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zebulon

Pinball Electrical 101....an Addendum

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Recently, while reading the threads and troubleshooting issue with members, I’ve noticed a growing trend in faults coming back to the same issue. I’m hoping that this post will help to clear up some of the mystery.

Please bear in mind that I’m trying to write this to be as clear as possible for both experienced and novice builders…..:)

 

Electromagnetic Interference (EMI) / Radio Frequency Interference (RFI)


 

Something to be aware of when building a cab is the RFI that everything electronic and electrical produces. When tvs are decased, in most instances some of the shielding that is designed into them is removed. As well, when you have multiple electrical appliances (tvs, amps, computer, power supplies) plugged into the same source, everything shares the noise that they generate with everything else.  Microcontrollers like what is used on the ledwiz, the virtua-pin plunger etc are very susceptible to this noise.

 

A great primer on what RFI/EMI is can be found here…http://www.on4ww.be/emi-rfi.html

 

In a nutshell, anything that operates on or conducts the movement of electrons generates interference (noise).

 

Consider an actual pinball machine. 

 

If you ever look inside an actual pinball machine, you'll see that there is a ground bus that is used to bond everything metal inside the cab and up in the head. As well, in the head where all the circuitry is, they have metal lined it to shield it from the noise produced by all the sparking from the coils firing and switches opening and closing.

 

All of that activity is going on 3-4' away from the control boards.

Now look inside your cab……..

 

Bonding vs Earthing vs Grounding

 


 

 Bonding:

 

Bonding is simply the act of joining two electrical conductors together. These may be two wires, a wire and a pipe, or these may be two Equipments.

 

Bonding has to be done by connecting of all the metal parts that are not supposed to be carrying current during normal operations to bringing them to the same electrical potential.

 

Bonding ensures that these two things which are bonded will be at the same electrical potential. That means we would not get electricity building up in one equipment or between two different equipment. No current flow can take place between two bonded bodies because they have the same potential.

 

Bonding, itself, does not protect anything. However, if one of those boxes is earthed there can be no electrical energy build-up. If the grounded box is bonded to the other box, the other box is also at zero electrical potential.

 

It protects equipment & Person by reducing current flow between pieces of equipment at different potentials.

 

The primary reason for bonding is personnel safety, so someone touching two pieces of equipment at the same time does not receive a shock by becoming the path of equalization if they happen to be at different potentials.

 

The Second reason has to do with what happens if Phase conductor may be touched an external metal part. The bonding helps to create a low impedance path back to the source. This will force a large current to flow, which in turn will cause the breaker to trip. In other words, bonding is there to allow a breaker to trip and thereby to terminate a fault.

 

Bonding to electrical earth is used extensively to ensure that all conductors (person, surface and product) are at the same electrical potential.  When all conductors are at the same potential no discharge can occur.  

 

Earthing:

 

Earthing means connecting the dead part (it means the part which does not carries current under normal condition) to the earth for example electrical equipment’s frames, enclosures, supports etc.

 

The purpose of earthing is to minimize risk of receiving an electric shock if touching metal parts when a fault is present. Generally green wire is used for this as a nomenclature.

 

Under fault conditions the non-current carrying metal parts of an electrical installation such as frames, enclosures, supports, fencing etc. may attain high potential with respect to ground so that any person or stray animal touching these or approaching these will be subjected to potential difference which may result in the flow of a current through the body of the person or the animal of such a value as may prove fatal.

 

To avoid this non-current carrying metal parts of the electrical system are connected to the general mass of earth by means of an earthing system comprising of earth conductors to conduct the fault currents safely to the ground.

 

Earthing has been accomplished through bonding of a metallic system to earth. It is normally achieved by inserting ground rods or other electrodes deep inside earth.

 

Earthing is to ensure safety or Protection of electrical equipment and Human by discharging the electrical energy to the earth.

 

Grounding:

 

Grounding means connecting the live part (it means the part which carries current under normal condition) to the earth for example neutral of power transformer.

 

Grounding is done for the protections of power system equipment and to provide an effective return path from the machine to the power source. For example grounding of neutral point of a star connected transformer.

 

Grounding refers the current carrying part of the system such as neutral (of the transformer or generator).

 

Because of lightening, line surges or unintentional contact with other high voltage lines, dangerously high voltages can develop in the electrical distribution system wires. Grounding provides a safe, alternate path around the electrical system of your house thus minimizing damage from such occurrences.

 

Generally Black wire is used for this as a nomenclature.

 

All electrical/electronic circuits (AC & DC) need a reference potential (zero volts) which is called ground in order to make possible the current flow from generator to load. Ground is May or May not be earthed. In Electrical Power distribution it is either earthed at distribution Point or at Consumer end but it is not earthed in Automobile( for instance all vehicles’ electrical circuits have ground connected to the chassis and metallic body that are insulated from earth through tires). There may exist a neutral to ground voltage due to voltage drop in the wiring, thus neutral does not necessarily have to be at ground potential.

 

In a properly balanced system, the phase currents balance each other, so that the total neutral current is also zero. For individual systems, this is not completely possible, but we strive to come close in aggregate. This balancing allows maximum efficiency of the distribution transformer’s secondary winding

 

 

What does this mean????

 

I’ll put it less confusing terms.

 

Bonding is running a bus, cable, sheet or braid of conductive material (let’s say wire) from part to part to ensure that all the parts in the chain are connected. Ever noticed that the standoffs on the back of the tv in your pincab are metal and screwed into the sheet metal of the frame of the tv? That’s bonding. The metal screw, metal standoff and metal frame are acting together to get the ground on the circuit board to the ground on the frame from where it will eventually bond to another circuit board.

 

Earthing and grounding are essentially the same. There is a minor variance, but we’ll ignore that as none of us are splitting atoms in our pincabs. Earthing for our purposes is providing a safe and effective path for the completion of the circuit so that the tv and computer turns on and we can push a button on the coindoor without becoming part of the circuit and frying ourselves like a strip of bacon.

 

OK…I’ve gotten this far…so what???

 

Many are ignoring the basics of what will make their cabs run efficiently and trouble-free.

 

Some tips……

 

Wiring

 

We’ve all seen these pics right?

 

Indian+power+lines.jpg

 

power-lines.jpg

 

 

Somehow this works for them…but probably not very efficiently. Amazingly enough, they are still able to get telephone communication signals through that mess so that they can answer your call for support for your Microsoft product that you’re having trouble with…:D

 

Wiring your cab like the above pics will not work for very long and will be impossible to troubleshoot.

 

When I was young, cute and full of promise I worked as a car audio installer. We had a rule of thumb that signal lines and power lines didn’t mix. Ever had a car stereo that emitted a whine in synch with the engines RPM? That’s why. Having heavy draw power lines next to lighter amperage signal lines allows for noise (EMI/RFI) to be injected into the signal line…it doesn’t have the strength to fight it off. Now we’re not worrying so much about sound signals in a pincab…but the theory still applies.

 

I recently built myself a cnc router with a variable speed inverter from China to power the spindle. Maxxsinner thoughtfully provided me with the shielded cable to connect the inverter to the spindle so I will go out on a limb here and guess that it wasn’t cheap cable (he is after all a cabbie in Queens, NYC who likes to affect an Australian accent and pretend to be an electrician…:D

 

The machine is bonded, the cable is bonded and still to this day, if I move one of the proximity switch wires in the cable carrier by even 2mm or more, the limit switch will trigger when the spindle turns on and throw the machine into reset.

 

The point I’m trying to make is to keep your wiring as tidy and logical as possible and that when things go wrong…try changing the placement of the component or wiring for the component. You may find that it’s just a matter of mm’s to get away from the noise that is giving you trouble.

 

Bonding

 

Back to that again????

 

Yup.

 

Bond the metal frame of your coindoor, the legs of your pincab, the frames of your noisy, decased tv’s, the can on the shaker motor, the case of your power supply, the metal casing of your pc……if it’s metal, connect it.

 

OK..one proviso here…don’t bond the metal of heatsinks…a lot of the time the heatsink is directly connected to the transistor and therefore live (has voltage on it)….you’ll blow the hell out of it.

 

When you bond large components or span large distances…don’t use 24awg wire and expect it to be effective. Use a decent braided copper braid available at most electrical stores and automotive electrical suppliers. Ensure that there is a good, clean metal to metal connection (scrape some paint if you have to) and use a star washer where possible.

 

Placement

 

Consider the possible effect of where you locate your components. Think of every wire as a potential route for noise to get into a circuit and place your components and wire looms accordingly. 

 

That contactor for the slingshot right above the ledwiz is an electromagnet strong enough to hold 6 or 8 heavy spring-loaded contacts together when it is energized. That magnetic field is going to play hell with the microcontroller on the ledwiz underneath it.

 

That shaker motor is an electromagnet when energized, the same scenario applies there.

 

That television without any shielding is broadcasting a huge amount of EMI/RFI. The inverters that fire the cathode tubes that are used for backlighting are a huge source of noise for the circuitry inside the tv, let alone what we are putting around it in our cabs.

 

Is the plunger too close to it? 

 

Should I really cable tie the power wires for my booster board to the underside of it? 

 

Shielding

 

If you have the option, keep any shielding (the metal flashing behind the tv, covering the pcb, or whatever) intact when removing bezels, cases, etc. This stuff was designed to be there for a reason, and that reason may be why you are having troubles with your ledwiz or plunger keeps cutting out.

 

Even better, if you have the ability, make some shielding for circuits that don’t have any (booster boards, dual H bridge drivers, etc). Light gauge, ferrous metal is all that’s needed. Place it around the component in question (without shorting it out) and add it to your bonding chain.

 

Metal project boxes from electronics supply houses and surplus shops, cough drop tins, coffee cans….all are great solutions to encasing and shielding circuitry from EMI/RFI if properly bonded.

 

Summary

 

Nothing is harder or more frustrating for you than to try and troubleshoot an issue that is/may be related to EMI/RFI. 

 

You’ve spent thousands on the best parts, read years worth of development and build experiences, spoken to many experienced and novice builders alike…..and still it doesn’t work. 

 

You’re looking for an issue that you can’t see without specialized equipment in a sea of components that have been modified or built to just work. 

 

The best defense in this case is a good offense and if you plan out your build with a few of these considerations in mind you could potentially be saving yourself hours of frustration.

 

I hope this helps some of you out there.

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