I like to buy surplus stuff 😀
When you know what to look for you can get very good components from recycling, surplus stores, hamfest etc...
I got quite a few very well constructed power transformers, some E/I laminations and some toroids for very little money.
Finding the primary, secondary windings, the voltage etc of an unknown transformer is fairly easy with just a DVM and a variac.
But how can I figure the current rating?
Of course just a rough examination of the size of the wires, the weight of the transformers can give an idea, but I would like to be a bit more precise.
My first idea is to monitor the AC voltage at the secondary with no load and then add resistors across until the voltage drops by say 10%. I can then measure the voltage at the resistor, then measure the resistor and ohms law will tell me what current is there.
I guess that would work but is 10% drop a good value? less? more?
Another way I am thinking is to devise a 'variable load' circuit and measure the DC current. I guess DC current will be the same value of the AC current.
Does someone have a better idea? 🙄
When you know what to look for you can get very good components from recycling, surplus stores, hamfest etc...
I got quite a few very well constructed power transformers, some E/I laminations and some toroids for very little money.
Finding the primary, secondary windings, the voltage etc of an unknown transformer is fairly easy with just a DVM and a variac.
But how can I figure the current rating?
Of course just a rough examination of the size of the wires, the weight of the transformers can give an idea, but I would like to be a bit more precise.
My first idea is to monitor the AC voltage at the secondary with no load and then add resistors across until the voltage drops by say 10%. I can then measure the voltage at the resistor, then measure the resistor and ohms law will tell me what current is there.
I guess that would work but is 10% drop a good value? less? more?
Another way I am thinking is to devise a 'variable load' circuit and measure the DC current. I guess DC current will be the same value of the AC current.
Does someone have a better idea? 🙄
A fixed value of 10% is useless. Bigger transformers have better regulation. The only thing you can do is guess from weight against similar known types and measure temperature rise under load
The 10% drop is useful when you have multiple secondaries and have no idea as to how the power is split between them.
It is only a rough guide though.
It is only a rough guide though.
The problem of guessing the total power of an unknown Xformer has been discussed on numerous occasions on this forum, here are some examples:
http://www.diyaudio.com/forums/powe...-transformer.html?highlight=transformer+power
http://www.diyaudio.com/forums/powe...-transformer.html?highlight=transformer+power
http://www.diyaudio.com/forums/powe...ctional-area.html?highlight=transformer+power
http://www.diyaudio.com/forums/part...-transformer.html?highlight=transformer+power
http://www.diyaudio.com/forums/powe...power-rating.html?highlight=transformer+power
http://www.diyaudio.com/forums/powe...er-va-rating.html?highlight=transformer+power
Once that is known, the rating of each winding can be computed using this method:
http://www.diyaudio.com/forums/powe...-transformer.html?highlight=transformer+power
http://www.diyaudio.com/forums/powe...-transformer.html?highlight=transformer+power
http://www.diyaudio.com/forums/powe...-transformer.html?highlight=transformer+power
http://www.diyaudio.com/forums/powe...ctional-area.html?highlight=transformer+power
http://www.diyaudio.com/forums/part...-transformer.html?highlight=transformer+power
http://www.diyaudio.com/forums/powe...power-rating.html?highlight=transformer+power
http://www.diyaudio.com/forums/powe...er-va-rating.html?highlight=transformer+power
Once that is known, the rating of each winding can be computed using this method:
http://www.diyaudio.com/forums/powe...-transformer.html?highlight=transformer+power
Thank you Elvee and the others.
I will look at these links later.
The transformers I have are all laminated core, no doughnuts😛.
I found while searching the net that these transformers are about 15VA per pound.
The other trick seems to put a load on them and wait for them to warm up to a point where you can barely hold your hand on them. At first that did not make sense to me, but thinking again, most transformers will have a temperature rise under load and many makers will state this temp at 60C.
I have read somewhere that 50C will cause first degree burns and produce blisters. I like my equipment to run as cool as possible, so the 'hand test' is fairly good one. It will tell me I need bigger heat-sink, or transformers, so I will tend to stay on the conservative side.😉
I will look at these links later.
The transformers I have are all laminated core, no doughnuts😛.
I found while searching the net that these transformers are about 15VA per pound.
The other trick seems to put a load on them and wait for them to warm up to a point where you can barely hold your hand on them. At first that did not make sense to me, but thinking again, most transformers will have a temperature rise under load and many makers will state this temp at 60C.
I have read somewhere that 50C will cause first degree burns and produce blisters. I like my equipment to run as cool as possible, so the 'hand test' is fairly good one. It will tell me I need bigger heat-sink, or transformers, so I will tend to stay on the conservative side.😉
- Status
- Not open for further replies.