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Quoted from PhilGreg:Hi,
from what I've read it would seem like it would be possible and not-damaging to do this as long as you add a diode like on SS or DMD games, but would it be practically feasible to drive non-holding Gottlieb EM coils with 24V DC power (as opposed to the normal 28V AC).
Cheers!
Phil
Yes, I did it too my Gottlieb '58 woodrail pop bumpers, used an individual bridge and diode on all 5 pops. Gives them a nice kick.
Quoted from Det_Deckard:They will pull more current with the dc your giving them. The voltage for the ac divided by square root of 2 would give you the equivalent dc voltage. I dont know if it matters. I would try it and feel the coils to see how hot they get.
Bear in mind that I did this with pop bumpers only, which are on for brief bursts. I wouldn't recommend this mod for coils that are on for longer lengths of time, such as flippers, and definitely not for hold coils.
Quoted from KenLayton:Yes, you can operate AC coils on DC. However, on coils driving things like slingshots, pop bumpers, hole kickers, etc you need heavier duty switch contacts. The higher momentary surge will burn up regular contacts. This is where you'd need tungsten contacts for longevity.
On my '58 Gottlieb the pop relays already have the larger heavy duty tungsten contacts in the pop bumper solenoid firing circuit, while on the same relays there are also sets of smaller silver contacts for ringing the bell and scoring.
Quoted from jrpinball:You don't need diodes for an EM application.
The diodes are used to save the bridge from damage when the coils magnetic field collapses, so I absolutely recommend them ...sure the game is an EM, but you just added a solid state component.
Quoted from PhilGreg:Hey I'm not saying I want to modify my EM, I want to drive EM coils with DC power for a different application, so I do want to use diodes.
I understand. Regardless of your application, the coil is a current operated device. It doesn't care what voltage (AC or DC) is impressed on it as long as the voltage level does not exceed the voltage rating of its magnet wire insulation. As such, you can apply a DC voltage to an AC coil. To get the AC coil to work on a DC system requires a sufficient DC voltage be impressed on the coil so that the same amount of current is drawn as when the coil is operated on AC.
When you apply a voltage to a coil is creates a magnetic field. When you remove the voltage the magnetic field collapses and creates a reverse polarity voltage and can be many times the value of the original applied voltage. This creates a transient voltage pulse that can damage other components in the circuit that are not rated for this polarity or the higher voltage created, things like semiconductor bridge rectifiers have a maximum voltage limit and breakdown if exceeded. Having a reversed biased diode across the coil allows the diode to conduct for reverse polarity voltages and creates a 'short circuit' across the coil that allows the pulse to be dissipated in the resistance of the coil wiring.
Quoted from jrpinball:I thought the diodes were added to protect the driver board and MPU board from back EMF damage in solid state games.
This is correct. In a solid state game, what are you protecting on the driver board? Transistors. In an EM game, If modifying an AC coil to use DC by using a semiconductor based bridge rectifier, it is also desirable to add the flyback diode. (transistors and bridge rectifiers are solid state devices)
http://en.wikipedia.org/wiki/Flyback_diode
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Berea, OH