Author Topic: 12VAC power adapter - power output is shorted to ground - is this normal?  (Read 424 times)

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Offline c0ntr4d1ct10n

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I have bought two 12VAC  (i.e. 240V AC to 12V AC) adapters from two different sources (they are different manufacturers).

They have the removable plugs, so when I use a multimeter with one lead into the power output hole and the other lead into the ground hole, I get a short (or about 2ohms of resistance). This is all without the adapters connected to anything (unpowered and all).

When I do this same test for a 12VDC adapter, there is no continuity. It is a few k's resistance which rises.

Can someone explain to me if this is normal for AC power?

The reason I am concerned to hook this up to a diy PSU that I built (which the instructions say that it runs on 12VAC and specifies clearly that I must use an AC/AC power supply), is because when I first hooked up the first 12VAC adapter, a large 4700uf electrolytic capacitor started smoking and leaking. Note that the circuit was still working during this (the LED display was still on and functioning). The interesting thing was, before I bought the AC adapter, I was using a 12VDC adapter which did not cause any problems and the circuit was actually working. After removing the capacitor, I noticed that there was a short between +12V and ground. This was when it was powered off (I didn't dare power it on again).

At first I suspected that it may have been a faulty AC adapter; then, I feared that something in my circuit may have damaged the adapter causing the short. When I bought a second 12VAC adapter from a different source, I discovered that this also had power shorted to ground. I still have not dared to power it on.

As I replaced the 4700uf capacitor, I realised that I may have had it the wrong way around, which explains why it blew up. I do not understand why when I was using the 12VDC adapter, that it was actually working and nothing blew up.

I am still too afraid to power it on for fear that it may blow up, until someone explains to me whether it is normal for an AC/AC adapter to have continuity between power output and ground (I cannot for the life of me find any information about this online and this can't be a coincidence with both AC/AC adapters from two different manufacturers).
 

Online Gyro

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Welcome.

A 12V AC output adapter is simply a transformer. When you are measuring across the output, you are simply seeing the Secondary winding coil resistance. This will be very low.

I'm not surprised your capacitor smoked - it was expecting DC! It may or may not have damaged the adapter, but probably not. Adapter transformers usually have an internal thermal fuse that fails if they overheat. Plug the adapter in without any load, check that it puts out in excess of 12V AC (it will be higher unloaded). Then leave it plugged in for half an hour and see if it overheats (this is a check for shorted turns in the windings). If it passes these these tests then it is fine.

To use the adapter to produce DC, compatible with your capacitor (which IS dead!), you need to pass it through a bridge rectifier first.

If this is a capacitor within a piece of equipment then you must ensure that you get the correct adapter type.


Edit:
Re-reading your post - I am not sure if you mean the adapter is shorted or the equipment. If it is the equipment then you have almost certainly done damage - you need to do a component level analysis and see what components are likely to have been damaged (eg. Voltage regulator). A picture would help.
« Last Edit: October 12, 2018, 11:20:04 pm by Gyro »
Chris

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Offline Hero999

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Welcome.

A 12V AC output adapter is simply a transformer. When you are measuring across the output, you are simply seeing the Secondary winding coil resistance. This will be very low.

I'm not surprised your capacitor smoked - it was expecting DC! It may or may not have damaged the adapter, but probably not. Adapter transformers usually have an internal thermal fuse that fails if they overheat. Plug the adapter in without any load, check that it puts out in excess of 12V AC (it will be higher unloaded). Then leave it plugged in for half an hour and see if it overheats (this is a check for shorted turns in the windings). If it passes these these tests then it is fine.

To use the adapter to produce DC, compatible with your capacitor (which IS dead!), you need to pass it through a bridge rectifier first.

If this is a capacitor within a piece of equipment then you must ensure that you get the correct adapter type.
Yes, it's normal that the output of an AC PSU will be a short circuit, since it's just a transformer and the polarised capacitor is designed to work of DC will smoke, if connected to AC. However:

The reason I am concerned to hook this up to a diy PSU that I built (which the instructions say that it runs on 12VAC and specifies clearly that I must use an AC/AC power supply), is because when I first hooked up the first 12VAC adapter, a large 4700uf electrolytic capacitor started smoking and leaking.
Now that doesn't add up. If the power supply is designed to work off AC, then it should work perfectly fine from a transformer.

Reversing the polarity of the capacitor is a bad idea, because it will blow up if powered from AC regardless of the polarity and it will now blow up, if powered from the correct polarity DC.

Do you have a link to the shop you bought it from, a part/model number & make or schematic of the power supply?
 
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Offline c0ntr4d1ct10n

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Thanks, that sort of makes sense in terms of just looking at the power adapter in isolation

However, when connected to this circuit, I am still confused.

I have attached the schematic of the power part of the circuit.

The diy circuit is this one: https://mutable-instruments.net/archive/module_tester/

So according to this schematic, if pin 1 is ground and pin 3 is power, does that mean that current would flow both from the ground pin and the power input pins of each of the voltage regulators?
« Last Edit: October 12, 2018, 11:35:39 pm by c0ntr4d1ct10n »
 

Online Gyro

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Yes, that supply does need an AC adapter, it uses the positive half cycle to produce the positive output rails (+5V and VCC), and the negative half cycle to produce the negative output rail (VEE).

The most logical explanation for the 4700uf capacitor (C29) burning is that D2 is connected backwards or is shorted.

Note that the 1uF capacitor, C30 is seeing 12V AC, that one cannot be an electrolytic (unless it's a bipolar one).


P.S.
Regulators IC8 and IC9 may have been damaged, however D8 and D6 should have protected the following circuitry.
« Last Edit: October 13, 2018, 12:12:05 am by Gyro »
Chris

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Offline Hero999

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Thanks, that sort of makes sense in terms of just looking at the power adapter in isolation

However, when connected to this circuit, I am still confused.

I have attached the schematic of the power part of the circuit.

The diy circuit is this one: https://mutable-instruments.net/archive/module_tester/

So according to this schematic, if pin 1 is ground and pin 3 is power, does that mean that current would flow both from the ground pin and the power input pins of each of the voltage regulators?
As mentioned above that power supply does need AC because it alternately uses the positive cycles to power the positive supplies +5V and +12V and negative cycles to power the -12V supply.

If this circuit were powered off DC then only the positive or negative parts of the power supply would work, depending on the polarity of the DC voltage. If it seemed to work off 12VDC, then perhaps you only measured either the positive or negative supply and the polarity just happened to match which supply voltage you measured?

Yes, the 4700μF capacitor is toast and D2 is either bad or connected backwards. Replace them both and ensure they have the correct polarity. I you don't know what the correct polarity is, then it can be checked with a meter using the continuity test function. The negative side of C29 should be near 0 Ohms to pin 1 and J9 and the anode of D2 should be near 0 Ohms to pin 3.
 
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Offline c0ntr4d1ct10n

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Thanks so much. Yes, I'm pretty certain I put the 4700uf on backwards. I remember not checking and assumed that it was the same direction as the 2200uf. D2 is definitely in the correct direction.

Am I still supposed to be connecting the ground pin (pin 1 of the DC 2.1mm power socket) if it is connected to the power output as part of the transformer? What does the arrow symbol mean (i.e. from pin 2 pointing at pin 1). Also, is there such thing as an AC socket (the BOM lists the DC socket).
 

Offline Hero999

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Yes, pin 1 should to to the power output part of the transformer.

The arrow part of the connector symbol indicates a switch. Pin 2 gets connected to pin 1, when the power connector is removed from the socket and disconnected when the power connector is inserted. This can be used to change over to battery power, when the mains supply is unavailable.

There's no standard for small cylindrical connectors, regarding polarity, voltage and whether it's AC or DC. In this case the connector can be used in a device running of either AC or DC. This is a mess because someone could connect a much higher voltage power supply to something not designed for it, causing it to be damaged. Ideally there should be a standard set of connectors for different voltages, which can't be connected to one another.
 
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Offline c0ntr4d1ct10n

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Thanks guys! I've just powered it on and it didn't blow up. I couldn't smell any smoke and the new capacitor I put in with the correct polarity this time doesn't look like its about to burst like last time. Seems to work quite normally now.
 


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