transformer question

[sam2]

I am building a resistance solderer( https://www.electro-tech-online.com/custompdfs/2008/12/solderer.pdf ) and have this transformer: 24V CT (12-0-12) 10A TRANSFORMER-MPJA, Inc. .

It has 2 primary windings, parallel them for 120v input , or series for 240v input. Output is 24v @ 10 amps.

It would be more usefull to me to have a 12 volt output and 6 volt from the center tap( as opposed to 24v and 12v center tap).

Can I wire the primaries in series( as you would for 240v input) but use 120v instead so I can get the 12v and 6v I want?
I know it will work, but will it cause harm to the transformer?

Thanks, Sam 2

 

[kchriste]

I hope you don't intend to solder electronic circuits with this device. The potential for ruining components is immense.

Can I wire the primaries in series( as you would for 240v input) but use 120v instead so I can get the 12v and 6v I want?

No. That is a bad idea because there will be too much resistance in the windings and it will overheat while delivering very little usable current. If you are skilled enough you could separate the two secondary windings at the center tap and put those two windings in parallel for a 12Vac output. Or try and find a transformer with a low voltage winding: 6Vac or lower.

 

[HiTech]

A quality solder pencil heats up in no time. Marlin P Jones sells a Weller knock off, iso-tip design, variable temperature with digital readout for a mere $39 US. That resistance solderer should be a real trustworthy gem around CMOS circuits!

 

[sam2]

I hope you don't intend to solder electronic circuits with this device. The potential for ruining components is immense.

No. That is a bad idea because there will be too much resistance in the windings and it will overheat while delivering very little usable current. If you are skilled enough you could separate the two secondary windings at the center tap and put those two windings in parallel for a 12Vac output. Or try and find a transformer with a low voltage winding: 6Vac or lower.

Thanks for your reply.
I will use a different transformer. Can you elaborate on why it would overheat and deliver low current?

Its for soldering wire to model railroad track rails. It can be used anywhere you need to solder heavy gauge materials quickly without heat damaging surrounding areas(like if you dont want to desolder previous work). The heat is very localized, kind of like spot welding without melting the base metal ( just the solder)
Thanks again ,Sam

 

[kchriste]

Basically, the gauge of the windings is designed for a maximum current capability. Yours would be apx 0.5A per primary winding. If you put the windings in parallel, you double the current capability. In series, the current capability stays the same at 0.5A. So, if you operate the transformer on 120V, you'll be able to get 120 VA of power from the transformer if you put the windings in parallel. But you'll only be able to get 60 VA of power at 120V from the transformer if you put the windings in series. (VA = watts into a resistive load)
But it's worse than that:
Lets say that the resistance of each primary winding is 2Ω. If you put them in series, then you'll have a total resistance of 4Ω as opposed to 1Ω if you had put them in parallel. So, for a given current, the resistive losses increase 4x as well.
There are other, more complicated losses such as core losses, which I won't go into here.

 

[sam2]

Basically, the gauge of the windings is designed for a maximum current capability. Yours would be apx 0.5A per primary winding. If you put the windings in parallel, you double the current capability. In series, the current capability stays the same at 0.5A. So, if you operate the transformer on 120V, you'll be able to get 120 VA of power from the transformer if you put the windings in parallel. But you'll only be able to get 60 VA of power at 120V from the transformer if you put the windings in series. (VA = watts into a resistive load)
But it's worse than that:
Lets say that the resistance of each primary winding is 2Ω. If you put them in series, then you'll have a total resistance of 4Ω as opposed to 1Ω if you had put them in parallel. So, for a given current, the resistive losses increase 4x as well.
There are other, more complicated losses such as core losses, which I won't go into here.

Thanks for the information.

There are alot of projects out there using either variacs or dimmer circuits ahead of a transformer to adjust the voltage up or down going into the transformer.

I have built a few foam cutters the same way myself ,and was going to use a dimmer circuit ahead of the transformer for my resistance solderer.
The foam cutters Ive built have always worked fine.

But that does not mean that it is a proper way to do it or that I have not been damaging the transformers. They seem to work and I didnt know any better.

If the transformer is doing its job ( cutting foam , soldering ect) and its not getting hot am I ok ?

thanks, sam

 

[Dean Huster]

A dual voltage primary (e.g., 120/240v) is designed such that each of the two primary windings is expecting 120v. In series with 240v applied, each gets 120v; in parallel with 120v applied, each gets 120v. With 120v, each primary winding will create the magnetic field necessary for the transformer to output its specified secondary voltage and current. Using only one of those two primaries at 120v will output the proper voltage, but the current output capability will be halved. You will have another 120v secondary available, but be very careful with that: it was intended to be a primary and will not have the primary-to-"secondary" voltage breakdown ratings that you'll have for a normal primary-to-secondary. In other words, that's not a recommended practice at all! Besides, adding that formerly-primary winding as a new secondary with only a single primary doing a half-assed job of injecting a magnetic field will really mess up the aggregate power availabilty from the secondaries.

If you apply 120v to the series-connected primaries (240v configuration), you'll indeed get half the voltage out. You'll also get tremendously-reduced current output.

Resistance soldering is not passe by any means. It's the best way for soldering many of the turret or bifurcated terminals of large connectors or PCBs and the model railroad track application is PERFECT! Of course, you don't use the thing for soldering ICs.

Dean

 

[ecerfoglio]

I am building a resistance solderer( https://www.electro-tech-online.com/custompdfs/2008/12/solderer-1.pdf ) and have this transformer: 24V CT (12-0-12) 10A TRANSFORMER-MPJA, Inc. .

It has 2 primary windings, parallel them for 120v input , or series for 240v input. Output is 24v @ 10 amps.

It would be more usefull to me to have a 12 volt output and 6 volt from the center tap( as opposed to 24v and 12v center tap).

Can I wire the primaries in series( as you would for 240v input) but use 120v instead so I can get the 12v and 6v I want?
I know it will work, but will it cause harm to the transformer?

Thanks, Sam 2

It will work OK- It won't cause harm to the transformer

It's output current may be the same (10 amps). Of course, with only a half of the the voltage you will have only half of the original power

No. That is a bad idea because there will be too much resistance in the windings and it will overheat while delivering very little usable current. If you are skilled enough you could separate the two secondary windings at the center tap and put those two windings in parallel for a 12Vac output. Or try and find a transformer with a low voltage winding: 6Vac or lower.

No, it won't overheat.

Each primary winding will still have the same resistence and the same current it was designed for.

The (series) total primary winding will have more resistance, but it will have less current than the (parallel) original 110V configuration.

Basically, the gauge of the windings is designed for a maximum current capability. Yours would be apx 0.5A per primary winding. If you put the windings in parallel, you double the current capability. In series, the current capability stays the same at 0.5A. So, if you operate the transformer on 120V, you'll be able to get 120 VA of power from the transformer if you put the windings in parallel. But you'll only be able to get 60 VA of power at 120V from the transformer if you put the windings in series. (VA = watts into a resistive load).

But it's worse than that:
Lets say that the resistance of each primary winding is 2Ω. If you put them in series, then you'll have a total resistance of 4Ω as opposed to 1Ω if you had put them in parallel. So, for a given current, the resistive losses increase 4x as well.

OK, but the secondary now has only 6V @ 10 A = 60VA (half the voltage, same current)

You have 4x resistenace at half the current, power losses = I²R are the same

There are other, more complicated losses such as core losses, which I won't go into here.

Core losses will be much lower with the primaries in series at 110 V (in fact, at half the voltage you have a cuarter of the core losses)

If you apply 120v to the series-connected primaries (240v configuration), you'll indeed get half the voltage out. You'll also get tremendously-reduced current output.

You will have reduced current output with the same load resistance.

If you reduce the load resistance you may have the same output current.

 

[MrAl]

Hi there,


The conclusion is that we can drive the transformer in this way, but
only if the output current is still restricted to the original 10 amps
as with the 'normal' connections.

It is possible to wire the two primaries in series and supply with
120vac, which should not be a problem. As mentioned in the
previous thread, the core losses go down as the excitation voltage
goes down.
There is a loss of output power however, in that the available output
power is reduced by about one half, and trying to draw more power
than the original full load current will cause more heating in the
windings.

The following set of numbers tells the whole story. These are taken
from a transformer like the one described with 2.5 ohms series
resistance in EACH primary winding, and 0.1 ohms series resistance
in the total secondary (from end to end, not looking at the center
tap). This would be a typical set of resistances for a transformer
like this.

From the numbers we see that we can go up to 10 amps on the
output but that's about it. Maybe a little higher, but if we try to
get the original power output at the reduced voltage we will most
likely burn up the transformer.
The 'normal' power in the transformer is about 16 watts, but if the
secondary is loaded to 20 amps (with series 120vac input) the power
in the transformer goes up to more than 4 times this normal operating
power dissipation, which is very very bad.
Also note however that loading to 10 amps is just fine.

Voltage (no load)
  Parallel 120vac input: 25.2v out
  Series 240vac input:   25.2v out
  Series 120vac input:   12.6v out

Power
  Parallel 120vac input:
    10 amps, Primary 5.6 watts, Secondary 10 watts, Load 236 watts, volts 23.6v
  Series 240vac input:
    10 amps, Primary 5.6 watts, Secondary 10 watts, Load 236 watts, volts 23.6v
  Series 120vac input:
     5 amps, Primary 1.4 watts, Secondary 2.5 watts, Load 59 watts, volts 11.8v
    10 amps, Primary 5.6 watts, Secondary 10 watts, Load 110 watts, volts 11v
    20 amps, Primary 11 watts, Secondary 40 watts, Load 189 watts, volts 9.5v

 

[sam2]

Thanks everyone for the replies/information ,and MrAl for the numbers.
I will do some testing myself with the same input voltages and loads and see what kind of tranformer temperatures I get.
Thanks ,sam