I have few questions about isolation transformer.

Can I use a low power rating isolation transformer to test higher rating machine? For example, most desktop PC has around 450W, which is maximum power output. My computer consumes between 20W to 95W.

The country I live has 220 AC power grid. If I want to build an isolation transformer with two identical power transformers, what should I look for? Below is specification of a power transformer on a supplier. 50 VA is equal to 50W power rating?

AC Power : 50VA
External Depth : 79mm
External Length / Height : 79mm
External Width : 66mm
Input Voltage : 230V
Output Voltage : 110V
Single Primary Voltage : 230V
Single Secondary Voltage : 110V

I found information that building an isolation transformer is just to connect the secondary winding of two identical transformers. I haven't find a through building instruction or guide yet.

If I plan to build an isolation transformer that has 200W power rating, do I only need buy two power transformers of 200VA and few basic components - power on/off switch, output terminal, a case, fuse?


Thanks in advance.

 

Can't you just buy a 220V-110V transformer? They are a commonly available and cheaper than using two back-to-back transformers.

A 50 VA transformer can handle a 50W resistive load. It's less for a load with a power factor less than one. Generally you probably don't want to connect a load with a power rating greater than about 75% of the VA rating.

 

The following could be your reference:

>50 VA is equal to 50W power rating?

They are the same, just represent in the different kinds of components.

A few basic components -

1. AC Power wire.
2. Power on/off switch.
3. Power on Indicator. (Neon indicator as : http://shop.rabtron.co.za/catalog/images/R777R.jpg)
4. output terminal.
5. A case.
6. Fuse.
7. Fuseholder. (https://www.egr.msu.edu/eceshop/Parts_Inventory/images/fuse holder2.jpg)
8. Transformer x 2.

Output Require :
Output Voltage : 110Vac.
Output Watts : 200W.

Output Current : 200W/110V = 1.818 A.
Add 20%~30% of calculation output current for spare.
Total Output Current : 1.818A x 20%~30% = 2.1816A ~ 2.363A
Total Output Watts : 110V x (2.1816A ~2.363A) = 240W ~ 260W.
Porbably you can buy two 220V/110V/2.5A transformers.

How many current you should buy, depends on how many watts do you want.

 

Can't you just buy a 220V-110V transformer? They are a commonly available and cheaper than using two back-to-back transformers.

A 50 VA transformer can handle a 50W resistive load. It's less for a load with a power factor less than one. Generally you probably don't want to connect a load with a power rating greater than about 75% of the VA rating.

Thanks crutschow.

I opened up my step down transformer and took photos.

Specification:
AC power: 2kVA
Frequency: 60Hz
Input voltage: 220V
Output voltage: 110V
Secondary current: 18.2A
Insulation type: E

Light blue wire is primary power wire to primary coil.
Brown wire connected to the fuse holder is ground.
Yellow and naked copper wire is power wire from secondary coil.

Am I understand correct?

How to connect two transformers for making isolation transformer?

Three wires go into the coil

Rear side

 

That transformer is NOT an Isolation T/F, it only has 3 wires its an auto T/F. An isolation T/F has 4 wires, 2primary & 2 secondary.

 

Here wiring diagram I just made.

The power on/off switch has three terminals: T1, T2 on top, T3 is bottom.
T3 is connected to blue wire of the cable.
T1 and T2 is connected even power off state.
When switch is on, T1, T2, T3 are all connected.

T1 is connected by yellow wire to O1. A coil wire C1 is connected to the O1.
C1 and yellow wire are soldered on the O1 terminal.

@debe

You are right. This one is 220V to 110V step-down transformer.
I like to know whether I can buy two of these for making a isolation transformer.

 

Two kinds of Isolation T/F:
1) 1:1 - 220V,0V to 220V,0V (One T/F)
2) 2:1 - a)220V,0V to b)110V,0V → c)110V,0V to d)220V.0V (Two T/F)

* b)110V connect to c)110V
* b)0V connect to c)0V.

 

This is an Isolation T/F. there should be 2 seperate windings to isolate the primary from the secondary. See test with megger, no resistance (leakage) betwean primary & secondary. No you cant make an isolation T/F out of 2 Auto T/Fs.

 

An isolation transformer, by definition, has no electrical connections between the primary and secondary.

 

@debe

Thanks. That makes me move on.