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Simple sequencing using common 5-pin auto relays

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Voyager

Joined: 30 Jun 2020
Posts: 77

Posted: Mon Aug 07, 2023 4:19 pm Post subject: Simple sequencing using common 5-pin auto relays

I have an idea that I am not sure how to implement or if it is even possible to implement simply.  I know it could be easily done using a microcontroller and fairly easily done with a small PCB with a 555 timer and a few discrete components, but I am wondering if this can be done using nothing but standard Bosch-style auto relays.  The application is the start sequence for a Rotax 915 that needs to have start power provided for a short time until the engine revs past 1500 or so.



I know this can easily be done with two momentary contact switches, one for the start power and the second to actuate the starter.  However, this requires manually operating both switches with one hand to keep the other on the throttle.  This can be made easier by mounting one switch where you can reach it with your hand on the throttle, or mounting one as a toggle switch sideways that can be pulled towards a guarded starter button using the guard as a place to pull against while waiting for the Lanes to self-test before hitting the start button.



I am trying to decide if this can be done using two buttons operated in sequence. Basically this is the sequence of operation:



Press button #1 - latch a relay to provide start power.

Release button #1 - NOP

Press button #2 - engages starter

Release button #2 - disengage starter AND unlatch the start power relay, possible with a short delay, but it appears that the Rotax engine will rev past 1500 so fast that by the time a human releases the start button the engine is fine also losing the start power supply.



I know how to latch a relay using two switches, one tNO to latch and another NC to unlatch.  What I am not sure how to do is to unlatch a latched relay using the trailing edge of a momentary contact push button/switch.



Anyone done this or have an idea how to do it with just a relay or three?



Matt

Eric Page

Joined: 15 Feb 2017
Posts: 244

Posted: Mon Aug 07, 2023 6:07 pm Post subject: Re: Simple sequencing using common 5-pin auto relays

I don't know if it's possible to do it with two buttons and ONLY relays, but how about using a single button, three SPDT relays, three diodes and a capacitor?  We can take advantage of the fact that relays will remain energized at voltages significantly below their rated coil voltage to hold START POWER engaged for a short time after the ENGINE START switch is released.  See the attached diagram.



When the ENGINE START button is pushed, all three relay coils are energized and capacitor C1 is charged to battery voltage.  The ENG STRTR relay will close a few milliseconds before the two STRT PWR relays (due to charging C1), but that may not make any difference if the Rotax Fuse Box needs to see START POWER before it will engage the starter.  That's an unknown.



When the ENGINE START button is released, diode D1 isolates the ENG STRTR relay (K1) from the capacitor, so it drops out immediately.  C1 discharges through the coils of the two STRT PWR relays (K2 and K3), until the voltage falls to their drop-out point.  A capacitor of 4,700μF should provide about half a second delay, assuming the relay coil resistances are about 90Ω each and they drop out at around 1.2V (values taken from a randomly selected automotive relay datasheet).  It would take some experimentation to find exactly the right value.



This was an interesting thinking exercise, but you implement my idea at your own risk.  Personally, I would stick with something very close to the Rotax installation manual diagrams.

Single Button Rotax Start.pdf

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Eric Page

Joined: 15 Feb 2017
Posts: 244

Posted: Mon Aug 07, 2023 7:04 pm Post subject: Re: Simple sequencing using common 5-pin auto relays

Upon further reflection, I definitely would not do this.  With a 9-Series iS engine, you need to be able to engage the starter with the BACKUP BATTERY switch closed in the event of dual charging system failure.  In that case, you don't want START POWER engaged as well.

Voyager

Joined: 30 Jun 2020
Posts: 77

Posted: Tue Aug 08, 2023 5:26 am Post subject: Re: Simple sequencing using common 5-pin auto relays

 	  Eric Page wrote: 	
		  Upon further reflection, I definitely would not do this.  With a 9-Series iS engine, you need to be able to engage the starter with the BACKUP BATTERY switch closed in the event of dual charging system failure.  In that case, you don't want START POWER engaged as well. 	




I am still learning about the 915 as I am fairly new to Rotax.  I took a week of training at Lockwood, but didn’t have my engine yet and didn’t know all of the questions to ask.  The schematic in the installation manual doesn’t preclude having both start and battery backup power applied to the engine as they are completely separate circuits so I am not sure what would happen if this was done. I would hope this wouldn’t be a fatal combination, but I don’t yet know what I don’t know.



I did spend an hour or two last night playing around on paper and have a configuration I think night work.  I don’t have it in a nice schematic yet as my MicroCap skills are pretty poor since retiring 8 years ago.  I may try to draw it up today and simulate it.  I ended up using 3 relays as you did, but with just one diode.  A schematic is worth a thousand words, but let me see if I can describe concisely in words.



I have S1 (start power momentary switch) connected between 12V and pin 86 on K1 (Relay 1). K1 P85 connects to K2 P87a.  K1 P30 connects to 12V. K1 P87 actuates the start power relay to provide the contact closure for both power and ground interconnect to the engine.  K1 P87 also is connected to K1 P86 to latch the relay and K3 P87 via a diode such that K1 can’t provide current to the starter engage relay.  K1 P87a is not used.



For the second relay, K2, whose main function is to unlatch K1, connections are: K2 P86 to K3 P87 to actuate K2 when K3 is actuated, K2 P85 to GND, K2 P30 to GND to provide ground path for K1 coil, K2 P87 not used, K2 P87a connects to K1 P85 to provide coil ground path when K2 not activated.



For the third relay, K3, I have S2 (momentary starter push button) connected between 12V and P86.  K3 P85 to GND.  K3 P30 to 12V.  K3 P87 connects to starter activation coil to provide contact closure to engage the starter.  K3 P87 also connects to K1 P87 via a diode.  This will allow K3 P87 to provide 12 V to both the start power relay and the starter engage relay,  K3 P87a is not used.



My main concern is timing as obviously the start power signal can’t be interrupted as the starter engage relay is activated.  Since most relays take several milliseconds to respond, the current from K3 should get through the diode nearly instantly and provide power to the start power relay before K2 can respond and unlatch K1.  However, before I try this I will need to relearn MicroCap enough to run a simulation to have some confidence in the timing.



Does this make any sense?  Assuming I didn’t make a mistake in the description above.



Matt

user9253

Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan

Posted: Tue Aug 08, 2023 5:30 am Post subject: Re: Simple sequencing using common 5-pin auto relays

Attached is a drawing from Kitfox that shows only ONE switch used for starting the 912iS engine.

The drawing also shows battery negative connected to the engine mount.  Don't do that.

Connect battery negative either directly to the engine or to a ground bus that is connected to the engine.

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Joe Gores

Last edited by user9253 on Tue Aug 08, 2023 7:59 am; edited 1 time in total

user9253

Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan

Posted: Tue Aug 08, 2023 6:19 am Post subject: Re: Simple sequencing using common 5-pin auto relays

Are you overthinking this?  True, the start sequence for a Rotax 915 that needs to have start power provided for a short time. 

But it is my understanding that the Rotax relay box does this automatically when the start switch is activated.

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Joe Gores

Voyager

Joined: 30 Jun 2020
Posts: 77

Posted: Tue Aug 08, 2023 7:19 am Post subject: Re: Simple sequencing using common 5-pin auto relays

 	  user9253 wrote: 	
		  Attached is a drawing from Kitfox that shows only ONE switch used for starting the 912iS engine.

The drawing also shows battery negative connected to the engine mount.  Don't do that.

Connect battery negative either directly to the engine or to a ground bus that is connected to the engine. 	




I get a 404 file not found message when I try to open the drawing file.

Voyager

Joined: 30 Jun 2020
Posts: 77

Posted: Tue Aug 08, 2023 7:24 am Post subject: Re: Simple sequencing using common 5-pin auto relays

 	  user9253 wrote: 	
		  Are you overthinking this?  True, the start sequence for a Rotax 915 that needs to have start power provided for a short time. 

But it is my understanding that the Rotax relay box does this automatically when the start switch is activated. 	




Probably.  That is what engineers do.    



I believe you are correct in that the Rotax relay box will disconnect the start power once the internal alternators take over, but I also believe it is bad practice to leave the start power applied and, particularly, to leave the ground interconnect in place. Everything I’ve read in the Rotax manuals says that start power should be removed shortly after engine start and the grounds isolated so that regulator A is not grounded to the airframe.

user9253

Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan

Posted: Tue Aug 08, 2023 7:39 am Post subject: Re: Simple sequencing using common 5-pin auto relays

About that 404 error message: I got an error message trying to attach the file.  It seems that the Matronics computer is running out of disk space.

Try this link: http://forums.matronics.com/files/joe_rotax_is__166.jpg

_________________
Joe Gores

user9253

Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan

Posted: Tue Aug 08, 2023 7:44 am Post subject: Re: Simple sequencing using common 5-pin auto relays

The schematic with single switch accomplishes your goal.

 	  Quote: 	
		  Everything I’ve read in the Rotax manuals says that start power should be removed shortly after engine start and the grounds isolated so that regulator A is not grounded to the airframe.

_________________
Joe Gores

user9253

Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan

Posted: Tue Aug 08, 2023 7:53 am Post subject: Re: Simple sequencing using common 5-pin auto relays

Schematic attached.

Rotax 912iS SCHEMATIC.png

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Joe Gores

Voyager

Joined: 30 Jun 2020
Posts: 77

Posted: Tue Aug 08, 2023 10:39 am Post subject: Re: Simple sequencing using common 5-pin auto relays

 	  user9253 wrote: 	
		  The schematic with single switch accomplishes your goal.

 	  Quote: 	
		  Everything I’ve read in the Rotax manuals says that start power should be removed shortly after engine start and the grounds isolated so that regulator A is not grounded to the airframe. 	
	




It doesn’t as I mentioned in my initial post.  You still need two switches as this schematic does not show the switch to engage the starter.  The switch in this schematic simply selected between start power and backup power, but doesn’t address the need to hold the start power switch while keeping one hand on the throttle and a third hand to engage the starter.

user9253

Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan

Posted: Tue Aug 08, 2023 5:13 pm Post subject: Re: Simple sequencing using common 5-pin auto relays

You are right Matt.  I did not read your original post carefully enough.

You wrote, "mounting one as a toggle switch sideways that can be pulled towards a guarded starter button. . . "

I think that is the best solution instead of complicating the circuit with relays or electronic timers.  

My 912ULS also requires 3 hands to start.  One hand pulls the choke, one hand turns the start key.  It starts within 2 seconds. 

Then I have to push the choke in and lock it while simultaneously advancing the throttle.  It is not easy, but doable.

_________________
Joe Gores

Voyager

Joined: 30 Jun 2020
Posts: 77

Posted: Wed Aug 09, 2023 9:09 am Post subject: Simple sequencing using common 5-pin auto relays

That is nearly what I have now.  The main difference is the diode connection to the start power is replaced by the delay relay.  I it will be fun to experiment when my battery box gets back from the fabricator and I can start putting things together.



Sent from my iPad



 	  Quote: 	
		   On Aug 9, 2023, at 12:52 PM, user9253 <fransew(at)gmail.com> wrote:

 

 

 

 How about this circuit? I am not necessarily recommending it, but I think that it does what you want.

 

 --------

 Joe Gores

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=511233#511233

 

 

 

 

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