View single post by Helmut
 Posted: Fri Nov 25th, 2016 07:08 pm
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Helmut



Joined: Sun Feb 17th, 2013
Location: Friedberg, Germany
Posts: 1189
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@Si
A solenoid's force is determined by the current you drive through the coil's windings. If that force doesn't suffice, you can do two things:
1. Use DC instead of AC - you do not have any inductive resistance - the current increases. But there are, as you noted, sometimes detrimental side effects.
2. Increase the voltage, because that leads to increased current.

Now, if you use a capacitor-diode combo as a half-wave rectifier, the capacitor charges to ~ 1.4times the effective voltage value. 16V become ~22V to operate with. As this is DC now, the current driving the solenoid is determined just by the coil's resistance, which is in the 12..30 Ohms range ( the Märklin ones have 22 Ohms for example )
So this gives you something between 0.7 and 1.8 amps. Nothing that will kill any, even the cheapest, pushbuttons IF the duration is not long. The capacitor's discharge through the coil is a RC net whose characteristics are given by the time constant T= R*C. After a time t = 2*T, the initial current has dropped to 14% of its starting value. Now let's assume 2200µF and 22Ohm coil. The initial current is 1A, the time constant is 0.05sec, so after 0.1 sec it has dropped to 140mA. Now in the 'worst case' of time: 4700µF and 30Ohms we have 0.7A to start with and after 0.3sec it dropped to 90mA
and of current: 4700µF and 12Ohms we have 1.8A to start with and after 0.1sec it dropped to 250mA.
After 3*T, it is neglegible in all cases, and the lamp current of ~60mA prevails. ( Under normal operating conditions, you don't even see the lamp glow in most cases )
There is no way to store enough energy in the magnetic field for a voltage spike to inititate arcing by contact interrupt because it is transformed into kinetic energy to move the plunger.
All this shows there can be no resisitive overload ( heating by current leading to contact destruction ) of the PB and/or cutoff contacts. As you have noted yourself, the normal pushbutton operation, be it AC or DC, can lead to contact failure due to this effect.

Last edited on Fri Nov 25th, 2016 07:12 pm by Helmut



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