# AC Transformer Substitution - Determining Current Handling Requirements

Thread Starter

#### jgilroy

Joined Jan 21, 2022
3
I am trying to repair a vintage computer terminal which has a fried power transformer. Most specs are not documented but I know the Primary(s) is 120V/240V AC, the Secondaries are 18V, 30V, 32V CT, and 20V CT. The input (in my case 120V) is fused at 1.25A.

I know I am extremely unlikely to find an exact replacement for this transformer, and I'm considering multiple parallel transformers to generate the required secondary voltages. I am not sure how to determine what the current rating on any transformer I source needs to be.

I have seen "The secondary current is equal to the product of the primary voltage and primary current divided by secondary voltage." Can I take the 1.25A fuse as the primary current (it must actually be somewhat less, no?) and use this in the calculations? For the CT secondaries, do I calculate (for example) the -10V -- +10V current, or do I need to also consider the 0 -- +10V current (which would be higher, if this calculation is right?)

My current thoughts are:

(120V * 1.25A) / 18V = 8.33A

(120V * 1.25A) / 30V = 5A

(120V * 1.25A) / 32V = 4.69A

(120V * 1.25A) / 20V = 7.5A

Thanks!

#### Papabravo

Joined Feb 24, 2006
20,411
There is a flaw in your thinking. The transformer will not support the idea of taking more power out than the power in provided by the line voltage. So you still have no idea what output power is required by each voltage. All you know is the sum of the power required by each of the secondaries will be less than the power provided by the primary. On top of that you have no idea why the transformer got fried. Replacing it will not likely solve that problem.

Thread Starter

#### jgilroy

Joined Jan 21, 2022
3
There is a flaw in your thinking. The transformer will not support the idea of taking more power out than the power in provided by the line voltage. So you still have no idea what output power is required by each voltage. All you know is the sum of the power required by each of the secondaries will be less than the power provided by the primary. On top of that you have no idea why the transformer got fried. Replacing it will not likely solve that problem.
Thank you. I understand that there are likely other faults and fixing this is probably the tip of the iceberg, but the transformer voltages feed the power supply and getting these voltages in will help me test the rest of the power supply. The transformer blows the 1.25A fuse even when secondaries are not connected to the power supply.

As for the flaw in my thinking - is there no way with the information I have to specify a safe maximum current rating for each replacement transformer?

#### Papabravo

Joined Feb 24, 2006
20,411
Thank you. I understand that there are likely other faults and fixing this is probably the tip of the iceberg, but the transformer voltages feed the power supply and getting these voltages in will help me test the rest of the power supply. The transformer blows the 1.25A fuse even when secondaries are not connected to the power supply.

As for the flaw in my thinking - is there no way with the information I have to specify a safe maximum current rating for each replacement transformer?
If you use the numbers that you computed then each of the transformers will have a significant safety factor. I'm assuming here that cost is not likely to be a consideration. I do have to wonder why there are four AC voltages with small differences between them.

Thread Starter

#### jgilroy

Joined Jan 21, 2022
3
If you use the numbers that you computed then each of the transformers will have a significant safety factor. I'm assuming here that cost is not likely to be a consideration. I do have to wonder why there are four AC voltages with small differences between them.
Cost is a consideration, and I'm not sure it's worth pursuing this plan, but wanted to be sure that if I do I am on the right track with regards to current with the limited information I have. I would be happy to hear if you have any suggestions to control cost. My assumption is that if the primary is fused at 1.25A, normal draw must be somewhat lower than that and I could at least round down the current rating for the substitute transformers. I suppose I could also take a chance on cheaper / lower current rated transformers, and fuse them for safety?

#### Papabravo

Joined Feb 24, 2006
20,411
Cost is a consideration, and I'm not sure it's worth pursuing this plan, but wanted to be sure that if I do I am on the right track with regards to current with the limited information I have. I would be happy to hear if you have any suggestions to control cost. My assumption is that if the primary is fused at 1.25A, normal draw must be somewhat lower than that and I could at least round down the current rating for the substitute transformers. I suppose I could also take a chance on cheaper / lower current rated transformers, and fuse them for safety?
I've never taken on a task like this, so I'm ill prepared to advise you.

#### Ian0

Joined Aug 7, 2020
8,439
I know I am extremely unlikely to find an exact replacement for this transformer,
Not so - you could easily have one made.

If you look at the size of the bobbin (printed on it) and the height of the stack of laminations you can easily find the power rating.
Then the sum of all the secondary powers must add up to less than the rated power for that bobbin and stack of laminations.
What you need to know is how the power is distributed.
Something must produce 5V at a lot of amps for the logic. Then you need to work out what all the rest of the secondaries actually do. The biggest clue will be in the wire size.
For the centre-tapped secondary you need to look what type of rectifier it has.