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Need Help with simple 9V circuit design.

RobH2

Aug 6, 2020
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@hevans1944 , I appreciate the input. Power or battery life is not an issue. I'm driving this with a regulated power supply...a real one... not a wall wart. My friend is my client and the project is a "proof of concept" for a mechanical mechanism that a shaft on the motor actuates. My client is trying to develop and idea for a product on a shoestring budget and is why we can't just hire an electrical engineer from the start. The important part of the project is not the electronic circuit, it's what the mechanical part (confidential at this point) does. I just need to turn the motor on and be able to vary it's speed. I could do that with the handle of a pencil sharpener...lol... but a nice little motor circuit is better obviously.

I'm a Product Designer but not an electrical engineer. So, I frequently wear a lot of hats and learn some cool stuff along the way, like I'm doing here. I realize my little project here is not brain surgery for you guys who know electronics well. That's why I figured with my little bit of knowledge and your help I could figure this out, and with your help I pretty much have done so.

I do have a cheap PWM arriving today. I will say, that the pot I currently have does not get hot. But at the low end, there is absolutely no torque. I'm thinking the PWM will help that from what I've read per you guys suggestions.

When this project is all over I "will" need someone to design the final circuit for me for a real product slated for manufacturing. If any of you would like to let me know you are available for hire, please PM me with contact info. The requirements will be to drive a DC motor with potentially something between 3V and 9V, have a touch pad that has on/off and touch sensitive spots for 4 various speeds. The switch/touchpad I think will be a custom circuit and board design and will be the hardest part. We can't find any online so we'll need to design it and get the overlay graphic screen printed to go over it.

Thanks again for all the help. I don't know if I'll need any more on this but will report back if I do. :)
 

ratstar

Aug 20, 2018
485
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Are MOSFETS an all or nothing event, I thought they were the same as bipolar junction, that they can accept an analogue input. I dont know tho, i'm not very experienced with transistors. Only ever got one to work once when I was amping up a photo-resistor into a motor, but it was actually working.

Im learning other things at the moment.

P.S. hats are cool =)
 
Last edited:

bertus

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Nov 8, 2019
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Hello,

@ratstar , Mosfets do have a "linear" part in the transition curve.
During the "linear mode" there will be power dissipation in the mosfet.
That is why you want to do fast switching, so the time in "linear mode" is as short as possible to keep the dissipation low.

Bertus
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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MOSFETs do have a linear region, but when used as current switches in an on/off application such a this thread describes, you don't want to stay in the linear region very long, especially if switching large currents that could cause overheating of the device.

MOSFETs can be constructed to have excellent high-frequency response over a linear range of output currents, which is useful for designing linear radio frequency (RF) power amplifiers. These have become quite popular in amateur radio applications because of their lower cost, compared to vacuum tube circuits offering the same power output.

MOSFETs are also often operated as fast on/off switches in Class D audio power amplifiers, switching at dozens or even hundreds of kilohertz to effectively pulse-width-modulate the audio input signal. A simple LC filter removes the high-frequency components, leaving just audio frequencies to drive loudspeakers directly. This application is popular in automotive entertainment electronics, again because it is cheaper than most other alternatives.

I would suggest you purchase a variety of MOSFET transistors to "play" with and to gain some familiarity with their characteristics. Bear in mind that most MOSFETs are sensitive to static, because their gate electrode is just a thin conductive layer deposited on a very thin insulating substrate located between the MOSFET source and the drain connections. It doesn't take much voltage applied between the gate and either the source or the drain to "punch through" that thin insulating substrate and ruin the MOSFET.

Wearing a high-resistance, a megohm is typical (to prevent your own electrocution) grounded wristband and working on a conductive, grounded, workbench mat may seem like an excessive precaution, but is certainly does no harm. Until the MOSFET is safely soldered in place in the circuit it is intended to serve, it is susceptible to damage from static electric charges. You will note that many circuits have a moderately high resistance connected between the MOSFET gate and drain terminals. This resistor serves to "drain off" any charge that may accumulate on the insulated gate electrode, thereby protecting the MOSFET even if no power is applied.
 

Harald Kapp

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Nov 17, 2011
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MOSFETs do have a linear region, but when used as current switches in an on/off application such a this thread describes, you don't want to stay in the linear region very long, especially if switching large currents that could cause overheating of the device.
I'll add to that: MOSFETs for switching applications are constructed differently from MOSFETs for linear applications. Switching MOSFETs may be used in the linear region only under restricted and well controlled conditions, see this application note.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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@Harald Kapp, thanks for the link. Although most folks don't get into trouble operating outside the SOA (Safe Operating Area) when using power MOSFETs as simple on/off switches, biasing and operating (or transitioning through) the linear region is fraught with perils. This applies especially to pushing the limits with PWM, where duty cycle and turn-on and turn-off times play an important role in die heating. Heat generation is always instantaneous (electrically) but heat dissipation (thermally) to the environment is a complex thing with many factors affecting the final die temperature. Experience is the best teacher of what an engineer can "get away with" but we never want to design that way. Best to do the research and calculations to know your circuit will always stay within the SOA, and then conservatively specify those boundary conditions.

For example, I for one would not be comfortable designing a 100 W, 1.8 MHz to 29.7 MHz, linear RF amplifier using MOSFETs. No experience in that area. But fortunately hundreds of hams before me DO have the requisite experience, so if I need to build one I can follow one of their examples. Or purchase a factory-assembled KXPA-100 linear power amplifier from Elecraft, which is what I did do.
 
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