Transformer

[mccreamark]

Hi
I know this is very basic but my toroidal transformer is 220 volt primary and 2 x 25 v secondaries on the label it says the red and yellow secondaries are 25v@4.5 amp and the black and orange are o .
the black and orange have a black dot beside them .
I plugged it in and tested the wires not connected to anything with my vom.
I have 50v from the red and yellow wires and 26v from the black and orange.

I must be doing something wrong here.
This transformer is new out of the box today.
I have searched and found nothing like this anywhere.
Any help here would be greatly appreciated
This for a Zen ,I was going to attach to the rectifier as shown in the Zen schematic.
Also wouldnt 26vx1.414 give me 36.76v not the 34 described in the Zen article. My house supply is 240v.
Any ideas would be a great help.
Thanks
Mark

[mccreamark]

Hi
I think i found my mistake Ihave measured the voltage difference between the positive and negative secondary wires and it is 28v ,I suppose that is because of the higher input voltage of 240v .

How do I reduce this28v to 24v to gt the right voltage 34v going to the capaciters?

Thanks Mark

[Pars]

The unloaded voltage of most transformers is higher than the spec'd voltage (under load) that you will get once you actually hook this up. IIRC, this is part of the % regulation that is specified for some transformers. You should be fine.

Make sure that you match the correct leads up with one another for each pari of secondaries if you are not using them in parallel.


For example, you would use the black and red leads for one secondary and the orange and yellow leads for the other in the drawing above.

[woody]

As was just pointed out that transformer will be putting out less than 28v when it is under load and you also lose ~2v in the bridge rectifier.

[mccreamark]

Thanks for the diagram and help /
I connected the secondaries as in the third diagram independently
to the bridge.
I have 24.9v and 24.8v dc from each bridge I was expecting 35v?
Does it change when it connected to board ?
Thanks again
Mark

[Pars]

I didn't mean to use that diagram... you need to use the one that came with your transformer. The diagram I posted was from Avel Lindberg, and was meant to be an example only.

[mccreamark]

Hi
Thats fine
I meant independently as in the 3rd example in the diagram
Do the voltages I got seem OK to you as I read somewhere the bridge would increase the voltage by 1.4 but it dropped a couple of volts here.

Thanks
Mark

[Pars]

Yes, I would expect 25Vac * 1.414 = 35.35Vdc - ~1.4Vdc (diode loss) = ~34Vdc

Are you using a full wave bridge?

[lineup]

Originally posted by mccreamark

I have 24.9v and 24.8v dc from each bridge I was expecting 35v?
Does it change when it connected to board ?
Thanks again
Mark

If you attach a capacitor across the bridge output you will see a change.
The voltage is different with and without caps.
One 1000 uF with 50 VDC rating will be enough.

When I design a power amp circuit, I use VAC * 1.33 = VDC. (4/3).
This is a worst case figure for a normal sized trafo, at high power output.
As mentioned, the recitifed DC voltage when trafo is unloaded,
can many times be 1.5 X VAC rating. (3/2)

The bigger transformer in relation to amplifier power,
the less it will drop when heavy loaded.
There are no rules that can tell this exactly. What final DC Voltage levels will be.
It can be very different for different brands of trafos.

[mccreamark]

hi
I have hooked up my 225 vac transformer to 2 x 63v 10000mf caps and still get 37v dc .I,m using a full wave rectifier.
Also I made a test Zen on a bread board and hooked up to the transformer and fried the r1 resistor.
i also put a 10r power resistor on the power supply to simulate load and fried it before it fried I got 456Ma of current and the dc volts where about .5v.
I know I am doing something dumb but I can,t work it out ,
I`m on my last r1 (Ive ordered more) but I can,t think of why this is happening.
Any ideas would be great.
Thank you
Mark