Surge protection & Fast Reaction Time - Page 1
Forum section: electronics / beginners

author: AmazingTrans
author_rank: Newbie
timestamp: September 05, 2014, 01:44:00 pm
content: Hi there,I always have been wondering how do we size these diodes & zener diodes to have the fastest reaction and also protect the output of IC Circuits.I have a schematic here with two options.Option 1 is when I have a PLC(Programmable Logic Controller) output that closed the circuit to power the 24VDC solenoid. The output of the PLC uses a opto-isolator output.The PLC can output 24 - 30 VDC.When I pulsed the output, the transient voltage might go as high as 4-6 time the voltage.Should I size my diode to 1N4001 (50v) OR 1N4002 (100v) ?Should I size the zener diode to IN4749 (24VDC) OR IN4746 (30VDC)?I would want the fastest cutoff time as well.Option 2 is similiar, but I have a contactor in between the output and solenoid.Do I have to put to put a surge protection on the solenoid if it is on a Contactor contact for faster reaction time? How should I size it as well?I am thinking of option 3, where I maybe should use a N channel MOSFET to power the solenoid?Hope someone can enlighten me through.

author: Simon
author_rank: Unknown
timestamp: September 05, 2014, 02:11:16 pm
content: What are you trying to protect from ?By the looks of it you are talking back e.m.f. in which case you just need a diode that has a higher breakdown voltage than the supply which in your case is no problem as 100+V diodes are the norm and you only have 24V, the diode needs to be able to carry at least as much current as the load and needs putting in parallel with the coil but in reverse so that it normally does not conduct. and that is about it most of the time.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: September 05, 2014, 02:30:26 pm
content: The device you need to protect is the opto isolator.If its max Vce voltage is 30volts you may have to get rid of the zener in circuit A.The zener is only there to speed up the de-energising of the coil.Or if you move to circuit B you can probably keep both the zeners in.I would say it would be easier to bread board this circuit than to mathematically evaluate the speed vs over voltage tradeoff.

author: mij59
author_rank: Frequent Contributor
timestamp: September 05, 2014, 02:33:30 pm
content: Option a works, option b not there is nothing to power the solenoid.Why do you need the fast response time?You don't need the zener diode, it only increases the voltage across the opto coupler when it switches off.The 1N4001 would do , diodes can withstand very high currents for a short time.

author: Simon
author_rank: Unknown
timestamp: September 05, 2014, 02:35:19 pm
content: Why does this need to operate "fast" give us times. I was recently working with a brushless motor driver that takes a 50Hz PCM signal, 1ms pulse means not to run and from 1-2ms means run 0-100% power.Because of noise issues between the driver and my system control board that was supplying the PCM signal to tell the motor driver what to do I had to add an opto coupler, all of a sudden I had the motor running at low speed when it was supposed to be stoped because the opto coupler was adding a delay that was making my slightly less than 1ms pulse just over 1ms.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: September 05, 2014, 03:06:49 pm
content: Oops didn't even see the lack of power in circuit B.Also it might be better to use common emitter to drive the relay rather than emitter follower.

author: AmazingTrans
author_rank: Newbie
timestamp: September 05, 2014, 03:08:16 pm
content: Thank you for everybody quick response.Simon: Yes, I am preventing back e.m.f and also wants a fast response.mj95: I need the fast response time because this solenoid coil powers a machinery that requires instant braking, and releasing brakes. So the user might pulse it continuously to move the machine in small increments.What if i were to used a 24VDC or lower Zener Voltage ?I also read sometimes in automotive, they use TVS Diode or MOV for faster response time and reduce emf ? Would that be an option for me?If yes How would I size them?Thanks!

author: Simon
author_rank: Unknown
timestamp: September 05, 2014, 03:14:24 pm
content: For back emf you just need a parallel reversed diode, I'm not sure how you turn opto isolators off quickly other than short their input with a low side switch or basically drive it with a half bridge.Again, what speed in realistic measurements is fast in your opinion ? it's all relative, in the human world seconds and milliseconds are fast, in electronic terms nano seconds and pico seconds are fast.I used to work on printing machinery that was 20+ years old that could stop pretty damn quick despite the sheer weight of metal going round. and yes we could pulse the machine back and forth in small enough steps to setup for thin paper, nothing new in your application I think.We need proper perspective here on what fast actually is.

author: AmazingTrans
author_rank: Newbie
timestamp: September 05, 2014, 03:34:06 pm
content: Simon,Our mechanical valve shift time is about 100ms and we would like the mechanical system to stop instantaneous if possible to maintain proper position but unfortunate we can't because it is a mechanical brake and not a servo system.I guess right after when the opto. turned off, what can i do to dissipate the power from the solenoid coil quickly ?

author: Simon
author_rank: Unknown
timestamp: September 05, 2014, 03:51:35 pm
content: so is a couple of ms in 100ms important ?Again, instantaneous in engineering terms does not exist unless it makes a very very loud bang caused by a small amount of energy. instantaneous is a term used to describe human perception.Can you explain how these valves work, what makes them start and stop ? just the power in a coil/solenoid ?

author: mij59
author_rank: Frequent Contributor
timestamp: September 05, 2014, 05:10:16 pm
content: Quote from: AmazingTrans on September 05, 2014, 03:08:16 pmYou could try option 3, use e.g. a 100V N-channel mosfet to control the solenoid, connect the gate via a 1k/1k voltage divider to the opto coupler.The voltage across drain and source of the mosfet needs to be less than 100V, to be on the safe side make it 80V.The max voltage of the zener will be (80-24-0.7)V, you could start with a zener of 33V, 2W.The time constant of a L-R system is L/R, thus a high value of R gives a small time constant, but also gives a high emf.

author: AmazingTrans
author_rank: Newbie
timestamp: September 06, 2014, 03:44:06 pm
content: Thanks guys, that's what i'm going to try with a MOSFET. Cheerio.