help needed

[Deltech]

The following would be to use instead of a spring-to-centre toggle switch (which I don't like).

I would like (if at all possible) to use a single push-button, which when pressed will send a short forward(?) pulse, and when pressed again will send a short reverse(?) pulse.

The final unit downstream requires pulses, NOT constant signal.

This would be either by the push-button having 3 (or more?) pins for common(?), forward(?), and reverse(?), so that alternate presses would send forward/reverse pulses (if such a push-button is available, which I doubt);

OR,

it having only 2 pins, which would send a pulse to some an intermediate component/circuit, which in turn would send forward/reverse pulses downstream on alternate presses of the push-button.

Is such an idea possible, and if so, what other components would I require.

Thanks.

 

[rjenkinsgb]

What is it connected to - what voltage and current does whatever it connects to need?

It could be as simple as a push-on/push-off button with a changeover contact and a couple of capacitors and resistors to do the pulse part each time it switches.

 

[Deltech]

rjenkinsgb,
sorry for late response; I got lost in other threads.

It's to control the operation of model rail points. Pair of solenoids, each having it's own live connection, and sharing a common neutral/ground connection.

Only 1 operating at a time to throw the points one way or the other. The most common method of operation is by centre-sprung on-off-on toggle switches, because they cannot be left with power applied or they burn out.

I personally don't like toggle switches, just a personal thing, so that's why I'm thinking of push-button.

Now I accept that I could simply use 2 x push-buttons (non-latching) to replace 1 x toggle switch, but if it's possible to do by 1 push-button (non-latching) then it would "streamline" things at the control panel, but obviously I'd need some sort of electrickery between the single push-button and the solenoid(s) so that subsequent operation of the push-button gives alternative operation of the solenoids.

They can operate on both AC (16-18v) and DC (19-22v).

No information from manufacturer on current, but I'm informed by the model rail community that it is </= 1.5A, and they "throw" in a fraction of a second.

 

[rjenkinsgb]

The simplest solution would be DC, with a simple push-push changeover switch, and a capacitor with a parallel resistor connected from each of the two outputs to the two points operating coils.

Power / positive to the switch common, ground / negative to the coil common. You should be able to easily find a 19V laptop charger (or universal replacement) on ebay to use as a DC power source.

I'd try eg. 1000uF 25V or 35V caps to start with and see if they give reliable operation. If not, try 4700uF ones.
They will pass current for a fraction of a second until they charge, then no current.

Add eg. a 1K half watt or preferably one watt resistor in parallel with each capacitor. That will allow about 20mA through the coil while powered which will not cause that to heat - the resistor will get warm though.

As soon as the switch changes to the opposite way, the resistor will discharge the cap ready for the next changeover. The only limitation with that is that you cannot switch back and forth very quickly, it will take 2 - 5 seconds for the cap to discharge to a low level.

If you used a two-pole changeover switch you could use each pole to switch one of the caps between a solenoid or lower value discharge resistor, to eliminate the continuous current and give faster "reset".

 

[Deltech]

Thanks for that.

I feel an experiment coming on.

Does this diagram reflect what ye mean? (polarity of capacitors omitted, and excuse rudimentary drawing from Microsoft Excel)

 

[rjenkinsgb]

Does this diagram reflect what ye mean?

Exactly!

 

[Deltech]

Ok, thanks.

What I don't understand is...... when a coil is powered during the charging phase of it's capacitor, why will it not continue to be powered (via the resistor) after the capacitor stops passing current?

 

[rjenkinsgb]

It will, but the current will from one or two amps to 1/50th of an amp, which is small enough to be irrelevant.

eg. 1A at 20V would be 20W of heat, which would rapidly cook the solenoid. It also implies a 20 Ohm coil.
At 20mA (using a 1K resistor) the power dissipation would be around 0.008 watts, which is meaningless heat-wise.

As the coil is likely lower resistance, the heat at 20V would be higher - and the heat at 20mA even lower.

 

[Deltech]

Ok, thanks.

So am I correct to think that the larger the capacitor, the longer it takes to become fully charged, therefore the longer the duration which the solenoid receives the "high level" current? So a lower value cap is better for the purpose of a shorter duration of "high level" current to the solenoid?

And if what I have just said is correct, it will be a case of experimenting with how low a value cap I can use (on the basis that if I go too low, the solenoid won't receive a long enough "burst" to throw it?

 

[rjenkinsgb]

Yes, again spot on.
The values I suggested will give very short high current pulses, a few milliseconds; you don't have to worry about the points overheating with caps in that type of range. If you find a minimum to operate I'd suggest three to five times larger to guarantee they work when less than perfectly clean.

eg. a 1000uF cap at one amp will charge 1V per millisecond, but also the current drops as the voltage across the solenoid increases so it's linear.
Also the inductance of the solenoid will mean the current builds up rather than starting at maximum.

At a rough guesstimate, around 20mS to get down to half strength with 1000uF and 100mS or so with a 4700uF.

 

[Deltech]

Thanks again.

If you used a two-pole changeover switch you could use each pole to switch one of the caps between a solenoid or lower value discharge resistor, to eliminate the continuous current and give faster "reset".

I can't picture what ye mean by this suggestion. Can ye assist with a diagram, or further explanation please?

p.s. how the hell do ye know all this stuff???

 

[phoenixcomm]

Are you trying to use an embedded controller? Yes please make sure that you use a TIL111 Opto-isolator between the controller and the solenoids!
~~Cris

 

[rjenkinsgb]

I can't picture what ye mean by this suggestion. Can ye assist with a diagram, or further explanation please?

OK, one half, for one solenoid:

The changeover switch common contact to negative end of the cap. Positive end of cap to positive power.
Normally open contact to a solenoid; normally closed contact to eg. a 100 Ohm resistor connected positive.

The second pole on the switch works exactly the same, but exchanging normally open/closed so the solenoids operate alternately.

(Several decades designing, building, fault finding and programming just about anything you can think of).

Is "deltech" from Deltic, by any chance?

 

[Deltech]

Thanks again.

The Deltech comes from Del-Tech, a portmanteau (of sorts) of Del (from Derek) and Tech, based on a wee sideline I've dabbled in from time to time since 1997, with computers (buy, refurbish, sell).

But lots of websites and forums won't allow "hyphens" in usernames, so it's been bastardised to drop the hyphen.

Just as a complete coincidence, I do happen to like the Class 55 Deltics, but I'd never heard of one until around 2000.

Only been driving Trains since 1993, and had no interest in the Railway before that.

 

[Deltech]

Is this what ye mean?

 

[rjenkinsgb]

Almost; break the centre of the left hand connection so the two switch commons are only connected to one cap each.
That means each cap is either discharged via a resistor, or charging via a solenoid coil & pulsing that when switched.

 

[rjenkinsgb]

Another point,for when you start constructing:

Add a diode (eg. a 1N4000 series rectifier) across each solenoid coil, with the diode cathode (banded end) to positive and anode to negative.

That's a "Flywheel diode", it prevents a high-voltage spike appearing if the connection is broken while current is flowing. It's a bit like an electrical equivalent of water hammer and can burn switch contacts or even break down insulation in coils.