Push-to-Make Switches

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noel
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Push-to-Make Switches

Postby noel » Wed Nov 03, 2010 3:58 pm

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I am looking for a cheap replacement for this push-to-make switch,
which are a dime a dozen. Heavier switches are about 3-4 Pounds.

The problem is using a CDU with a diode matrix to drive as many as
4 x PL-10's, exceeds the wattage for this switch.

I have not experimented to find the failure load for the switch
- I fully expect any "creep" to give me a jab!

Push-to-Make.jpg
Push-to-Make.jpg (3.87 KiB) Viewed 1440 times


I do not want to have the expense of dedicated relays to take the
increased wattage, at about 3-4 pounds each.

Any ideas?

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bigbob
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Re: Push-to-Make Switches

Postby bigbob » Wed Nov 03, 2010 4:19 pm

Growing old is compulsary, growing up is optional!


ALL CLASS 50`S SHOULD BE CUT UP INTO SMALL PIECES AND USED FOR SOMETHING MORE USEFUL LIKE A PAPER-WEIGHT

noel
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Re: Push-to-Make Switches

Postby noel » Wed Nov 03, 2010 4:33 pm

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bigbob:- Thanks for the reference - problem is they are latching, and rather large.

I think I am going to be driven to a pen & stud solution.

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bigbob
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Re: Push-to-Make Switches

Postby bigbob » Wed Nov 03, 2010 4:39 pm

Have a look on that website, loads of PTM switches! Pen and stud is another option :D
Growing old is compulsary, growing up is optional!


ALL CLASS 50`S SHOULD BE CUT UP INTO SMALL PIECES AND USED FOR SOMETHING MORE USEFUL LIKE A PAPER-WEIGHT

m8internet
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Re: Push-to-Make Switches

Postby m8internet » Wed Nov 03, 2010 5:11 pm

Most miniature push to make switches are rated at 125 VAC 1A
That is more than suitable as most supplies you will use are just 12VDC or 16VAC, furthermore just a short burst (especially if you use a CDU)
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Flashbang
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Re: Push-to-Make Switches

Postby Flashbang » Wed Nov 03, 2010 6:05 pm

Hi
So long as a CDU is used, the low cost push to make non locking push button switches should be fine to use.
Contact arcing is caused mainly when the contacts open. So as a CDU is used the current flow once the point solenoid motors have moved will be in milliamps.
At around £0.30p each these push buttons should be ok and I would always order a few extras for replacement use, to avoide future P & P charges!
Rapid low cost push button switches
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noel
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Re: Push-to-Make Switches

Postby noel » Wed Nov 03, 2010 11:33 pm

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m8internet wrote:Most miniature push to make switches are rated at 125 VAC 1A


These switches tend to be too big for the track plan on my control panel.

The example I gave was for a miniature switch rated as 125VAC at only 0.25A

bigbob:- Flashbang's example, also as you suggested from Rapid, is a miniature
switch rated as 125VAC but at 0.5A - I think this is going to be my prefered choice.

Thank you all.

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Gordon H
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Re: Push-to-Make Switches

Postby Gordon H » Fri Nov 05, 2010 12:14 am

Flashbang wrote:Hi
So long as a CDU is used, the low cost push to make non locking push button switches should be fine to use.
Contact arcing is caused mainly when the contacts open. So as a CDU is used the current flow once the point solenoid motors have moved will be in milliamps.

Whilst this approach may allow an underrated switch to be used, it does require that a disciplined approach to switch actuation is employed to avoid eventual failure.
In other words, you must ensure that you press the switch positively and hold it in for a sufficiently long period to ensure the CDU discharges to a safe level. Accidental brief actuation could easily result in welded contacts.
Also beware of plain ordinary contact bounce, which is a phenomenon all mechanical switch contacts suffer from to some extent, and could give the same result.

m8internet
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Re: Push-to-Make Switches

Postby m8internet » Fri Nov 05, 2010 9:06 am

noel wrote:as many as
4 x PL-10's, exceeds the wattage for this switch

The power rating is typically 125W
Please advise how you calculate that using up to 4 PL-10 point motors would exceed this?
The CDU would fail
Equally, the wiring would probably melt with 125W passing through it

I am using these switches with up to six PL-10E point motors on 16/0.2 wiring
The PL-10E produces a burst of up to 24W
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Bufferstop
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Re: Push-to-Make Switches

Postby Bufferstop » Fri Nov 05, 2010 9:32 am

There are two types of damage caused by carrying excess current. Fortunately the CDU takes care of both. First is the heating effect caused by the current flowing over a period of time. The CDU stops this as it delivers a single pulse of energy, then its output voltage falls too low to do any damage. The other effect is contact burn caused by breaking the circuit with an inductive load, the CDU prevents this happening as its output has dropped to almost nil before the switch starts to open. So flashbang is correct in saying that the low cost miniature switches will be OK. My own experience of these switches is that they require a relatively strong push to make (at least for a single finger). Accidental operation is unlikely. if you listen for the motors to throw the power's gone and any contact bounce will be immaterial.
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Re: Push-to-Make Switches

Postby Gordon H » Fri Nov 05, 2010 12:36 pm

m8internet wrote:
The power rating is typically 125W
Please advise how you calculate that using up to 4 PL-10 point motors would exceed this?


This is a misinterpretation of the voltage and current figures quoted in the switch ratings. Switching a load of 125W can only be achieved by the combination of both maximum figures. You cannot trade off volts against current by simply using a 'Volts x Amps' Wattage calculation from the ratings given.
1A is 1A max current no matter what the voltage.

Equally, the wiring would probably melt with 125W passing through it


Again, a misunderstanding. Watts don't pass down wires - Amps do. The Watts that result at the far end depend on the Voltage as well.

The CDU stops this as it delivers a single pulse of energy, then its output voltage falls too low to do any damage. The other effect is contact burn caused by breaking the circuit with an inductive load, the CDU prevents this happening as its output has dropped to almost nil before the switch starts to open.


But can you guarantee that the current has fallen sufficiently far every time? That is the point I was trying to make - accidental brief actuations of a few milliseconds may still cause damage if the capacitor has not discharged completely.

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Re: Push-to-Make Switches

Postby noel » Fri Nov 05, 2010 5:54 pm

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The whole burst has to be borne by the push-switch
before each individual PL-10's - that is why the number
of PL-10's is important.

The output of the CDU is given as 12v at 5A. - in my rough
and ready calculation, that is 60w available to be DELIVERED
elsewhere.

The switch I show at the opening of this thread is rated
125v at 0.25A - which in my ignorance I equate to 30W.

The Rapid Electronics switch IS rated 125V at 0.5A
- which again in my ignorance I equate to the 60W CONSUMED
in the elsewhere.

In the "W=V x A" formula, what corrections have to be
made for practice?

Suitable wiring will be used, and is not at issue.

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Flashbang
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Re: Push-to-Make Switches

Postby Flashbang » Fri Nov 05, 2010 6:12 pm

Hi
I have used these low cost push buttons and only ever had one fail and that fell apart internally at an exhibition!!
That said, I had pre wired all my point push buttons and that included one set of PBs used via a matrix, so that if one should ever fail I had replacements readily available.
I used a multi way connector terminal block - 2 way with each switch, pre wired with a length of 16/0.2mm wire which then was connected into the appropriate terminal blocks. Should one have failed for whatever reason all I needed was a screwdriver and pliers. Screwdriver to undo and tighten the two connection terminals and pliers to release the securing nut on the push button. Spare PB switches where pre wired with identical lengths of 16/0.2mm wire so swapping over could be carried out in minutes if required.

I no longer use PBs as I have changed to non locking sprung to centre off toggle switches. :D
Shortly I'm going to convert all my panel switches to MERG DCC operation using their Accessory Encoder and Decoder kits all operated via locking toggle switches!
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Broken? It was working correctly when I left it.

noel
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Re: Push-to-Make Switches

Postby noel » Fri Nov 05, 2010 6:44 pm

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Flashbang wrote:
Shortly I'm going to convert all my panel switches to MERG DCC
operation using their Accessory Encoder and Decoder kits all
operated via locking toggle switches!


This would be very nice, but as a pensioner I am not likely to
achieve this.

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Bufferstop
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Re: Push-to-Make Switches

Postby Bufferstop » Fri Nov 05, 2010 6:46 pm

Writing and reading the specification for components is fraught with complications. What does the 125V claimed for this switch mean? Is it the voltage between the contacts at which the air gap is no longer effective and arcing will ensue or is it the voltage at which its insulation from the outside world starts to break down. What we don't know is what the open circuit, charged voltage of the CDU is. In all probability we are unlike to reach the break-down value of the insulation. On the other hand we may well be able to break down the resistance of the air gap,if we could cause the contacts to open slowly. The point I made about the single finger is relevant to this. The switches of this type that I have come across require a firm push to close and don't lend themselves to being released slowly, so we aren't likely to cause arcing. What about its current carrying capacity. The specification says 0.5A. Without adding other parameters this is fairly meaningless. If it were a lever or slide switch we could quote a maximum continuous rating, that would be the current that it could carry indefinitely without the contacts melting (same goes for the connecting wires), but this is a switch intended for momentary operation, being used with a supply that is intended to give a short spike of current. If we knew the component values within the CDU we could calculate the charge built up, and the time over which it will take to discharged through the switch, and thus the average current, but we don't have that information. So how do we decide if it's up to the job. We're talking about a control panel for a model railway, not life and death dependency in a space capsule! We're not even talking about enough power to do any damage to the operator's finger (at least not as long as it stays on the button), so try it and see, or use others' experience. That's all that flashbang is doing, quoting his experience, and thats good enough for me.
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