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.

[OldReno]

Thanks for the information, I do appreciate it very much.  So, then the diodes help with collapsing coils then...and keep spikes from that from getting to the machine.
The parts may be from a pinball, as I don't recall this setup on other em's.  But then again, it has been a very very long time since I worked on a double progressive, and they MAY have used this set up.  I don't know, because I don't have schematic on it either.
I do notice that the step up relay controls not only the odds step up unit (disc) but also simultaneously steps up another odds type looking unit that allows for the progressives to advance, one step for each coin inserted.  So then, our step up relay is actually running two fairly heavy duty coils (solenoids) at the same time.  Is this perhaps why the diodes are there???  Certainly there are no IC's or transistors in the machine that I have seen.  Perhaps bally could have had one coil going off allowing the other coil to turn on.  Or some kind of delay.  I notice that the coils that are working in this whole set up are VERY loud and do seem to use quite a bit of energy.
I do very much appreciate your tutorial on all this, it is most informative...!!!  Thanks again.

[rokgpsman]

Like The Fatman said earlier, if those relay coils with the diode across them don't look like original parts and someone has replaced them somewhere along the way they may have used whatever relay they could get and perhaps it was made for more modern slot machines or pinball machines. Nowadays a lot of coils come with the diode already on them since the newer game equipment that has ic's and transistors would need the diode on the coils. You can snip out the diode that's across the coil if you think it is causing a problem if there are no circuit boards or ic's and transistors in your machine that are connected to the coil. Without the wiring diagram or schematic it is difficult to know for sure.

If you have another similar machine or know where one is maybe you can look at it or someone here on NLG that has one can take a picture and see if the coils in it have the diodes. If you want to check the diode you can use a meter that has a diode function or an ohm (resistance) function. With power off and one end of the diode disconnected from everything (to eliminate stray paths that will affect the reading) you measure the resistance thru the diode by putting a meter lead on each side of the diode. Then reverse the meter leads and measure again. The resistance should be much higher one way than the other, like 10 times as much or even greater. This test works on most types of diodes (but not zener). The diodes are inexpensive and can be replaced fairly easily. But maybe they shouldn't even be in your machine and can be removed.

I know you've got a lot of experience working on these older slot machines, any idea what year this machine is from?

[Op-Bell]

It's a delayed dropout circuit The parallel diode acts like a flywheel, allowing current (created by the collapsing field) to continue to flow in the coil and hold the solenoid in. You've seen something like it a hundred times, in the fat resistor across the coil of the win relay on EMs. In that case the resistor acts both to hold the relay in during momentary breaks as the wiper moves on the disk, and also to limit the voltage and reduce arcing that could damage the tracks. A resistor is less than ideal, but it's all you can use on AC. The series diode in this case converts the circuit to DC so that the parallel diode can do the job.

Since there are two solenoids on that circuit and the other one doesn't have a diode, I suspect that there's a timing race condition, that the other solenoid has to complete its stroke momentarily before this one does. Since the wipers move on release, a slight delay on this one means the other should have settled and completed its circuit before this one moves.

Note! Normally you should never run an AC solenoid or relay on DC. AC coils are very low DC resistance and rely on the inductance to limit the AC current. If you power them with DC they quickly overheat and burn out. You'll probably find this coil is different from the others and is rated for 24V DC.

[slotsteve]

we used the same ac coil in a  late model pinball that took  dc coil for years  by just adding a  diode , on a old org  em slot there is no dc  that I ever saw  unless it was a conversion  like a summit

[OldReno]

Op-Bell, thank you, stellar information, and I'll digest that a couple of times.
Makes sense.
It appears that this 4 reel fruit double progressive advances one penny every nickel put in.
That's a 20% loss right away, excluding the non p[rogressive hold % which I haven't calculated yet.  Gotta be in the negative territory,
So I suspect the odds increase % is incorrect.  20 cents of a dollar in going to jackpot seems a bit whack.
The odds disc step up solenoid fires at about the same time as the progressive controller unit and board.  They're both beefy and certainly draw a lot of energy in a short time. 
I wonder if the diodes were an aftermarket fix for that...?
I still recall seeing nickels welded to the frame by shorting across the coin lockout coil.
And this helps explain too, some of these exotic coil numbers.
Appreciate your input on all this, and I will read it again.
Thank you Steve also.  So guys, AC coils run longer as DC?  Do they use less current?
Could I cut down the kick on those solenoids by putting a diode in to one side of lead in series?  I'm just thinking it seems so harsh how hard they kick.  AND for every coin in, too.
Fascinating...

[Op-Bell]

A few more words on the difference between AC and DC coils and relays. First some definitions.

The force applied by an electromagnet (magneto-motive force, or MMF for short) is measured in ampere-turns. That is, the current in the coil multiplied by the number of turns of wire - the number of times the current is made to go round the coil. Each time it goes round, a unit of current adds a unit increment of MMF. This is a design constant, based on what you expect the coil to do for you, and the number of turns and the wire gauge are selected accordingly. In a DC relay, the current is limited by the resistance of the wire.

In an AC relay or solenoid, the current is limited not only by the resistance of the wire, but also by the inductance. The inductive impedance is 2.pi.f.L, or at 60 Hertz, approximately 380 ohms per Henry (the Henry is the unit of inductance). This is quite a significant amount - in the case of a large 110VAC solenoid its DC resistance may be something like 20 ohms but its inductance could well be 5 Henrys, so the effective resistance to AC (called the reactance) would be more like 2000 ohms. Most smaller AC relays and solenoids don't have such a large difference, but it's still nearly always more than 2 to 1. So if you apply 50 volts DC to a 50 volts AC solenoid it will be pulling twice the current it's designed for, and will have a merry but short life. If the DC is supplied through a single diode, a half wave rectifier, it's only conducting half the time and the average current is half, so this generally works out ok. BUT - it won't work so well as if you drive it with AC, because ...

There's another characteristic of AC solenoids that makes them particularly suitable for our purposes. The full inductance is only achieved when the magnetic circuit is completed, that is, when the armature is fully home. In the unenergized state the inductance is only a fraction of its final value. That means when you first apply AC to (say) a step-up coil, the current is only limited by the DC resistance, so it's much higher than it would be if the inductance were constant. More current, same number of turns, means the initial MMF is much higher than it would be with a DC coil and the armature moves in with great power. When it's fully home and hits the stop, the inductive reactance dominates and the holding current falls to a lower value.

Here's a worked example. I have here a 50V pinball score unit. The step-up coil has a DC resistance of 28 ohms and unenergized, an inductance of 55 milliHenrys. When 50VAC is applied the initial current is approx. 50/(28+10) = 1.3A. Once the armature is fully home in the core, the inductance rises to 200mH, so the hold-in current is approx. 50/(28+40) = 0.75A. The difference is not that great with this small coil, but in the case of a large coil with thick wire and not much resistance the difference can be five or ten times. That means if the armature gets stuck and can't pull in, the coil will rapidly overheat and burn out. It's a common cause of failure.

The other difference between AC and DC relays and solenoids is magnetic remanence, the tendency to behave like a permanent magnet and stick in the energized position. DC relays and solenoids go to some trouble to avoid this, through special selection of magnetic alloys and other tricks. AC, not so much, because the constant reversal of the magnetic field tends to demagnetize the core. So they tend to be made with less expensive iron that, if not constantly demagnetized, can turn into permanent magnets and leave you with a solenoid that won't let go when you turn off the current. It's happened to me, and that's the main reason I wouldn't put an AC relay in a DC circuit.

[David B Fowler]

OldReno ... you do know that the double progressive increases 1 penny for every nickle, but it alternates the jackpots. That means it is only a 10% JP payout because it only awards the one that is selected when the JP is hit.
Let me know if my mind is working well today or I am in a fog. 
Dave

[slotsteve]

also it may have been a quarter slot at onetime

[OldReno]

Op-Bell, that's great information.  I will read this whole thread a couple more times, and will certainly have a better understanding of this all.  Thanks again.
Dave, yes I realized it shared the progressive between two top wins, just after I posted.  Still, it seems extreme, and I'll have to calculate the odds I guess and see.
Steve, it makes more sense to have been a quarter machine,

Thank you ALL for your wonderful input on this, I am very appreciative.

Now I have to figure out why the odds step up and odds reset relays are staying on... Those old progressive were a PITA, but they got some play.