 8V @ 40A (>300w) is an achievement in itself. Neither approach is for the inexperienced.

Maybe it was a typo and you meant 1A to 4A ?????

 

Hello,

Perhaps you can use parts of the schematic on this dutch site:
http://home.tiscali.nl/homepagevictronic/electronica6.html
OP2 is a triangle generator.
OP1 is a comparator.
The mosfets are to switch enough current.

Greetings,
Bertus

 

What switches are you talking about? (in PWM)

what is your suggestion?

The switches are the FETs that you are using to pulse the current to the motor. I say switches because there are a variety of different types of parts that could be used, not just FETs.

I like the SgtWookie's idea of using an n channel FET due to the low Rds on. If the 555 or whatever you have doesn't turn it on fast enough, you could use a "FET driver". These are parts designed to provide a lot of current to the gate of a FET to turn it on quickly. Also, paralleling the FETs like in that Dutch site is a good idea, but you need to make sure your gate driver can drive all of the current quickly enough.

 

Hi fasteddie911,

You asked for some schematics. Here is something I built several years ago with 5 mosfets paralleled top and bottom. I don't think you can just copy it for your application, but it might help you see one way to do it.

I used a TPIC 2102 to generate the PWM. You will see a lot of extra stuff, which is there for controllable speed and soft start. There were also some interlock devices for safety which I have deleted from this schematic and just called it "on/off." The important part is the LT1158 controller/driver for the mosfets, of which there were 10 in all. The schematic ends with a 7-pin header. I have included a picture of the power board and its design, which was done in a cad program. The other end of the 7-pin connector is shown. I have grown to like the LT1158. There are more wires to connect, but it has feedback sense from both the top and bottom mosfets, and I have never had a problem with it.

I hope this helps to give you some idea of how to do it. The motor was considerably more powerful than the one you are using. Mine was about 1.5 KW at 12V. Each mosfet had its own gate resistor (15 ohm) and the tabs were soldered directly to the PCB for heat conductivity. The high-current cables are bolted to the left side of the power board, and all power is derived from them. That's why there is a small filter circuit in the upper right corner of the schematic so the logic voltages stay clean.

I notice on the Dutch site that the gate drive resistors were 1k, which seems a bit large. I am used to using something more on the order of 10 to 22 ohm.

My apologies for not having better drawings. This was done when I had no idea I would ever be posting it. If anything is illegible because of the file conversions, let me know and I will send something of higher resolution. John

Edit: See corrected schematic, next post. Had to show where the +12V came from correctly.

 

Here is the corrected schematic. John