combined 2 transformers for 24v-0-24v

[DarpMalone]

Hi all,

I combined two identical 24v-0 75VA transformers to create the required 24v-0-24v supply for a chip amp... It does work, but is this safe practice? Also, I've currently got a fuse on the main AC feed before it splits to both transformers. Is this correct or should each transformer be on a separate fuse?

Thanks

[paulb]

Should work fine. It sounds like you have the phasing for the transformers correct, otherwise you wouldn't get much voltage out of them in a series connection.
I would fuse them together because you want both off if a fuse blows.

[DarpMalone]

Thanks for your reply, Paul... Am I correct in thinking that I should be using a 7A fuse if I'm feeding (2) 24v 75VA transformers?

[MikeBarton]

Originally Posted by DarpMalone

Thanks for your reply, Paul... Am I correct in thinking that I should be using a 7A fuse if I'm feeding (2) 24v 75VA transformers?

I'm no expert but...

If the fuse is on the primary side, then the total (max) current flowing (if maxxed out which it shouldn't be) will be:
total watts = 2 x 75va = 150va
current = 150va / 110v = 1.4amps (where 110v is your mains voltage? )

For the secondary side, then the current in the +/-24v lines would be:
75va / 24v = 3.1amps.

so 7amps is too high for either situation.

Hopefully someone more qualified than me can confirm this.....

[DarpMalone]

Thanks for the guidance, guys!

[dodo]

Originally Posted by DarpMalone

Hi all,

I combined two identical 24v-0 75VA transformers to create the required 24v-0-24v supply for a chip amp... It does work, but is this safe practice? Also, I've currently got a fuse on the main AC feed before it splits to both transformers. Is this correct or should each transformer be on a separate fuse?

Thanks

identical doesn't have to = the same , (winding tolerances )
Did you checked voltage level on the smothing caps ( with no load ) on the + and - side , are they identical ??

You should also consider , every transformer is some sort of inductor , so you have to follow the rules regarding placement two inductors near eachother .

[DarpMalone]

Hi dodo

I checked the secondary voltage on both transformers before combining them and they were equal. I'll check for variance on the +/- this evening.

My case is quite small and currently the 2 transformers are physically mounted parallel to one another... From building x-overs, I know that it's good practice to place inductors in perpendicular orientation... I actually hadn't considered transformer placement. I'll reposition one of them.

Thanks