Old Reno is stumped
OldReno:
I'm working on a real nice 4 reel double progressive. The odds step up relay, and the odds reset relay both will lock up (energized) when the coin switch is actuated. It's probably the odds disc reset solenoid, which I will fix, but here is a question someone good in electronics might be able to answer for me.
The odds step up relay has a diode across the coil, and a diode inline with the coil (series). Will someone explain, please why this is????
rokgpsman:
I'll take a stab at this to get the ball rolling.
The diode across the relay coil is common in circuits or systems that have more delicate electrical parts like transistors, ic's, etc along with rugged things like relay coils. It is a protection diode. This parallel diode connected across the coil suppresses (shorts out) the voltage spike that occurs when the relay is de-energized (powered down). When a coil is energized it has a magnetic field around it due to the electrical current (electricity) flowing thru the wires wrapped in circles around the coil. When the power is removed from the coil that magnetic field collapses quickly and that temporarily induces a reverse current to flow in the coil for a short period of time, even though there is no power applied to the coil. (It is similar to turning a motor by hand and generating electricity in the windings. Magnetism can create electricity in wire if either the wire is moving or the magnetic field is moving/changing). The reverse current flow in the coil is accompanied by a voltage spike and this voltage is called "back emf" or back electromotive force. It can be dangerous to other electrical parts since it is a much higher than normal voltage, it is higher due to how quickly the magnetic field collapses. The diode across the relay coil isn't to protect the relay coil, it is to protect other electrical parts in the system such as ic's, transistors, etc.
This voltage spike that is caused by the coil's collapsing magnetic field could flow back thru the power wire connected to the coil and damage other circuits, like if a transistor or ic was controlling the coil power wire. The diode connected across the coil will conduct and short out this voltage spike, keeping it from going anywhere. Notice that this diode is connected in reverse polarity so that it does not conduct when the coil is energized. The banded end (negative end) of the diode is usually connected to the same coil terminal as the power wire which will be a positive voltage. Having a diode connected like this across the coil also implies that the power to energize the relay coil is DC, since if it was AC power the diode would conduct when it is not supposed to, as the AC goes from positive to negative cycles.
This is where the other diode comes in. It is connected in series to change the power voltage in the wire that goes to the coil from AC to DC. The diode in series is a rectifier, only allows electricity to flow in one direction, that's how it changes the AC to DC. The DC voltage out of this series diode goes to the relay coil to energize it.
So you have a series diode that changes the relay coil power voltage from AC to DC. And another diode connected across the coil (in parallel with the coil) that conducts for an instant or two to short out the voltage spike when the relay coil is powered down. This parallel diode is a protection diode, if it fails you can have damage to the circuits that control the coil since voltage spikes can travel back down the coil's power wire every time the coil is powered down. A meter isn't fast enough to measure or "see" these voltage spikes, but an o'scope can clearly show them and they can be 4 or 5 times higher than the normal voltage on that coil power wire.
The relay coil would energize if it had either AC or DC voltage applied to it, doesn't care. But the designer of the machine wanted to have the protection diode on the relay coil to protect something else in the circuit, like a transistor or ic. But to have the protection diode across the coil means the coil has to be powered by DC. That's why they also added the series diode. So if you have a failure of the relay coil driver (transistor, FET, ic, whatever) you would want to check & replace the protection diode across the relay coil since it may have also failed, allowing the voltage spike to damage the relay coil driver.
Since I don't have the schematic or wiring diagram of your circuit some of this may not exactly apply but I think it will be pretty close and give you an idea of what's going on. Others here can add to or correct any of this, it's been a while since I was involved with this stuff.
ramegoom:
^^Exactly right, as rokgpsman stated. And very easy to understand. I might add, though, that if there is a wire on each side of the coil terminals, in addition to the series diode, it would indicate that there may be a secondary source to fire that coil in addition to the primary source. The "forward" diode would be there to prevent electrical feedback to the primary source, if and when the secondary source is energized. And vice-versa.
Clear as mud, right?
rokgpsman:
Yes, that would be like an isolation diode, to allow more than one circuit to control the same relay coil without interfering with the other control circuit. We used to call those "wired OR" circuits since it mimics the way an OR gate works.
David B Fowler:
Could be that someone replaced the original coil with a pinball coil from a solid state machine and didn't remove the diode from it. That is if the machine is a EM slot.
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