Isolation transformer do most of you....

[sam67]

Isolation transformer do most of you guys own one ..I want to try some voltage measurements on a monitor i have just picked up ,do i need to have an isolation transformer or can i open the back of the lcd .. I know where i am going to test for voltages and i do have a steady hand but wondered about using one but i cannot seem to find that many that are cheapish over here in uk.

[PlainBill]

Originally Posted by sam67

Isolation transformer do most of you guys own one ..I want to try some voltage measurements on a monitor i have just picked up ,do i need to have an isolation transformer or can i open the back of the lcd .. I know where i am going to test for voltages and i do have a steady hand but wondered about using one but i cannot seem to find that many that are cheapish over here in uk.

In most cases an isolation transformer is unnecessary. A monitor uses a power supply design that isolates most of the circuitry from the power line. As is usually the case, there are exceptions. The input side of the power supply is not isolated, obviously.

Even then, if you are using a battery operated DMM you can still measure the voltages present at any point in the supply without problems. It IS recommended you use the 'one hand in pocket' rule, of course.

The only time an isolation transformer is absolutely necessary is if you are using a line-powered piece of test equipment (oscilloscope) and are checking signals on the primary side of the power transformer, or you are working on one of the designs that does not isolate the circuitry from the AC line (most TVs from the 70's had such a design).

I would point out that depending on an isolation transformer is not a good idea. For example, if the line voltage is 220 volts, the filter cap in a monitor will have more than 300 volts on it. Isolation transformer or not, touching one of the leads could be painful.

PlainBill

[sam67]

Thank you plainbill ,

You are a very knowledgeable man and i often read your posts on other threads when you are trouble shooting for others.Could i ask you a quick question about the lcd monitor.

When doing voltage tests with a dvm is it better to test the monitor while connected to the pc or does it not matter .Just curious about this in case it could affect any out come.

Thanks

sam67

[Krankshaft]

It depends on what circuits you're checking.

For instance on some monitors the mainboard sends a signal to the PSU to come on. If a video source isn't detected it shuts the inverter down within a few seconds. So having a source is the best way to go if you want the inverter to stay on during testing.

As for an isolation transformer while it's true that it's indispensable when using a scope or other line powered stuff on unisolated electronics.

It can also protect you from live heatsinks like the B+ 130+ voltage regulator (for the flyback HOT collector) in CRT TV sets. I have quite a few sets that have this guy on a nice large aluminum heatsink plenty of surface area to brush up against it.

For a more relevant example on most computer PSUs and switchers in LCD monitors the switching FET sinks can also be live. Unless the FET has a fully insulated package (plastic encapsulated all around the front and back) then assume the heatsink is live.

An isolation transformer isn't perfect either it will only protect you from one point of contact. If you were to touch a live heatsink with one hand and another point in the circuit with another simultaneously you would still get a shock.

If you're ever in the market for one this is the one I use:

http://www.mcmelectronics.com/produc...-6670-/72-6670

Inexpensive and comes in a nice self contained case 4 Amps of isolation is more than enough for most consumer electronics. If you ever decide to get a variac be sure to get one of these as well. Since variacs are unisolated.

[Krankshaft]

Some boards will also have a silkscreened line across the board one will say COLD and the other HOT.

This denotes the Primary (HOT) and Secondary side (COLD). The cold side is isolated while the HOT side is not. With an Iso plugged in the HOT side will be isolated as well.

Without it touching one point on the HOT side will result in a shock.

As long as you keep in mind where your hands are on live equipment you will be fine.

The only real reason I have an Iso is because I work on old hot chassis tube radios and use my scope regularly. The scope cost me 250 bucks so I'd be crazy to not back it up with 63 bucks worth of insurance .

I hope I explained this clearly enough there is so little info on ISOs on the net short of "they make servicing safer". Any information out there is very fragmented.

[sam67]

Thanks you have been more than helpful ..I am just about to open this acer monitor and check a few things ..thats if i can work out how to open the dam thing as there are no visible screws that i can see lol.

[Krankshaft]

If they are not visible any screws will be under the monitors base mount. After the base is off they usually like to hide some screws under it.

From there the case is probably snap together I've used a the bottle opener on a Swiss army knife to open Dells LCDs before.

Stuck it through the vent holes which gave me access to push the tabs one at a time to release the back.

[stj]

well a monitor (crt type) can kill you even with a transformer,
and a shorted device could damage one anyway.

people i know put a 500w halogen floodlamp inline with the power as a surge/short protector.

and i use an earth-leakage breaker.
it probably wont trip if you touch the dc side, but touch the ac and it will cut the power before you even feel it - guess how i know!

[PlainBill]

Originally Posted by stj

well a monitor (crt type) can kill you even with a transformer,
and a shorted device could damage one anyway.

people i know put a 500w halogen floodlamp inline with the power as a surge/short protector.

and i use an earth-leakage breaker.
it probably wont trip if you touch the dc side, but touch the ac and it will cut the power before you even feel it - guess how i know!

I'm not so concerned about even feeling it. I'm more concerned about never knowing I felt it!!

PlainBill

[Krankshaft]

I wouldn't put so much faith in earth leakage breakers we call them GFCIs in the US.

I had an AC induction motor connected to one once where there was a hot to ground leak. When I touched the motors body (it was running) I got a shock and the GFCI never tripped. I tested the GFCI later and it was fine.

Turns out an old knockout clamp was overtightened on the Romex and just pulling out the wires in the box to work on them was enough to compromise the already damaged insulation.

No protection device is 100% safe so be careful.

[stj]

well just remember that if i get a shock, it's 230v - not the pussy-110v that you get hit with.

last time i got thrown about 5 feet backwards!

[Krankshaft]

Haha I forgot about that .

I have a 220 circuit in my house for baseboard heating and I was real careful when I had to measure it. Never been bit by 220 and never plan to.

The US pole transformer secondaries are wired like this with the center tap grounded.

------------ 220V


------------ Center Tap GND (grounded by grounding rod on pole)


------------ 220V

To get 220 you simply tap the two ends of the secondary. Any 120V circuits run on either end of the secondary winding and the center ground tap.

[Krankshaft]

Well 240 is really more accurate since the average line voltage is 120 volts here now.

[stj]

i like 415v 3phase - that really makes you pay attention to where your screwdriver is!
we have that on industrial motor circuits.