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I have an 20v power adaptor. But I noticed that the output +ve and -ve terminals are not isolated. I mean if I measure the resistance between +ve and -ve terminals, it tells 10k ohms and keeps on rising. If I could find out anything on google it was some comparison between isolated and non-isolated converters. But I could not relate the info in my scenario. Any help would be great.

Edit: If I measure by keeping multimeter on 200k limit, it starts at 10k but if I switch it to 2000k limit, it starts from 100k.

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4 Answers 4

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The rising resistance is because the voltage at the meter is increasing as the supply output capacitor is charging from the meter. Behind the capacitor will be some sort of regulator which will be un-powered and therefore be high impedance.

A meter on ohms range applies a small fixed current to the load and then measures the output voltage. This gives resistance. If you connect your meter to a discharged capacitor it starts at a low reading because the capacitor voltage is low and then increases as the capacitor charges.

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I have an 20v power adaptor. But I noticed that the output +ve and -ve terminals are not isolated.

That's right; they shouldn't be isolated from each other. If they were isolated from each other then forming a circuit by plugging it into a load would cause zero amps to flow. You don't want that from any power supply and you don't want the output terminals to be isolated from each other.

You want the output terminals to be isolated from the input AC terminals but that's a different story.

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An ideal voltage source has output impedance of 0 ohms. Your power supply is just unpowered.

The multimeter measures resistance usually by feeding small constant current into the measured device, and then measuring the voltage. Thus voltage is proportional to the resistance connected.

What happens is that the multimeter is back-feeding current into the output capacitor and it slowly starts to charge up voltage. The multimeter just measures this voltage and displays a number based on that.

Basically, what you are seeing is expected from a power supply that is unpowered, and the output is not short-circuited.

So that does not measure if the output is isolated from mains or not.

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  • \$\begingroup\$ But if I plug it in, it starts from 20v and slowly rises again. Shouldn't the capacitor be charged immediately? It rises up to 30v. It was slow so I stopped measuring after 30v. \$\endgroup\$ Commented Jun 24, 2020 at 12:06
  • \$\begingroup\$ How does that relate to measuring resistance any more? You are now measuring output voltage. If you have a new question, start a new question. Yes, it should be 20V, but we don't know what power supply it is or how you are measuring it, so it's your responsibility to add details. \$\endgroup\$
    – Justme
    Commented Jun 24, 2020 at 12:12
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Power supply isolation refers to isolation between the input and the ouput, not between the two terminals of the output as you've assumed.

There is no way to determine of a power supply is isolated or not by only measuring the output terminals. Whether it is isolated or not makes no difference on the output alone. Whatever you're measuring is going to be meaningless anyway, as the multimeter is being fooled by passive and likely active circuitry inside the power supply that is being engaged ever so slightly by the tiny amount of current the multimeter is injecting into the power supply's output (at least in resistance measuring mode). Regardless, there is nothing particularly meaningful to any resistance measurement on the output of a power supply like that. It is not really measuring resistance.

The difference between an isolated and non-isolated supply occurs strictly between the input and output of the power supply. With an isolated supply, you should measure the resistance (with the power supply unplugged/turned off) between either of the inputs and each output as open circuit. If it is mains powered and has a protective ground connection, this is typically bonded to the ground of the output as well (in a non-isolated supply).

Another example of an isolated supply is a battery. It provides energy by itself, and has no input connection, and thus there is no input to which it can or cannot be isolated from. So it is isolated by nature. This is why you can connect battery cells in series to make a higher voltage battery.

If you attempted this with non-isolated power supplies and their inputs were in any way connected to each other (like both plugged into the wall), then their outputs are also effectively going to be connected (as if in parallel), so if you try to put them in series, you'll just short the positive to the ground, but between two power supplies.

Another way to check is with two power supplies turned on (as isolation doesn't really have much meaning if you don't have two things with which there can be isolation between), check the voltage between the ground/negative terminal of one power supply, and the positive terminal of the second power supply. If they aren't isolated, you'll see a voltage across those terminals, even though there is no connection visible (there is one, just internal and back through the inputs). However, if you don't see any voltage, that doesn't necessarily mean they are isolated. You need to make sure there is no voltage between a terminal of one supply and both terminals of the other supply, one at a time. You'll likely see either no voltage between one pair of terminals, but will see a voltage between the second pair, depending on relative polarity of the power supplies. If you see a voltage between the two supplies but their output terminals are not connected in anyway to each other, then the supplies are not isolated.

Two isolated supplies will behave, for all intents and purposes, like two batteries (which are themselves isolated).

The safest way is to check for continuity between all combinations of terminals but always between two power supplies (multimeter leads should always be going to a terminal of a different power supply than the other multimeter lead) while they are turned off. If they are isolated, all combinations should be open circuit.

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