Too high AC output--How to lower?

[rfj5106]

In a mental lapse I purchased a transformer with 60 volts ac output for a project that required 70 volts dc. After rectification, I have 85 volts which is way to much. I thought about a resistor to lower the DC after rectification but I realize that is going to get very hot and create a lot of other problems. So then I thought about a step down transformer to lower AC from 60 to 45. Does anybody make such a thing? Would I need one for each secondary? My suspicions are I should just buy a new transformer. Any opinions?

This transformer (Hammond 182N60) has 2 primary coils. If I ran the primary coils in series on 117 vac and the secondaries in parallel would output voltage be a little less than 30 vac? The transformer is 500 VA. Normal operation, the 2 primaries in parallel gives 60 volts @ 4.17 to the secondaries. What will the secondaries deliver in current with primaries in series?

[luka]

I whink I would add turns to primary, to lower secondary voltage
secondary will deliver as much current as the load is, no matter how you wire primarys (But voltage will change, so would load need too, to draw same current)

[VictoriaGuy]

Originally Posted by rfj5106

This transformer (Hammond 182N60) has 2 primary coils. If I ran the primary coils in series on 117 vac and the secondaries in parallel would output voltage be a little less than 30 vac?

Yes, that's the way I read the Hammond info.

Originally Posted by rfj5106

The transformer is 500 VA. Normal operation, the 2 primaries in parallel gives 60 volts @ 4.17 to the secondaries. What will the secondaries deliver in current with primaries in series?

I read the spec as 60V @ 8A ?

[luka]

Originally Posted by VictoriaGuy

I read the spec as 60V @ 8A ?

Wrong, since voltage will be half that, so with the same load, so will be current only half

And for 500VA you can get 60v at 8A, but not with || and -- primary connection

[sofaspud]

85 volts seems on the high side to me, even with no load. Is this measured at the rectifiers or after the filter capacitor(s)? There could be a significant difference between the two. The 85 volts you see may just be the peak reading.
Otherwise, a voltage regulator seems to be the easiest solution.

[DF96]

Rectifying 60V RMS to get 85V off-load sounds about right to me. 60x1.414=84.84, less a few diode drops, plus a rise because off load. It will fall when a load is applied. You might get 80V on-load.

I would not advise messing with the primaries, as this has safety implications. If 80V DC really is too high (how accurate does the 70V have to be?) then you could use a buck transformer. Google will help you.

[luka]

if primary is a problem for safety, then unwind secondary, anything else wil generate heat

[VictoriaGuy]

Originally Posted by VictoriaGuy

I read the spec as 60V @ 8A ?

[tauro0221]

Hi,
Used a voltage regulator to drop the high voltage. I used the output from a TL783 voltage regulator to feed the gate of a IRF640 mosfet transistor. You have to read the instruction for the adjustment resistor. There is a schematic on how to use it. The max voltage of the TL783 is a 120volts.

[VictoriaGuy]

If you have something suitable in your transformers collection, you could wire up a bucking transformer to drop either your primary or secondary voltage.
Rod Elliott to the rescue!