Can someone explain the significance of the isolation transformer?

[Hoagie_one]

I'm having a little trouble wrapping my mind around why these are necessary for arcade monitors.  I mean, power goes in, the same power goes out.  Why not just wire the arcade monitor direct?  Am I missing something?

[CheffoJeffo]

Randy Fromm wrote an article about it that I should check before posting, but my imprecise recollections will have to suffice for now.

The mains power coming out of the wall has the neutral connected to earth ground. "Hot chassis" monitors have the chassis connected to the hot through diodes, so plugging them directly into the wall can either short the diodes or leave the user prone to shock if the chassis became grounded. The isolation transformer provides the same voltage (or could be different, depending on the windings), without the connection between neutral and earth ground. Or something like that.

 

Hopefully someone better versed with electricity will fill in the gaps or correct my errors before I have to go look it up.

[Mario]

I'm having a little trouble wrapping my mind around why these are necessary for arcade monitors.  I mean, power goes in, the same power goes out.  Why not just wire the arcade monitor direct?  Am I missing something?

See http://en.wikipedia.org/wiki/Isolation_transformer
and this pic.

My understanding is that there is no physical connection between the two sides of the coils. The current is transferred from one coil to the other, but without the physical connection there is no chance (or reduced chance) of a ground short.

Hope this helps.

Mario

[castlesteve]

I can clear this up pretty easily.

1)  Older monitors that do not have switching power supplies require the inherent properties of an ISO T because you can sink a limited amount of current from them and then they will "brown out".  A cold monitor requires a good deal of current to get it up and running.  I've seen plenty of k7000s plugged directly into AC power and work.  The only reason is because the tube is old and requires less current to power up.  This is really a dumb idea though.  Most k7000 monitors blow their parts off and lift traces because of the amount of current that they sink during cold startup.  An ISO will limit the current and save the board during powerup.

2)  An ISO is a good way of isolating high frequency noise back into AC and the other components

3)  Most people say ISOs are put in for safety reasons... but it is just a bonus feature.

[MonMotha]

This gist of the actual answer is what Cheffo Jeffo said.  Older monitors simply take the input AC line and dump it into a rectifier to generate many voltages on the board.  This means that both power ground on the monitor and B+ (as well as other secondary voltages) are all referenced to the incoming AC line.  This means that they are all (including the metal frame of the monitor) "hot" in that they are live relative to earth ground, and one can pull a lot of current off of this.  This will happen no matter which way you hook up the AC line due to the way a bridge rectifier works.

Clearly, this is problematic.  It's a safety hazard, of course, and if your cabinet has power ground tied to earth ground (like many PCs do), you'll basically short out your AC input via your video wiring, monitor, and arcade boards.  That's likely to result in a pretty bad smell and some smoke.

The isolation transformer allows the AC input to "float" relative to earth ground.  This makes it safe to tie the frame of the monitor to earth ground, making everything safe to the touch.  It also removes the possiblity of the aforementioned ground loop, removing the smoke problem.

TVs often just deal with having a "hot chassis", and they use various techniques like high frequency AC coupling on video inputs (especially on TVs with only RF inputs) and plastic enclosures to make it safe.  An isolation transformer is standard equipment for most TV repair techs because it makes it safe to work on the monitor while plugged in, even though normally touching just about anything inside it would zap the bejeezus out of you if you were in contact with earth ground.

Monitors with a SMPS (switch mode power supply) can easily incorporate the necessary isolation into this power supply, and they therefore do so.

The inrush current and HF noise injection problems are just minor bonuses of using an isolation transformer, not the primary purpose.