Quick transformer connection question

[qa9b]

I have a dual 115V primary, dual 40V secondary transformer.

The connections are shown here:
https://avellindberg.com/transformers/y23_range_connections.htm

I would like to see +40v, 0v, and -40v on the secondary. Am I allowed to connect the primaries in parallel and the secondaries in series to achieve this?

Thanks!

 

[Jaguarjoe]

Absolutely, just be mindful of the phasing on the primary side as well as the secondary side. Use the left side of the top diagram for the primary and the right side of the middle diagram for the secondary.

 

[ronsimpson]

You can connect the primary in parallel (watch the phase) and the secondary in series.
When you say "+40v, 0v and -40V" I hope the +,- are phase and not DC.

 

[KeepItSimpleStupid]

If you connect the primaries in series wrong, you'll get 0V out.

 

[ronsimpson]

If you connect the primaries in series wrong, you'll get 0V out.

That is what fuses are fore.

 

[qa9b]

Thanks! I just wanted to double-check before I go making connections to this 500VA transformer! Needless to say, it will be fused.

 

[crutschow]

To get ±40Vdc out, connect the output in series as you proposed. Ground the center tap and connect the two windings to a bridge rectifier. One output of the bridge will then be a full-wave rectified plus 40V and the other output will be a full-wave rectified minus 40V. (With a filter the actual unloaded outputs will be close to the peak voltage of 56V, of course)

 

[KeepItSimpleStupid]

ronsimson:

You don't need a fuse. Connecting the primary in series wrong means you get 0 volts on the output. Connect the primary in series and you'll get 0V or 20 V out with 120 input.
Once you have the 20 V, you can now mark the phases. They may even be marked on the transformer.

You can now determine the phases of the secondaries. Connect them in series until you get 40 V out across the series combination. You can now phase the secondaries.

Now you also know how to parallel the phases.

It's OK to power a 240 primary with 120 V.

 

[Diver300]

You don't need a fuse. Connecting the primary in series wrong means you get 0 volts on the output.

With a 500 VA transformer, it will take a huge amount of current if you connect the primaries in series if they are in opposite directions. There will be no back-emf to oppose the input voltage, so all you have is the primary resistance. Transformers that size are normally about 95% efficient, so all of the winding resistances are small. Putting two in series will make little difference. I realise that tiny circuit board mounted transformers can be wired like that and survive, but big transformers will blow fuses.

There are two ways that I would uses for checking the connections. One is to wire the transformer in series with a handy electrical load, like a 500 W halogen lamp. With no load on the transformer, the lamp should be off, or really dim. If the transformer is wired wrong, the lamp will light but you only have 5 A flowing which won't do too much damage, certainly not quickly.

Alternatively, you can power just one of the primary windings. With no output load, there is no need for reduced resistance of the two windings in parallel.
If your primary connections are A, B, C and D, and you have tested and found that A and B are one winding, and C and D are the other, then connect A and C together, and connect neutral to that. Connect live to B. Measure the voltage between B and D. If it is less than 10 V, you can also connect live to D. If it is more than 200 V, swap C and D.

 

[KeepItSimpleStupid]

Diver300:

How much current would a transformer draw when no load was connected? Now make that load a 10 Meg Voltmeter?

A transformer transforms Power. VpIp=VsIs. If Is = 0 what happens?

I=V/R; therefore VpIp=V^2/R or VpIp = (40^2)/10E6 or 16 mW (the 40 assumes the sec is in series too because the primary V is reduced by a factor of 2.

16 mW won' do ANY damage. I'll let you figure out the primary current and when the secondary current is 5% lower to account for effficiency it 5%.

What;s the temperature rise of the transformer when it dissapates 16 mW for 30 sec? ...which is much longer than the time to make the measurement.

I totally disagree with your answer except for the efficiency term.

 

[Diver300]

Diver300:

How much current would a transformer draw when no load was connected? Now make that load a 10 Meg Voltmeter?

A transformer transforms Power. VpIp=VsIs. If Is = 0 what happens?

There will be iron losses, and there will always be some primary current even if there is no load.

A 500 VA transformer is about 95% efficient at full load, so it produces about 25 W of heat at full load. It will produce less, probably a few watts.

**broken link removed**
That gives a figure of 2.3 W at no load for a 550 VA transformer. The power factor will be quite bad under those conditions, so the current will be quite a lot more than 2.3 / 120.

My main point is that connecting all but the smallest dual-primary transformer with the primaries in series, but reversed, will cause something to fail, even on a lower supply voltage.

That is like winding a coil with half the turns one way and the other half the other way round. There is no net magnetising effect, so the arrangement will have hardly any inductance. It is the inductance that limits the current, so without that, the current will be huge.

 

[Reloadron]

If you have not yet wired it, the circuit would be wired like the attached based on the link to the transformer. The color code scheme for the transformer is taken from the link. The mains fuse is not shown. The bridge rectifier chosen should be chosen to handle the load and the filter caps also are not shown but I am sure you get the idea.

Ron

 

[Diver300]

I have a power supply that contains a dual primary 330 VA transformer, so I thought that I would do a test. The two primary windings are 2.3 Ω each at DC.

I put two 470 Ω power resistors in series (so 940 Ω), in series with the mains, and tested the transformer with virtually no load in three conditions.

Normal, both winding in series. There was 24 V across the 940 Ω ohms, so about 25 mA

One winding reversed, and both in series. There was 235 V across the 940 Ω, and 1 V across the transformer. The current would have been 235 / 940 = 0.25 A, so the transformer impedance was about 4 Ω, which is all accounted for by the resistance of the windings.

Finally, one winding open, the other connected. There was 139 V across the transformer, 113 V across the 940 Ω. As the windings are 120 V each, the core would have been saturating, which accounts for the current of 113 / 940 = 120 mA. There would have been much less current at 120 V, but I don't have an 120 V supply here.

Anyhow, reversing one winding reduces the transformer inductance to nearly zero. With no safety resistor, on 120 V, the primary current would have been about 30 A, and the transformer heating would have been about 3 kW.

If you don't know the phasing of the transformer primaries, do not guess and then connect straight to the mains.

 

[RCinFLA]

Connect one of the primary winding to 120 vac with no load on transformer. With an AC voltmeter there should be 120 vac on other primary winding. Connect one of the leads of the second winding to one side of the other 120 vac connected winding. Measure the AC voltage between the open end and the other 120 vac powered winding.

If you measure 240 vac you got it wrong. If you measure near zero vac you got it right.

You can do the same procedure for secondary windings.

 

[colin55]

What is your supply voltage?

 

[qa9b]

If I may add one more question:

I'm using the 500VA transformer I mentioned to drive some power mosfets. However, I need a way to power some opamps and other circuitry whose supply voltage is around +-15v. I've been able to find both linear and switching regulators that can work at supply voltages as high as 60v, but there's been no luck on the regulation of the negative side.

I think it would be easier to just purchase a small 30V center tapped transformer that, after rectification and filtering, would give me about +-20VDC. Obviously, I would send the center tap of the transformer to ground. Would it be OK to connect the center tap of this second transformer to the "0v" (Common 0V connection in Relaodron's drawing) output of my 500VA transformer?

 

[colin55]

Yes. You need to do that to create +20 and -20v. Otherwise the +20 0v -20v will be floating and you will be able to "push" and "pull" the other circuit to get it working.

 

[KeepItSimpleStupid]

Yes, you can connect the center taps together. You can buy a nice board from Jameco which has the regulators and caps on it.

 

[Diver300]

If it is a toroidal transformer that has hole in the middle, you could add another secondary winding. You will get about 0.5 - 1 V per turn, so for +/- 15 V you need between 15 and 30 turns.

For a centre-tapped secondary you need 30 - 60 turns, but you might be able to get away with half wave rectified, and use larger smoothing capacitors.

As the current you need is tiny compared to the transformer rating, you do not need to use enamelled wire which has very thin insulation, and is normally used on transformers to get the windings very closely packed. You can use ordinary PVC insulated wire.

 

[KeepItSimpleStupid]

Remember, that if you Bifilar wind this it's easier. Get some magnet wire which is usually Formvar insulated. If you know the turns, or aprox # of turns, then figure the amount of wire necessary to wind that many turns based on xformer dimensions.

Then wind it on say a 1 foot stick that will pass through your torroid making sure there is enough slack to access the wires. Do something to hold the wire in place. Kapton tape, electrical tape, fishing line. Wind your # of turns.

Since it's bifilar wound, you can take the ends and put them together. Have a load a 2x the current you need. Measure the unloaded and loaded windings. You would at this point have an inductor in series with a a transformer at this point which should be OK. Add or subtract windings based on your measurements.

It's possible, also to make a test with one or two windings to get an idea of volts/turn.

Cut and mark the windings at this point, A, B for one end B and B' for the other.

For the center tap, you need to connect A & B' or B and A' together.