2017 January 11    Tech   Electronics


The Hot Chassis article mentions the use of an "isolation transformer".

There are different types of isolation transformers out there, and some of them are not suitable for radio repair.  So, this article talks about simple continuity tests you can do.

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Continuity Test



In North America, the AC "ground" and "neutral" wires are bonded at the service entrance. 

That means, technically, that both wires will always have continuity.  Even though they look like distinct wires, they are really extensions of the same electrical conductor.

There's a simple reason why they do that.  If the "neutral" ever gets interrupted or has a bad connection, then the "ground" wire can still provide a return path for current.

This is a safety consideration, but sometimes it has no benefit.  If you have one hand on a "hot" wire, and you put the other hand on something that makes a connection to ground, you can get electrocuted.  Your heart could stop.  There are a lot of people who found this out the hard way;  they're dead now.  Water pipes, metal cabinets, radiators... anything that connects to earth ground can complete the circuit.

Even if you use an isolation transformer, you should get in the habit of keeping one hand in your pocket when working on anything that could have mains voltage.  It's still possible to complete the circuit within the radio, which means you could STILL get electrocuted.  That is why an isolation transformer is not foolproof.  Keep this in mind throughout this article.

If basic wiring and electricity are not your forte, it's a bad idea to work on line-powered electronics.  (Some of the guys on radio forums will pounce on you, but realize they just don't want you to fry yourself.)  Start with basic circuits using AA batteries and such, and learn electricity first.  If you really want to work on radios, start with battery-powered transistor ones that don't plug into the wall.  It's all about knowing where current flows, and why.

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What An Isolator Can't Do

Realize that an isolation transformer cannot eliminate all modes of electrocution.  You could still get fried if you make a mistake.

The radio will still have components at some AC voltage, relative to other components within the same radio (or the chassis).  If it's more than about 30 volts AC, consider it very dangerous.  If you bridge those objects, current will flow and you could get electrocuted.

So what's different when you use an isolation transformer?  Now, picking up a screwdriver that's on your radiator, metal table, etc, will no longer complete a circuit back to the mains ground.  Normally that's such a special danger because we don't think of "everything in the room" as potentially being part of a gigantic AC circuit.  A proper isolation transformer eliminates that problem.

The other reason for having an isolator is so that you don't destroy your oscilloscope or other test equipment.  Hot chassis radios can fry these devices, because they may have ground clips that attach to the radio itself.  And if the radio chassis is live, there goes your oscilloscope.  The thing is, even with an isolator, remember YOU could still get electrocuted.  A LIVE CIRCUIT CAN STILL BE LETHAL, EVEN WITH AN ISOLATION TRANSFORMER.  The isolation transformer does not render the radio harmless.  The oscilloscope chassis itself could be at 120 volts relative to the radio chassis, or other components.  That could still kill you if you do the wrong thing.

This is why I keep saying you shouldn't work on radios if you don't know basic electricity.  Not trying to be mean here, and I'm not trying to sound like a broken record.  You have to get this right.  Also, I recommend getting a quality multimeter like this one, or preferably this one, not some $10 knockoff that malfunctions.  The multimeter is your core piece of equipment. 

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Random Electrical Quiz!

Quick!  Here's a quiz.  Let's say I have a multimeter.  And let's suppose I'm about to test a live wire.  It's a copper wire, and for some reason it has no insulation on it for a few centimeters (not safe, but this is hypothetical). 

Now let's say we know that wire is supposed to carry line voltage, 125 volts AC.  And let's say I'm going to put the red multimeter probe on one end of that wire.  Then I'll put the black multimeter probe about a centimeter away, on the same wire.

What voltage do you think it will display?  You tell me:

A.  125 volts AC
B.  110 volts AC
C.  62.5 volts AC
D.  0 volts AC

Next question:  does that mean it's safe? 

I'm not going to make you guess that one.  The correct answer is "No".  However I'll leave you to explain why that is.

This is just one example of the types of knowledge you must have before you even think of working on mains-powered electronics. 

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Continuity Tests

There are two basic types of isolation transformer.  The common ones isolate RFI noise, but the secondary ground / neutral is still bonded to the primary ground / neutral.

Those are no good for radio repair.

Thinking of buying one?  Look at the schematic.  If you see a chassis-ground symbol on the secondary side of the transformer, and a chassis-ground symbol on the other side, it's no good for radio repair.  Probably if the schematic shows anything at all called "GROUND" or "GND" on the secondary side, you can assume it's connected to the primary ground.  That means it goes through the third wire to your service ground... which means it's not isolated.
A proper isolation transformer isolates the secondary side completely from the primary.  They are still inductively coupled, which allows the device to transmit the power to your electronics.  What is isolated, though, is the ground reference.  The ground on the secondary side is not at all linked to the earth ground.

Now, the continuity tests.  Quite simply, primary "anything" to secondary "anything" should be OPEN LINE.  No matter what you pick on the primary side-- be it hot, neutral, or ground-- it should be OPEN CIRCUIT to anything on the outputs.  No continuity whatsoever.  DC resistance should be infinite or "open line".

On a proper isolation transformer, neutral / ground on the secondary should not be connected at all to the primary.  The AC voltage on the secondary should have no earth reference. 

When it's powered on, turn your DMM to "volts AC".  Put one probe to the secondary hot and the other probe to the primary neutral.  You should read zero volts.

And by the way, proper isolation transformers often have two-prong outlets.  A third prong would be useless on the secondary. 

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In this article we saw how to test an isolation transformer.  The surest way to get the right kind is to buy a vintage one that was designed for radio repair.

And once again, don't overestimate what one of these can do.  It is very useful-- actually essential-- but it doesn't remove all danger of electrocution.  Be smart, be safe.

Hopefully you found some of this info useful.  If so, please help me keep this site on-line by using the links on here to purchase your gear!

Thanks for reading!


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