Opto-isolators are slow to switch on and off. Darlington pairs are slow to switch on, and even slower to switch off. This results in a system thats slow to switch off, so by extension right as the inductor is kicking a huge current spike down to it, the opto-isolator is least equipped to handle it, being somewhere in the active region with high resistance. This also makes them unsuitable for driving the gate of a mosfet by the way, which is a capacitive load.
In cases 2 and 3, you just drive an N mosfet directly. Heck, in all cases you just drive an NFET. I dont understand why anyone uses an SSR these days, when 200 amp mosfets can be had for <3$ each. Mosfets are cheaper, faster, and can handle more current safely. For your circuit to work, there must be a common ground. Ultimately the ground on your micro-controller and your load meet up no matter what you do. If you use an N mosfet, then the mosfet is on the low side of the load. I dont understand your reasoning for why you dont just drive the load off a mosfet. On Fri, Oct 10, 2014 at 7:44 AM, Joe Sommer <[email protected]> wrote: > > On Thursday, October 9, 2014 9:23:34 AM UTC-4, Frank Pittelli wrote: >> >> Agreed. I would never drive an inductive load directly with an >> opto-isolator. Fortunately, there are plenty of cheap FETs with >> built-in protection now that can be used. >> > > I disagree. I always use an opto-isolator between a microprocessor > that reads RC servo signals and an inductive load. The opto-isolator > protects the microprocessor from nasty inductive spikes. > > 1) small inductive loads (<150 mA steady-state current) > Drive the LED side of the opto-isolator with the microprocessor. > The Darlington side can safely drive a small inductive load directly. > > 2) medium inductive load (<2 A steady-state current) > Drive the LED side of the opto-isolator with the microprocessor. > Use the Darlington side to drive a BJT or MOSFET for the load. > > 3) large inductive loads (<40 A steady-state current) > Drive the LED side of a solid-state relay (SSR = big ass opto-isolator) > with the microprocessor. > The SSR output is a very large MOSFET with heat sinks that can drive the > load. > > Driving an FET directly from the microprocessor does not > protect it from large inductive spikes because there must > be a common connection between the microprocessor GND, > the FET source pin and low-side power for the load. > > Joe > > > -- > -- > You are currently subscribed to the "R/C Tank Combat" group. > To post a message, send email to [email protected] > To unsubscribe, send email to [email protected] > Visit the group at http://groups.google.com/group/rctankcombat > > --- > You received this message because you are subscribed to the Google Groups > "R/C Tank Combat" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > For more options, visit https://groups.google.com/d/optout. > -- Isaac Goldman 5142334423 This message contains confidential information and is intended only for the individual named. If you are not the named addressee you should not disseminate, distribute or copy this e-mail. -- -- You are currently subscribed to the "R/C Tank Combat" group. To post a message, send email to [email protected] To unsubscribe, send email to [email protected] Visit the group at http://groups.google.com/group/rctankcombat --- You received this message because you are subscribed to the Google Groups "R/C Tank Combat" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
