# Estimating wattage of unknown autotransformer

Update. As a result of the answers. I was able to buy this triad 100VA autotransformer. It weights exactly 1kg. so for autotransformers, it's 100VA per kg versus 50VA per kg for regular two winding transformers. Now about estimating the wattage of that unknown autotransformer. Here's the comparison with Triad branded one.

Top picture:

This is back of the unknown autotransformer

Well. I used my multitester and able to measure conduction in all leads between primary and secondary so it's 100% an autotransformer (the leads from the terminals was gone because I cut it so short). At least you know that in asia, autotransformer have leads coming from both sides unlike the Triad brand where it only comes out at front.

The unknown autotransformer weights 1.32kg. So it may be more than 100VA? using the formula, it's 132VA?

However, the construction seems to be different. Do you think they are same construction or really different so I can more accurate estimate the wattage of the unknown autotransformer.

original:

I need to acquire this autotransformer from another circuit for use in other circuit that requires only 30 watts. But it's not written how many wattage capacity is the autotransformer. Based on your experience, what is usually the range of wattage of this size (it measures 3 inches by 1.25 inches)?

50 watts? 100 watts? What is your estimate (just rough estimate)?

Here it shows there is conduction measuring 18.7 ohms between the primary and secondary leads so it's an autotransformer design. They separate the leads so it's clear which is 220v, 120v neutral, and where are the lower voltage. In the US, they put the wires in one sides only right.

It's an autotransformer. I'm just asking what wattage do you think is the above?

• Comments are not for extended discussion; this conversation has been moved to chat. Commented Jan 23, 2019 at 3:57

This transformer is not designed to be an autotransformer. It is designed to provide 50V, 30V, 18V, or 19V from either a 110V or a 220V supply. If using a 110V supply then you would get 50, 30, 18, or 19V on their respective secondaries by connecting your supply between the 0V and 110V primary taps. If using a 220V supply then you would connect your supply between the 0V and 220V taps. If I am understanding you correctly you are ignoring the secondaries and are using all three primary taps in an autotransformer configuration. This is a very bad idea, as it removes all isolation from the mains, introducing a severe shock hazard. I am willing to bet this is not how it was done in the original design, and I cannot recommend you proceed. If you need a step-down transformer, get a dedicated one with a 220V primary and a 110V secondary. Please don't go the autotransformer route.

Anyway, to answer your question, a general rule of thumb is about 50 VA per kg. If you weigh the transformer you can multiply the mass by 50 to get a rough VA value. More precise numbers can be acquired by diving into transformer design math, but I doubt you'll want to go that far. Also, I cannot guarantee that this 50 VA/kg is accurate when you're trying to use a standard transformer as an autotransformer. That's just asking for trouble since you give up the transformer's isolation ability, so I will re-iterate, please get a dedicated 220V:110V transformer instead.

• One won't say one waste time when one learns. What you have learnt today is that in china and asia where we don't even use any grounding or EGC. Manufacturers are into cost cutting so they use autotransformer design as as step down transformer. You can know this because all primary and secondary outputs have one lead connected, the common. And you need to use the 0v in primary to use multi taps secondary. I have done studies of this a lot last year. Also all our appliances like metal washing machine uses two prong only.No grounding. We never use grounding. We are living in third world country.
– Jtl
Commented Jan 23, 2019 at 1:44
• I agree - I've always used 25W/lb. (pretty much the same as 50VA/kg). An autotransformer with a 2:1 (or 1:2 step-up) would be able to handle about twice the power as half of the power would be supplied directly rather than through transformer action. Commented Jan 23, 2019 at 1:47
• 50 VA per kg estimate is for double winding. But for autotransformer design where only one winding is used. Please give what VA per kg estimate. Autotransformer is much lighter than double winding one.
– Jtl
Commented Jan 23, 2019 at 1:49
• Living in a country where 100% of appliance step down transformer uses one autotransformer/single winding design, and we don't use ground either. I'd like to know something. Even if your country has EGC. And even if you use autotransformer. If you put ground to the secondary. Then it's safe too, isn't it? In my country. Since we don't have any grounding in primary. Then no effect if there is no grounding either in secondary and primary and secondary not isolated. Of course isolation much safer that is why I ordered a $150 one just to see how good it is. – Jtl Commented Jan 23, 2019 at 1:53 • I struggle to understand why you're paying so much for your transformers. This transformer (digikey.com/product-detail/en/triad-magnetics/F-273U/…) from digikey is probably higher power than your one, and costs$30. Like every transformer that isn't an autotransformer, It also has isolation. This one (digikey.com/product-detail/en/hammond-manufacturing/1182P240/…) is \$150, but it's also about 10 times bigger than yours Commented Jan 23, 2019 at 2:05

It is a conventional step-down transformer with a primary that can be powered by either 220VAC or 120VAC. Based on the dimensions you gave the core cross-section would give it a rating of ~75 watts, or 125VA.

There are 4 other wires at back of it or further step down to 50v, 30v, 18v, 19v ac.

This transformer isolates the high voltage primary from the low voltage secondary taps which should have their own common return wire, possibly black in color. Obviously a multi-purpose transformer, nothing more than that. This means the 50 VAC output could safely supply ~1 amp, possibly a bit more, but heavy loads close to its maximum rating will make it get very hot. If too hot to touch then it is being overloaded.

Any transformer you use to experiment with should be fused. For this transformer use a 1 amp MDL (slow-blow) type fuse if powered by 120VAC. Use a .5 amp MDL type fuse if powered by 220VAC. A useful extra is to add a thermo-disc cut-off and mount it to the transformer. It should open at no more than 220$$\^o\$$F or 104$$\^o\$$C.

NOTE: Due to uncertain core cross-section values I could be giving this transformer a 50% or more safety margin. It may handle 2 amps at 50 vac. You have to go by the "too hot to touch" test. If it gets that hot seriously consider using only half as much current to drive a load.

EDIT: A 'true' isolation transformer often has a copper shield between the primary and secondary windings, so more cost. This type of transformer also has a green or green/yellow wire from the copper barrier. When this wire is Earth grounded it lowers the amount of capacitive leakage current, often reducing it to medical-grade levels of 50uA or less. Another proven isolation technique is to have windings next to each other instead of the primary winding being on top of the secondary winding.

• Do u mean it's an isolation transformer? Do u have diagram how the transformer isolates the low voltage secondary taps? As I understand it. It's jus a regular cheap autotransformer with this diagram.bing.com/images/… isolation transformer is 5 times expensive
– Jtl
Commented Jan 23, 2019 at 1:05
• @Jtl This is a standard transformer diagram with multiple secondary taps: i.postimg.cc/50sY7dtW/l21756701-1.png . Notice that the primary is not electrically connected to any of the secondaries. This means there is isolation between the primary and secondary coils Commented Jan 23, 2019 at 1:08
• You seem to be stuck on the transformers that are marketed as "isolation transformers". Once again, these transformers are designed for special purposes and they do cost a lot more. But a vast majority of basic transformers still provide electrical isolation. The transformer in your original question provides isolation between the primary and the secondaries, if you use it the way it is intended to be used (put the mains across only two of the primary taps and use the secondaries for their own purposes). That does not mean anyone would market it as an isolation transformer Commented Jan 23, 2019 at 1:14
• @Jtl Look at the second bullet in the description: "Step Up 110V/120V To 220V/240V Or Step Down 220V/240V To 110V/120V (Switch Is On The Back Of The Unit)". The switch changes which taps are used on the built-in transformer. That does not automatically make it an autotransformer. But, even if it was an autotransformer, I go back to my original comment: It is a very poor, dangerous design and should not be used. If you value your life and your safety, you will take the time to find the correct transformer which does not have the primary connected to the secondary. Commented Jan 23, 2019 at 1:26
• I have wasted way too much of my time on this. I gave you an answer to your question about power rating, as did Sparky. Do what you will, but be smart and don't get yourself killed by using a transformer in a stupid way. TAKE THE TIME TO DI IT RIGHT. It's worth your life. This is the last I'm saying on this topic. I'm done. Commented Jan 23, 2019 at 1:28