Circuit design help needed for soft start/stop application - Page 1
Forum section: electronics / beginners

author: DavidDaedalus
author_rank: Newbie
timestamp: February 26, 2019, 03:47:13 am
content: Hello, I'm a beginner here and lifelong tinkerer, and I've recently begun the dream of starting my own business. It's all happened fast, and there have been many bumps along the way, but now I'm up against a problem that I desperately need help solving. Any input is welcome.I make a product that has a normal PWM speed control that operates a 12v DC brushed motor at up to 0.5 amps, and is speed adjusted via built in knob. Direction is controlled via a DPDT switch in the motor leads. The problem is that there is no soft start or stop built into the speed control, and now I've found that my product requires both. Without them, the abrupt start or stop causes big problems, and my product is being justly criticized at this point.I do not have room in the machine itself to change speed controls (it was designed around this one to the mm), and very little room to add much circuitry but it's conceivable. The solution I've been pursuing is by designing a foot pedal in the 12v power supply leads that when pressed, will perform the soft stop and again will soft start (approximately 4 seconds of ramp time). So in my mind, the foot pedal needs to slowly reduce or increase the input power from 0v-12v after pedal press. I did some research on "soft starting dc motors" and found a couple of diagrams that showed a mosfet being used in conjunction with a small 1uf capacitor and pair of resisitors. If this is applicable, it would be perfect to be small enough to fit into my foot pedal. However, I have no idea of the values needed, and I have my doubts that a slowly charging and draining capacitor would affect the output of a mosfet (because it's a switch and can only reduce output effectively by being switched via a pwm signal). And this would be cascading transformers since a similar circuit is in the internal speed control.I also came across a reference to a 555 based sensing circuit solution but want to keep it as simple and small as possible. I wish I could express how important this is to me, having just made the leap into self employment and trying to get my business off the ground. If anyone can help guide me to a solution, I'd appreciate it.Thank you,David

author: jazper
author_rank: Regular Contributor
timestamp: February 27, 2019, 12:03:14 pm
content: Hi David,Congratulations on starting your own business!If you google soft start Arduino DC motor, you will find there are lots of designs and ways of doing it. You could potentially use a bare board Arduino to make it small and easy to construct.What you're asking for is certainty a simple design and a small design.This is the basic idea/layouthttps://goo.gl/images/aUufPjYou would make sure the pin is a pwm pin...the other tip is stick with an atmega328p if this is your first microcontroller, so you can implement easily. You also ideally want a logic level MOSFET.Another tip is Arduino nanos are cheap and small..

author: DavidDaedalus
author_rank: Newbie
timestamp: February 27, 2019, 06:55:09 pm
content: Jazper,Thanks for your response, and that's the logical direction (microprocessor) for the project to go. I was hoping there was a simpler solution with jellybean components, like a 555 being signaled by a slow charging and draining capacitor, and the 555 triggering a mosfet.However, I ordered a nano v3 (I have some C programming exp and very intro arduino exp so far) and some P channel T-220 mosfets to play with. In this arrangement, tuning the ramp speed can be easily achieved with a knob, where the jelly bean concept requires very specific sizing of values through trial and error.Now I have to learn about the diff in P and N channel mosfets, and if I can use just it on a sink (light 7w load) or if I need to use a glorified version of it as a "brushed motor speed control" that takes pwm input (either arduino-centric ones or RC hobby grade ESC). Any advice?Thanks again, really need the guidance.David

author: jazper
author_rank: Regular Contributor
timestamp: February 28, 2019, 01:02:05 am
content: OkSo N channel mosfets are similar to NPN transistors, P channel mosfets are similar to PNP transistorsN channel mosfets switch the negative rail, P channel mosfets switch the positive rail - When voltage is applied to the gate that is.Mosfets have a Nonlinear region and a linear region. When you drive the gate voltage through the nonlinear region, they act as a current limiting device (resistor), once you hit the linear region they pass nearly all the current.When they act as a resistor, they heat up (there are many formulas for this)Typical mosfets that people use with Arduinos are IRF530N and IRF540Ns (both of these being N channel) because their linear region starts at or around 5V where most mosfet linear regions start about 10v.The IRL540N from Vishay is a good one for logic level switching tooLogic level mosfets linear region start at logic level (2.5/3.3/5v depending on the FET). The linear region is typically noted as the Vgs voltage on datasheets.Unlike BJT transistors, in mosfets as they heat up the resistance increases. The Fet will typically only heat up if it is in its non linear region or is passing a lot of current - this doesn't mean you shouldn't put a heatsink on it, remember that you're going through the linear region each time you pulse it as the voltage has to increase over time each time.Using PWM to drive a mosfet into it's linear region you will care about gate capacitance - as higher gate capacitance means slower switching speed (it's like each time you're turning it on, you're charging a cap). This obviously isn't a huge issue if you have plenty of current, but just something to be mindful of.You can do what you want with a 555, but it'll take more effort for little savings in actual costs.