Hi all,
Can you safely connect an autotransformer or buck boost to the secondary of an isolation transformer?
I have a hefty 1:1 transformer with a 6.3v tap.
Can I connect an autotransformer to its secondary so I can have the benefits of an autotransformer and still have isolation from the wall?
Thanks
Can you safely connect an autotransformer or buck boost to the secondary of an isolation transformer?
I have a hefty 1:1 transformer with a 6.3v tap.
Can I connect an autotransformer to its secondary so I can have the benefits of an autotransformer and still have isolation from the wall?
Thanks
Yes you can. Just make sure not to exceed the ratings of either one in use. This is exactly what some power stations do like the Sencore PR-57 shown. I do forget which unit comes first, transformer or variac (autotransformer) but it really doesn't matter.
Upon thinking about it, I think the isolation xformer is first in line.
Upon thinking about it, I think the isolation xformer is first in line.
Attachments
Last edited:
Thank you HollowState.
I know that auto transformers have many advantages, plus multiple taps. Plus autotransformers can be wired using standard transformers...so it's good to find a way to use them while still having isolation.
I know that auto transformers have many advantages, plus multiple taps. Plus autotransformers can be wired using standard transformers...so it's good to find a way to use them while still having isolation.
May I have your thoughts on why it would be questionable?
I know Buck/Boost transformers are installed to the power before the isolation transformer. these auto transformers adjust the voltage by 20% of the input voltage...to bring 115V to 120V or visa versa.
What I am looking to do is connect a control transformer, (120V to 480V) wired as an autotransformer to get 600V. I think the 1:1 isolation transformer should be first, as the it is a lower voltage. Plus I can use the 6.3V winding for heaters.
I figure wiring the autotransformer would be better than reverse feeding a 600v primary, 120V secondary control transformer. However, I have a 15lb. multi-tap transformer I could reverse feed to get the right voltage (about 350-380V) for a voltage doubler.
You must connect the isolation transformer before the step-up in this case, to comply with
the ratings of the isolation transformer.
Well, you question *everything* to begin with. 😱
Should we worry about that?
Don´t think so. 😉
OF COURSE making the system safe as soon as possible is the sane way to go.
I quite often use transformers back to back in tube applications.
My current pre amp drops the mains to 12VAC for heaters but also to drive an 18VAC mains transformer backwards to get an isolated 160VAC.
Works a treat.
I don't know if you could use the same technique to get a higher voltage than the rating of the transformer.
I'm trying to get 600v+. at about 100ma. I'm not sure if back to back would work for this voltage. My wall supply is 120v.
Is it just not wise to back feed transformers directly from the mains?
I have seen control transformers be reversible...just not all of them.
If you need 600Vdc, then you can work back from that.
You want about 420Vac as your final output.
Step down from 115:50+50Vac or 55+55Vac. Then step up from 60:115+115. from one 50Vac winding and the same 60:115+115Vac from the second 50Vac winding.
You now have four 115Vac windings to connect in series to give a nominal 460Vac. But you have fed 50Vac or 55Vac into two 60Vac windings so you get out 460*50/60 = ~383Vac
You are pretty close and you have isolated your supply from the mains and no winding is stressed above it's rated voltage.
The extra 5Vac could be some added turns to the secondary of the first isolation transformer.
You don't have to come all the way down to 50 or 55Vac, just use ratios that are available cheaply/locally.
But this has cost you three transformers.
There is probably a cheaper way.
There is one more complication:
Transformer regulation.
Take a 1:1 isolation transformer. It is not wound with a 1:1 turns ratio.
The secondary voltage droops when under load. The Turns ratio is 1:1+regulation
if you have a fairly large 500VA toroid transformer with a regulation of 4.5%, then the turns ratio would be very approxiamately 200:209, if there are 1.74Turns per Volt for that core.
You want about 420Vac as your final output.
Step down from 115:50+50Vac or 55+55Vac. Then step up from 60:115+115. from one 50Vac winding and the same 60:115+115Vac from the second 50Vac winding.
You now have four 115Vac windings to connect in series to give a nominal 460Vac. But you have fed 50Vac or 55Vac into two 60Vac windings so you get out 460*50/60 = ~383Vac
You are pretty close and you have isolated your supply from the mains and no winding is stressed above it's rated voltage.
The extra 5Vac could be some added turns to the secondary of the first isolation transformer.
You don't have to come all the way down to 50 or 55Vac, just use ratios that are available cheaply/locally.
But this has cost you three transformers.
There is probably a cheaper way.
There is one more complication:
Transformer regulation.
Take a 1:1 isolation transformer. It is not wound with a 1:1 turns ratio.
The secondary voltage droops when under load. The Turns ratio is 1:1+regulation
if you have a fairly large 500VA toroid transformer with a regulation of 4.5%, then the turns ratio would be very approxiamately 200:209, if there are 1.74Turns per Volt for that core.
Last edited:
Thanks all,
Oops, I don't think I was clear.... I'm trying to get 600V ac, before being rectified. It will have to be a bridge rectification which means it needs to be 145ma to make up for the current loss. This will give 108ma after going through the bridge.
This will consume 65.25W.
I hooked up the 480pri, 120sec. as an autotransformer. 120V mains connected to 120sec. (for testing purposes only) and came up with 570V. So I'm still short using that method.
I'll do some math using some transformers I have for back to back operation.
Oops, I don't think I was clear.... I'm trying to get 600V ac, before being rectified. It will have to be a bridge rectification which means it needs to be 145ma to make up for the current loss. This will give 108ma after going through the bridge.
This will consume 65.25W.
I hooked up the 480pri, 120sec. as an autotransformer. 120V mains connected to 120sec. (for testing purposes only) and came up with 570V. So I'm still short using that method.
I'll do some math using some transformers I have for back to back operation.
using a back to front transformer cannot get you above 115+115Vac.
If you transform above that you are taking the transformer insulation beyond what it was designed for.
If you want <460Vac you can use two transformers, i.e. four 115Vac windings.
If you want 600Vac, then each winding would need to be 150Vac, that is ~ 118% of the normal maximum of 127Vac.
I don't know enough about transformers to be able to recommend that.
Talk to the manufacturers about trying to get 150Vac from a nominal 115Vac primary.
Do you see that stating "600V" led to the confusion. Use the units unambiguously.
If you transform above that you are taking the transformer insulation beyond what it was designed for.
If you want <460Vac you can use two transformers, i.e. four 115Vac windings.
If you want 600Vac, then each winding would need to be 150Vac, that is ~ 118% of the normal maximum of 127Vac.
I don't know enough about transformers to be able to recommend that.
Talk to the manufacturers about trying to get 150Vac from a nominal 115Vac primary.
Do you see that stating "600V" led to the confusion. Use the units unambiguously.
Last edited:
Yes, I see that. My apologies 😱
I have transformers using 2500 RMS VAC insulation.
One type runs 115/230vac primary to 10vac secondary and 18vac secondary.
They are 100va rating. I have a lot of these...maybe I could calculate something out...just for theory...
I would need 7 tansformers
2 100VA transformers in front... 120vac in, 10vac out.
5 100VA transformers in back wired in series... 10Vac in, 240vac out.
5x 240=1200VAC ct. @ 200VA
A little much to sit on a chassis....along with the wire trail
post15 is perfect. No confusion between voltages and VA
The 2500Vac insulation is for a short term overvoltage. usually due to HV spikes on the mains supply.
A dual primary with two universal windings will have a 110/120 rating on each. They can be used in series or in parallel, so you know that the two windings are insulated to at least 250Vac.
You need to ask your manufacturer if you can take those a little higher, or a lot higher?
If your manufacturer confirms they are good for >300Vac from the two windings in series, then you only need two transformers to get to your 600Vac target.
A 110/120:10Vac transformer does not have many secondary turns. It would be relatively easy to add on the same number of turns to get a 20Vac secondary. Or any lesser number, if it gets you to your target.
But 100VA and 10Vac means an output of 10Aac.
That may not be a single thick wire winding. It could be a dual (bifillar) or triple (Tri-fillar) winding. Have a very close look at the secondary wires.
If it's bi, or tri, then you can separate the windings and series connect them for either 20Vac or 50Vac.
Find out what you have.
The 2500Vac insulation is for a short term overvoltage. usually due to HV spikes on the mains supply.
A dual primary with two universal windings will have a 110/120 rating on each. They can be used in series or in parallel, so you know that the two windings are insulated to at least 250Vac.
You need to ask your manufacturer if you can take those a little higher, or a lot higher?
If your manufacturer confirms they are good for >300Vac from the two windings in series, then you only need two transformers to get to your 600Vac target.
A 110/120:10Vac transformer does not have many secondary turns. It would be relatively easy to add on the same number of turns to get a 20Vac secondary. Or any lesser number, if it gets you to your target.
But 100VA and 10Vac means an output of 10Aac.
That may not be a single thick wire winding. It could be a dual (bifillar) or triple (Tri-fillar) winding. Have a very close look at the secondary wires.
If it's bi, or tri, then you can separate the windings and series connect them for either 20Vac or 50Vac.
Find out what you have.
Last edited:
- Status
- Not open for further replies.