PSU Repair- Transformer outputting wrong voltage. Help much appreciated =)

[HellasTechn]

Also the transformer is rated to give 2, 18v rails anyway, so how comes it would give me somthing different to whats specified?

Thanks for helping me out with this

A transformer is by it nature a device that it's output voltage is directly affected by the load connected on it ! So i find it normal for a rated 18volt transformer to produce 21 volt unloaded.

 

[HellasTechn]

For example a few years ago i built a 555 timer circuit designed for 12volt supply and i connected it to a wall transformer rated 12Volt 1000ma.
and it blew the 555 chip. When i measured the transformer it gave me 18 Volt and the 555 blew because its load was not enough to bring the voltage down to 12 Volt.
The transformer was not bad, it is just that it was designed for a device that would draw at least 500ma.
Do you get the point ?

That is what we call tolerance in electronics.

 

[(*steve*)]

Because then how would you know what the static voltage rating should be? And dont most appliances say they require a power supply that GIVES a certian voltage, not the rating that the supply comes down to after being connected.

Also the transformer is rated to give 2, 18v rails anyway, so how comes it would give me somthing different to whats specified?

Thanks for helping me out with this

21V is only 17% more than 18V. That's not bad for a small transformer.

In the days when power supplies (wall warts, plug packs) consisted of a simple transformer, rectifier and filter, it was totally normal for the output voltage to be significantly higher than what was marked until a significant load was placed on it. (I've seen almost double!)

These days most such devices are switchmode regulated and supply an almost exact voltage over the entire load range.

The transformer is not rated to give you two 18V rails. It will be described as having two 18V windings. If you were able to get a datasheet on the transformer you might find that it would describe the change in voltage with load.

If your preamp has a regulator, that will take care of differing voltages under various loads. If it does not have a regulator (and that is not uncommon -- especially in power amplifiers) then the circuit will be designed to handle the varying supply voltage. 18VAC will give A DC rail of about 25.5V. With your off-load voltage reading of 21V, that will probably rise to about 30V unloaded.

 

[jamman1125]

21V is only 17% more than 18V. That's not bad for a small transformer.

In the days when power supplies (wall warts, plug packs) consisted of a simple transformer, rectifier and filter, it was totally normal for the output voltage to be significantly higher than what was marked until a significant load was placed on it. (I've seen almost double!)

These days most such devices are switchmode regulated and supply an almost exact voltage over the entire load range.

The transformer is not rated to give you two 18V rails. It will be described as having two 18V windings. If you were able to get a datasheet on the transformer you might find that it would describe the change in voltage with load.

If your preamp has a regulator, that will take care of differing voltages under various loads. If it does not have a regulator (and that is not uncommon -- especially in power amplifiers) then the circuit will be designed to handle the varying supply voltage. 18VAC will give A DC rail of about 25.5V. With your off-load voltage reading of 21V, that will probably rise to about 30V unloaded.

Okay great, well i had a look at the data sheet (here if youre intrested http://docs-europe.electrocomponents.com/webdocs/13f5/0900766b813f5047.pdf) and it it does refer to voltage fluctuation and and important deail would be that it supplies 18v under a full load.

So this is fine, as long as i can connect the PSU without risk of the appliiance occouring damage and it functions im happy. Question regarding the mA rating, the unti will simply draw how much mA's it needs, am i right? Even if the PSU was specified at a different higher mA

 

[(*steve*)]

As far as I can tell, you should be fine.

And yes, a device will only draw the current it requires (assuming no faults).

 

[jamman1125]

Well thanks for the help everyone, tested it with the first centre tap configuration and it seemed to work fine with the mixer so all is well =). All soldered up and now just need to throw it into a project box for some insulaton.

Cheers again! And im sure ill be back again with another thing for you all to solve

James