Sounds like wiring the coil of the new relay between L and +12V will do it – just like they show.
They don’t show a diode there so hopefully they have one inside the regulator, between L and +12V. Or they are switching L with a relay contact and accept the arcing on that. But you can still put another one across your relay coil to make sure.
Might be worth checking the current rating of whatever is being used to switch L.
This is not an issue if you use a SPDT pressure switch or relay. The common contact is connected to ground, while the NO contact is connected to the hobbs, and the NC contact is connected to the transponder.
Sure – that’s a good solution if you can arrange the two items to work with opposite senses.
Thanks again for the sage advice. Below is a scan from the Pioneer wiring schematic showing use of the “L” terminal though this is not yet wired on my aircraft. I only have a single terminal oil pressure sensor so may as well go with plan A. The RS miniature relays Peter suggests look to be just what I need.
In general one may not be able to use a single switch to switch both the hobbs and the transponder, because these devices generally have internal resistive pullups to their internal supply rails. These rails may not be the same voltage e.g. one could be +12V and the other could be +5V. Also if the power supply on either device is removed (by the pilot pulling a CB, or via a failure) then the non-functional device could pull down the shared signal, permanently, or result in intermittent behaviour.
This is not an issue if you use a SPDT pressure switch or relay. The common contact is connected to ground, while the NO contact is connected to the hobbs, and the NC contact is connected to the transponder.
The if the airspeed is low (or zero) the NO contact is open, so the hobbs has no ground, and won’t run. the NC is closed, so the transponder wheel sensor wire feels ground and the transponder is in GND mode.
It the airspeed increases the NO contact closes, so that the hobbs has a ground, and starts to run. The NC contact is now open, so the transponder wheel sensor wire does no longer feel ground and goes it ground mode.
With this ground switching it doesn’t matter what the supply voltage is, it doesn’t need isolation diodes either, as one is on the NC contact, while the other is on the NO contact so there is always isolation.
semiconductors might be preferable as switches – no spikes, no noise, no sparks.
This is not an issue here if you use a minature relay and with this very small current (a few milli-amps) You won’t notice any noise or spark with that. Don’t forget that this relay will only switch twice during a normal flight, it is not a relay which carries a lot of current, or requires a lot of current to be controlled, and it is not switching constantly (like an old voltage regulator circuit would do).
Why not run the hour meter from the air pressure switch and then it reads clean airborne time, which is what one should be using to log for maintenance purposes.
This is what is suggested at my first reply, but doesn’t seem to fit the needs for this Rotax installation
If I install a hobbs which is to be used as flight timer, I always have the pressure switch in series, and still use the oil pressure switch as well. This will make the hobbs a flight timer under powered flight. You might wonder why, as it is likely that all flights are being under engine power. This prevents the hobbs from counting when an pitot static test is performed on the aircraft and it doesn’t cost much extra to improve the circuit like that.
For this Rotax installation It might be easier to use oil pressure switch alone to switch the hobbs in the ground, instead of using the generator solution.
Do note that you need a SPDT oil pressure switch for this hobbs applications! Pure oil pressure switches could be SPST NC contact only!
With this SPDT oil pressure switch you use the NO contact for the hobbs (oil pressure = NO contact closed = hobbs counting) and the NC contact for the oil pressure warning light (no oil pressure = NC contact closed = oil pressure warning light on).
starting the engine(s) with the avionics master switch on is considered deadly sin, almost on par with a wheels-up landing?
There have been cases of people blowing up their radio gear that way, yes. You get spikes of a few hundred volts on the supply bus.
It’s the same issue in automotive electrics, and they solve it in two ways: there is an ISO spec for the transients and the gear has to withstand that (if you look up the data sheet on say a LM2940CT-5.0 you will see references to it) and the ignition switch disconnects the radio gear’s supply during starting.
The later TB aircraft have an extra relay on the avionics master relay board which turns off the three avionics buses when the starter is powered, so they do the same as in cars. But most planes don’t have this.
Yes it is stupid to not have voltage clamping in there…
you’ll excuse me for not being impressed with the heat argument regarding mosfet’s – surely today’s mosfet’s with their very low Rdson will dissipate less power than any kind of relay coil?
Yes – I agree. But that’s only if the FET is turned hard ON. If anything funny happens with the gate drive, you might get a lot of heat very quickly. Personally I would use MOSFETs but I don’t like to suggest that in the context of a brief forum post, because of the other issues to get right.
It looks like Maplin here in the UK have a suitable SPDT miniature relay:
Steve – Maplin sell mostly cheap and nasty stuff. On an aeroplane, why not spend an extra few quid on a proper sealed relay? Superglue won’t guarantee sealing, and its very aggressive vapour is going to corrode the relay internals. The cheap sealed relays are sealed with epoxy at least. This one is two quid, is sealed, and has a 10G functional shock rating which is reasonable. But you need to check that you can drive a relay coil from the existing circuitry…
the general approach seems to be to run the Hobb’s timer off an oil-pressure detector, the same that actuates the “low oil pressure” light… Some even have the Hobb’s timer only active when ASI comes above stall speed or such, not sure how that is implemented.
Why not run the hour meter from the air pressure switch and then it reads clean airborne time, which is what one should be using to log for maintenance purposes.
Well, the general approach seems to be to run the Hobb’s timer off an oil-pressure detector, the same that actuates the “low oil pressure” light… Some even have the Hobb’s timer only active when ASI comes above stall speed or such, not sure how that is implemented.
On a general note: distrust the Ducati voltage regulator, mine blew out after less than 50 hours TT and it wasn’t the first; I replaced it with a German free-market alternate which has done fine up till now but that is only a short while. That is on a 912UL, 80HP so not the S variant. In all honesty I must add that my airframer, though generally quite knowledgeable and acclaimed, was less than impressive on the electrics, mounting the Ducati regulator in the horizontal, thus disabling all and any airflow along its cooling ribs. Sure enough I mounted the alternate vertically against the firewall, on the cabin side.
Jan I can solder components and have made up simple circuits on vero board but have little idea how they work.
It looks like Maplin here in the UK have a suitable SPDT miniature relay:
I should be able to seal it up with a dab of superglue on the case joints :)
I’m afraid I didn’t explain things well above. This is for a Rotax 912S engine. It has a Ducatti voltage regulator with a “L” terminal that goes to ground when the alternator is not running but the master is on. The charge warning light is normally wired to 12v and the “L” terminal makes the circuit but goes open circuit when the alternator kicks in. I plan to wire 12V to the hobbs positive and use the relay/circuit in the ground wire. The hobbs must measure engine run time as bizarrely that is what Rotax specify for service intervals not airbourne time.
Peter, you will remember I am unacquainted with the world of IR-equipped high-end private planes. But didn’t I understand that
-) starting the engine(s) with the avionics master switch on is considered deadly sin, almost on par with a wheels-up landing?
-) to protect the precious avionics against said error, a crowbar circuit on the avionics bus is a self-evident necessity in this category of plane, because we are all only humans after all?
BTW you’ll excuse me for not being impressed with the heat argument regarding mosfet’s – surely today’s mosfet’s with their very low Rdson will dissipate less power than any kind of relay coil? Apologies to the non-electronics members…
Among the issues to watch with semiconductors might be bus spikes e.g. a 12V bus could go to 300V due to starter motor inductance.
If you try to start the engine with avionics ON, it is somewhat likely that some boxes will get blown up.
Now, if you used a MOSFET to switch something, and its Vgs(max) is say 20V, you will blow it up. So you have to put in a means of clamping that, so you end up with a few components around it. All easy, but it isn’t as trivial as popping in a small relay.
One could debate whether it is OK to bundle a small relay into a harness. It’s not ideal but if there is good access, it is properly insulated, has no sharp edges (or has overall clear heatshrink), and you have documented it, then it should be OK. With a MOSFET, you can’t be sure it isn’t going to overheat under fault conditions… which may mean mounting it separately somewhere.
As Peter writes (cautiously between the lines) semiconductors might be preferable as switches – no spikes, no noise, no sparks. But some more analysis would be required. If you could post a schematic of what you intend to do, I’ll be glad to suggest a solid state equivalent.. However, use of semiconductors would involve soldering – as you state electronics is not your cup of tea you might well prefer the push-on connectors..?
I would use the differential pressure switch to swith both the hobbs and transponder (via resistor or relay)
In general one may not be able to use a single switch to switch both the hobbs and the transponder, because these devices generally have internal resistive pullups to their internal supply rails. These rails may not be the same voltage e.g. one could be +12V and the other could be +5V. Also if the power supply on either device is removed (by the pilot pulling a CB, or via a failure) then the non-functional device could pull down the shared signal, permanently, or result in intermittent behaviour.
Without having the circuit diagrams of the two instruments, one cannot be sure whether the control line can be simply interconnected and operated with a single shared switch contact.
In my article I show how one can use two diodes to solve the problem. That particular installer told me he had been wiring up stuff like that for years, mostly in rather high-end piston aircraft, and he says it works, which is true – until something stops working…
OTOH, if you are switching it with a relay, you may as well get a 2 pole relay and use 1 pole per instrument… easy.
