Electronics > Beginners
Classic don't-blow-up-my-scope question
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joeyjoejoe:
I have a 12V AC wallwart. Since it has a transformer inside, would it be safe to scope it? Would it be effectively isolate from ground?

Second question... if not, and/or for other devices, is there a cheap way to buy or make an isolation transformer? Amazon has a 250W isolation transformer for 200 bucks! A bit pricey.
Psi:
Yep, it's safe to scope the low voltage side. Not on the high voltage mains side.

If you keep looking your will probably find a cheap isolating transformer on ebay eventually.

You can make an isolating transformer from two mains transformer of the same type.
But you will need to put it in a box and make it safe etc..

It's usually easier to find two random mains transformer than one isolating transformer.
If you found two 110V ->50V transformers you could connect them in a step down then step up combo.
input 110V -> 50 output then feed that into 50V of second transformer.
The output of the second one would be 110V again.

Ya just have to be careful about the wattage, and obviously make sure you get them around the right way. 110V into 50V winding will end badly.
Also don't use anything under like 12V on the low voltage side, ideally you want as high as possible.
The amps gets really high if the voltage is too low.
sambonator:

--- Quote from: joeyjoejoe on February 19, 2019, 11:19:40 pm ---I have a 12V AC wallwart. Since it has a transformer inside, would it be safe to scope it? Would it be effectively isolate from ground?

Second question... if not, and/or for other devices, is there a cheap way to buy or make an isolation transformer? Amazon has a 250W isolation transformer for 200 bucks! A bit pricey.

--- End quote ---

Digikey has one for $87.
https://www.digikey.com/product-detail/en/hammond-manufacturing/171A/HM2103-ND
rstofer:
Safe?  Maybe not...  Watch Dave's video on "How Not To Blow Up Your Scope"

If one side of the secondary is grounded then that is the lead that you need to hook to probe ground.  Maybe one way to check is to measure, with a DMM, the voltage on each lead with reference to earth ground.  On a grounded supply, one lead will read very close to 0V (grounded) and the other won't.  I would expect the transformer to be ungrounded so both leads will measure 'something' relative to earth ground but the value will be nonsensical and perhaps erratic.

I wouldn't expect one side of the secondary to be grounded but it is possible.

It isn't the probe that is the problem, it is that darn ground lead.  By definition, it is earth grounded through the BNC connector and through the power cord.  You have to be very careful where you place it.
joeyjoejoe:
Checking resistance between the tip/barrel and both prongs of the wall wart show open circuit, so probably good.

However, thanks for linking that transformer. That's a reasonable price for something that will prevent me blowing up my scope. Arrow has it a few bucks cheaper, and has a 200VA model for only a few bucks more. Trying to think of if the lab warrants one and how much I would use it.
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