Post by thread:[EE]: Another relay question

I probably have this figured out right, but just checking There are two relays to control a motor. An SPST for power and a DPDT for direction. I've found the relays I want, based partly on physical size. The SPST is 8A continuous and the DPDT is 2A continuous but are close in size and low profile (important). The motor draws 0.4A when running. If the direction (2A) relay is closed before the power (8A) relay, then the power relay will take the brunt of the start-up surge ?? -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

At 12:44 PM 10/30/02 +1300, Jinx wrote: >There are two relays to control a motor. An SPST for power >and a DPDT for direction. If the direction >(2A) relay is closed before the power (8A) relay, then the power >relay will take the brunt of the start-up surge ?? Yes. dwayne -- Dwayne Reid <spam_OUTdwaynerTakeThisOuTplanet.eon.net> Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 18 years of Engineering Innovation (1984 - 2002) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

So long as the smaller relay can handle the full load current and the larger relay can handle the start and stop surges, you should have NO problems. Watch out for contact arcing on the larger relay, though. We used to build controls for variable pitch fan blades used on large earth-moving equipment using almost exactly that technique. While it worked fine, we wound up going to a somewhat more complex arrangement with 2 relays switching the +28V supply and 2 sense FETs switching the ground side in order to handle specific customer requirements. But as far as I know, there are still a bunch of the relay only versions still out there working - we occasionally get units in for repair (mechanic: "I tried to clean all the gunk off the box with the steam cleaner and I guess it got wet inside. Dunno how that happened.") dwayne At 03:20 PM 10/30/02 +1300, Jinx wrote: {Quote hidden}> > >(2A) relay is closed before the power (8A) relay, then the power > > >relay will take the brunt of the start-up surge ?? > > > > Yes. > > > > dwayne > >Cheers. Wating for the "but......." &or other shoe to drop.......... ;-) > >-- >http://www.piclist.com hint: PICList Posts must start with ONE topic: >[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads -- Dwayne Reid <.....dwaynerKILLspam@spam@planet.eon.net> Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 18 years of Engineering Innovation (1984 - 2002) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> > >(2A) relay is closed before the power (8A) relay, then the > > > power relay will take the brunt of the start-up surge ?? > > > > > > Yes. > > Agree Thanks Dwayne and Russell - and 1/2 point to me I did purposefully choose an over-specced relay for the power as it will do all of the active switching. By luck it just turns out that the SPST matches the package size of the DPDT, which keeps the PCB tidy Is there any mileage in trying to use something like an LM317 (but bigger) to limit the initial motor current, or is it normally unnecessary or not worth the effort if the power relay is over- specced by a wide margin anyway ? This is for a 12VDC motor running on a battery, and will run satisfactorily down to ~9VDC so perhaps a bypass transistor as current limiter ? The person who I'm doing this for has had experience with failing semis (FET H-bridge) in the previous version made by someone else, the main reason I chose to use relays instead -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Jinx wrote: > > > > >(2A) relay is closed before the power (8A) relay, then the > I did purposefully choose an over-specced relay for the power > as it will do all of the active switching. > Is there any mileage in trying to use something like an LM317 > (but bigger) to limit the initial motor current, or is it normally > unnecessary or not worth the effort if the power relay is over- > specced by a wide margin anyway ? This is for a 12VDC motor > running on a battery, Have you measured the stall (or start) current of the motor?? That is where you will get the potential problems and maybe with accel/decel current, ie it might be wise to actually monitor motor current. If energy efficiency is not critical you could use a 5% or 2% drop resistor, same principle as the light bulb resistor it wont do much when the motor is running but will make a huge difference at startup and in the case of a stall. Likewise if the motor will be stopped/started a lot you will always be switching the relay at the full stall current of the motor when starting it, and your 8A relay might not be up to the job. :o) -Roman -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

> Have you measured the stall (or start) current > of the motor?? That is where you will get the > potential problems and maybe with accel/decel > current, ie it might be wise to actually monitor > motor current. I've found out that the motor pulls 1A at the load limit. The client is off to Sydney for a few days from tomorrow morning and we'll do some measurements when he gets back. In the meantime maybe I'll look through mine and see if I've got something similar to twiddle with as per your suggestions Inrush suppressors (as found in monitors) have been mentioned before in connection with anti-blow protection for light bulbs and I was having a look at some in the RS catalogue Specifically 210-673 to 210-752 (p643), 50R-2A to 5R-6A example info http://www.todmcpg.com/inrush/inrush.html Now, these are said to be suitable for "small" electric motors. I'm perfectly happy with the series resistor on the light bulb, so wonder, without rehashing the light bulb debate, whether anything more than a series resistor for the motor is needed. In this case, I'm trying to make life easier for the relay contacts by reducing or eliminating arcing -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

On Wed, 30 Oct 2002, Jinx wrote: *>I probably have this figured out right, but just checking *> *>There are two relays to control a motor. An SPST for power *>and a DPDT for direction. I've found the relays I want, based *>partly on physical size. The SPST is 8A continuous and the *>DPDT is 2A continuous but are close in size and low profile *>(important). The motor draws 0.4A when running. If the direction *>(2A) relay is closed before the power (8A) relay, then the power *>relay will take the brunt of the start-up surge ?? Yes if you allow enough time between them. Same thing applies when breaking (open the big relay first). Using a hefty snubber should help, and using a NTC or other current limiter even more. Peter -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

At 05:45 PM 10/30/02 +1300, Jinx wrote: > > > >(2A) relay is closed before the power (8A) relay, then the > > > > power relay will take the brunt of the start-up surge ?? > > > > > > > > Yes. > > > > Agree > >Thanks Dwayne and Russell - and 1/2 point to me > >I did purposefully choose an over-specced relay for the power >as it will do all of the active switching. By luck it just turns out >that the SPST matches the package size of the DPDT, which >keeps the PCB tidy One thing that I should mention: all relays that I destroyed as part of my testing way back then failed because of arcing that occurred when trying to STOP the motor. We used dynamic braking (using a SPDT relay to switch a load resistor across the motor coil after disconnecting from the supply) and the contact arcing was ferocious. Clamping diodes reduced the problem to the point where we had relay failures only if a dead short at the load happened. dwayne -- Dwayne Reid <dwaynerKILLspamplanet.eon.net> Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 18 years of Engineering Innovation (1984 - 2002) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

> There are two relays to control a motor. An SPST for power > and a DPDT for direction. I've found the relays I want, based > partly on physical size. The SPST is 8A continuous and the > DPDT is 2A continuous but are close in size and low profile > (important). The motor draws 0.4A when running. If the direction > (2A) relay is closed before the power (8A) relay, then the power > relay will take the brunt of the start-up surge ?? Yup. It would be a good idea if the power relay is open whenever the direction relay is being switched. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

> One thing that I should mention: all relays that I destroyed as part > of my testing way back then failed because of arcing that occurred > when trying to STOP the motor Hmmm, good point. I'll check my code to make sure that the 2A direction relay is off before the 8A power goes off. The motor has back-to-back zeners and filter caps across it, but that was intended to reduce EMR, not address back EMF -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics