N Channel MOSFET Low Side Switching Questions - Page 1
Forum section: electronics / projects

author: UPI
author_rank: Regular Contributor
timestamp: April 09, 2016, 08:48:41 pm
content: I am pretty new to MOSFETS and would like some advice on device choices/values for the schematic below.I am adding Q200 to take over for a motor driver circuit when power fails. I will be using Super Capacitors to supply my emergency motor driver for about 500 msec. I have also added Q1 and T1 to break the existing driver circuit so that my Q200 can takeover during as the 24V/5V power is browning out.My main questions are:Will a MOSFET work in this position just like a relay would?Will 35ma @ 5V be enough to drive Q200 directly or should I stick with T1?What value of pullup resistor is required on R3?Any advice is appreciated.

author: UPI
author_rank: Regular Contributor
timestamp: April 10, 2016, 12:17:41 am
content: The intention of this circuit is to reverse a stepper motor 80 steps ASAP when the power is going down to protect other parts of the equipment.Everything on the left side of the stepper is existing circuitry within the machine except for my additions of T1, R3 and Q1. I am also adding Q200 on the right side to takeover motor drive (grounding of each phase). Q1 is there just to break the connection to the existing Darlington drivers. A relay would be much simpler but I hate to stoop so low.The MOSFET I have chosen is an FQD13N10LTM and the logic level would be provided by an ATmega328, but I like the simplicity of the MCP1406. Add a couple of caps, wide traces and a ground pour and hopefully call it a day.My own Q200 Darlington (I copied the OEM circuit since it has worked for decades) will always be in parallel with the machine's Darlington. I will disconnect their Darlington using the Q1 MOSFET and will start toggling my Q200 driver at the same time.I'm not quite sure how to handle snubbing of this when disconnecting. Since my Darlington is already in parallel, I essentially already have in place the same circuit that has always been solid under normal operating/switching conditions. I'm not sure why my Darlington driver circuit wouldn't handle it as usual business.Thanks for your very valuable suggestions.

author: UPI
author_rank: Regular Contributor
timestamp: April 10, 2016, 01:54:33 pm
content: My original prototype was a daughter board attached to the rear of the existing motor driver board that took over the logic of the existing drivers. The surgery was relatively invasive since I had to cut traces to inject my own standby power. This original prototype method would require the end user to send me their board for modification.My new prototype will be a separate board that will basically cut the 20 pin ribbon cable to the stepper motor. I will inject my own power into the two center taps of the stepper motor and will then ground each of the 4 stepper phases with 4 new Darlingtons.In other words, the existing Darlington that the supplies power to the two center taps will continue to be connected out to the motor. I will parallel in a new center tap voltage (super caps) but will break the existing 4 phase grounding lines (with MOSFETS). So, I cut 4 lines in the ribbon cable using MOSFETS and then ground these lines out to the motor in succession using 4 Darlingtons. In total, I will have 4 grounding Darlingtons, 2 power Darlingtons and 4 MOSFETS to break the existing phase grounding lines.I have attached an incomplete schematic showing the ribbon cable and new power/phase Darlingtons. It still needs the MOSFETS in line with pins 6, 8, 10 and 12 to connect these lines during normal machine operation. These MOSFETS will be opened when the power fails and my new driver will takeover.