momentary output

Hi guys - and please excuse my obvious lack of knowledge!
I am trying to re-configure the overdrive activation on my 1965 Sunbeam Rapier so it can be activated by a press of a momentary push button switch in the gear knob provided that either third or fourth gear is engaged (and can be deactivated by the same button) BUT also deactivates automatically when moving out of gear. I have sorted the main operation by having the gearlever switch give a negative signal to a latching relay which sends positive supply to terminal 30 on a changeover relay that is earthed by either of the gear sensing switches and thus powers the overdrive solenoid from the 87 terminal. When the gear is deselected the changeover relay then powers from the 87a terminal to another relay to send a negative signal to the latching relay to unlatch it but of course it needs to be momentary signal. I have found the relevant relay diagram on this wonderful site but know nothing about capacitors or resistors so do not know what to ask for and my RS Components store insists I have to give them full specifications (of the 10k fixed resistor and 1k uf capacitor) before they know what to look for.   
Help please!


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stevew

I'm not sure why the 10k resistor is there, but any 1/4W or 1/2W 10k resistor will be fine.
The 1,000 uF cap needs to have a voltage rating of 16V or higher; I'd suggest 25V (or higher, but 25V is fine). Any electrolytic will do (low ESR not required.)

And tolerance on any of the above is not critical.

Much obliged!

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stevew

Help!

I put together a trial rig (using a warning light rather than the overdrive solenoid) based on relay diagram 44 and it worked perfectly a couple of times then it just simply stopped working.



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stevew

Have you tested it externally? It may be a blown voltage source.

It seems to be something to do with the capacitor and resistor - if I remove them and simply earth the terminal I can get the whole rig to work perfectly by simply touching the input terminal momentarily with the feed from the changeover relay. Maybe your doubt about the resistor is well founded but as I understand it (I don't really understand the theory at all though!) the resistor is needed to leak the charge stored in the capacitor when the input feed is removed. Is it a question of time perhaps, because the input feed is there all the time that overdrive is not engaged.

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stevew

My "I'm not sure why the 10k resistor is there" assumes the switched +12V is grounded when not switched to +12V. That's usually true where the switched +12V is IGN or ACC where there are usually other loads with a total resistance reasonably lower than that of the relay (coil) being used. IOW the switched +12V when off grounds and hence discharges the cap thru the relay coil which is typically 250 Ohm or less thereby making the 10k cap resistor insignificant.
But yes, the 10k resistor is to discharge the cap.

You haven't wired the cap back to front? It should be with its "+" end to relay coil 85 (with 86 going to +12). Typical electrolytic caps are polar (tho non-polar versions are also available).
Wiring a polar electrolytic cap back to front leads to failure - usually in short (circuit) mode hence which thus reduces the circuit to a typical grounded relay which is on whilst the switched +12V is on (+12V).

Still struggling - I tried wiring a new capacitor the other way round but it's no different. I'm still back to the situation that I can make it all work by touching the 87a output from the changeover relay briefly onto the 86 terminal when the gear sensing switches are opened but can't get it to do it automatically.



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stevew

So taking 86 to +12V doesn't work?
It sounds like its fine to me but that your gear switching is not switching +12V.   
Will the gear switches turn on the relay with the cap shorted (ie, a normal relay circuit)?

Yep - I can get everything working except a momentary output when the relay is wired up with the capacitor and resistor. The gear switches do indeed give a +12v output from 87 on the changeover relay when "engaged" (to power the solenoid) and from 87a when "disengaged" and if I wire up the other relay with 85 to -12v and touch that output from 87a briefly on 86 the latching relay is "unlatched" by the -12v momentary output from 30.

I can see the logic of relay diagram 44 but somehow it doesn't want to play ball - about ten years ago I had another Rapier and did the same modification but a (now deceased) friend made me a circuit board that did it all for me so I can't even blame a failing memory for my failure to get it working!

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stevew

Make sure you test it using a 12V supply - eg, the car battery. If it doesn't work, there is something wrong with your circuit.

You may need a bigger cap - say 10,000uF. A 1k uF may only give a ~0.1 second pulse. 10k uF should give closer to 1 second. That's for a typical automotive relay with ~60 - 120 Ohm coil resistance.

Thank you for your continuing patience!

I've got my "test rig" all set up with a car battery so it's looks like you're right about the capacitor - I'll try a bigger one

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stevew