Transformer Power Output Mystery

[briesmith]

I have bought a 240vac to 24vac transformer for a project I am building using one of the Ali Express 30vdc power supply circuits (based on the Hiland design).

I need 30vac input to drive the circuit so the 24vac transformer should be fine. Except it outputs 57vac,

only outputting a useful voltage if I use the centre tap.

I have uploaded some pix showing the transformer and voltage readings. If someone could confirm that Ali Express have sent me a mis-labelled transformer, or not, I would be very grateful.

Best wishes

Brian

 

[rjenkinsgb]

It's probably a 24-0-24 winding, so you can get a full wave output with two diodes, or + and - outputs using a full bridge, with the centre tap as 0V.

Using one green and the black would give 24V AC, but at half the total power rating of the transformer.

 

[audioguru]

The 24VAC outputs of the transformer when rectified with only two diodes and filtered produces 33VDC peak. Then the circuit produces a maximum output voltage of about 25VDC at (30W/33V=) 91mA, not 3A with your transformer that is much too small.

The little transformer has terrible voltage regulation with 28VAC at a low output current. 28VAC outputs when rectified with only two diodes and filtered produces 38.9VDC and the opamps have an additional -6V supply then their total supply is 44.9VDC. But their absolute maximum allowed total supply is 36V so the opamps will be destroyed.
Frequently your 220VAC will be 240VAC or higher then the opamps will burn out quicker.

9 years ago I redesigned the circuit so that many of its overloaded parts survive and it meets is specs of 30V at 3A output using a 28V 120W transformer and higher supply voltage opamps.

 

[Diver300]

Then the circuit produces a maximum output voltage of about 25VDC at (30W/33V=) 91mA, not 3A with your transformer that is much too small.

I agree that 3 A isn't achievable with that transformer, but I think you mean 910 mA, not 91 mA.

 

[Diver300]

The little transformer has terrible voltage regulation with 28VAC at a low output current.

All transformers of that size have regulation that is near that. 28 VAC from a 24 VAC transformer at no load is about 16% regulation. Two 30 VA transformers that I found on Farnell had regulation of 14% and 15%, so 16% isn't much worse than those.

On any transformer the voltage change between no load and full load has to be considered.

 

[audioguru]

I have a little 9V/200mA transformer that has an 18V output with no load. Its output is 9V with a 200mA load.
Terrible voltage regulation!

 

[Nigel Goodwin]

I have a little 9V/200mA transformer that has an 18V output with no load. Its output is 9V with a 200mA load.
Terrible voltage regulation!

As expected, the smaller the transformer the worse the regulation - generally Efficiency is much poorer as well.

 

[briesmith]

Thanks to everyone who replied but I'm left with a conundrum; can I use this transformer for my power supply project?

As I said, I'm using a DIY circuit from Ali Express that outputs up to 30vdc at 3A based on the Hiland design and before I bolt it together I'd like to know if it's going to explode as soon as I turn it on.

Regards

Brian

 

[audioguru]

The transformer has a maximum rated output current that is too low. An important input fuse is missing.
If you turn up the project's current control and add a load that draws high current then the transformer will get VERY HOT and burn up.
The Chinese copy of the defective old circuit also has many overloaded parts and other problems.

 

[briesmith]

The IEC mains socket is fused. The project is intended to provide DC power to devices under test, bypassing the DuT's integral power supply. Hopefully, the current drawn won't ever need to be >1amp. Still not sure that I should be using this transformer?

 

[Diver300]

Fuses on the input aren't much use for protecting transformers. A thermal cut-out mounted to the transformer is far better.

That transformer should be OK for 1A, but especially when you are testing stuff you want to make sure that when there is an overload, there is some sort of overcurrent protection.

If the voltage regulator has a current limit that can be set for 1 A, that will probably protect the transformer. You could also put a fuse rated at about 1/2 A in series with each of the two green wires.

 

[audioguru]

I would never buy a cheap no-name-brand transformer from AliExpress. It might blow up.

 

[briesmith]

Before I started building this power supply I built a dim bulb tester and I plan to plug every device under test into the dim bulb test unit before doing anything else. Hopefully that will identify any overloads? The DIY power supply circuit I'm using has two potentiometers: one for volts (max 30vdc) and the other for amps (max 3a) with a Current Limit indicator LED in the circuit.