Understanding volt amps and transformers

6/20/2010 2:47:47 PMBrian

I have two class 2 transformmers. The primary is 115 volts and secondary is 24 volts @ 10 volt amps. Would this equate to be 440 milliamps in current and could I connect two of these transformers back to back for use in a B power supply, provided the load doesn't exceed 440 milliamps?

6/20/2010 3:51:53 PMNorm Leal

Brian

24 volts at 10 amps is 240 watts. Connecting 2 transformers back to back should give over two amps at 115 volts.

There are losses in transformers. Depending on the transformer you will not get 115 volts drawing 2 amps. Voltage will be less and current limited.

Regardless this is a good for making an isolation transformer for working on radios.

Norm

:I have two class 2 transformmers. The primary is 115 volts and secondary is 24 volts @ 10 volt amps. Would this equate to be 440 milliamps in current and could I connect two of these transformers back to back for use in a B power supply, provided the load doesn't exceed 440 milliamps?
:

6/20/2010 3:57:34 PMBrian

Thanks Norm,

Being class 2 transformers, would that restrict use in certain applications?

:Brian
:
: 24 volts at 10 amps is 240 watts. Connecting 2 transformers back to back should give over two amps at 115 volts.
:
: There are losses in transformers. Depending on the transformer you will not get 115 volts drawing 2 amps. Voltage will be less and current limited.
:
: Regardless this is a good for making an isolation transformer for working on radios.
:
:Norm
:
::I have two class 2 transformmers. The primary is 115 volts and secondary is 24 volts @ 10 volt amps. Would this equate to be 440 milliamps in current and could I connect two of these transformers back to back for use in a B power supply, provided the load doesn't exceed 440 milliamps?
::
:

6/20/2010 11:29:14 PM10 volt-amperes

Nope.

6/20/2010 11:27:08 PMReddy Kilowatt

Almost. 10 volt-amperes at 24 volts is about 417 ma. Keep in mind that this is the maximum current at 24 volts. The max current at 115 would be 24/115 X 417 or about 87 ma. using them back to back will give less due to losses. So expect 110 volts or so at 80 ma or so (both numbers are swags).

6/20/2010 11:43:10 PMMe Again

Remenber that watts is watts. Watts out can never be more than watts in. The secondary (24 volt) side max power would be 0.417 X 24 = 10 watts into a resistive load (the volt-ampere rating is for all loads, regardless of power factor). Therefore the max power available is 10 watts. When you connect the second transformer, 24 volts is stepped up to 115 volts but the max power available is still 10 watts. The max current available is (24/115) X 417ma or about 87 ma. These numbers assume 100% efficient transformers. Real life isn't that efficient and losses lower the power available at the output. How much depends on winding resistances, coupling coefficients, core losses and the phase of the moon (not really but we need to have some fun here).

:Almost. 10 volt-amperes at 24 volts is about 417 ma. Keep in mind that this is the maximum current at 24 volts. The max current at 115 would be 24/115 X 417 or about 87 ma. using them back to back will give less due to losses. So expect 110 volts or so at 80 ma or so (both numbers are swags).
:
::I have two class 2 transformmers. The primary is 115 volts and secondary is 24 volts @ 10 volt amps. Would this equate to be 440 milliamps in current and could I connect two of these transformers back to back for use in a B power supply, provided the load doesn't exceed 440 milliamps?
::
:

6/21/2010 1:30:36 AMNorm Leal

I took it as 10 amps. If really 10 volt/amps small transformer, less than half amp. Current will be much less. More like 80 ma as others said.

Norm

:Remenber that watts is watts. Watts out can never be more than watts in. The secondary (24 volt) side max power would be 0.417 X 24 = 10 watts into a resistive load (the volt-ampere rating is for all loads, regardless of power factor). Therefore the max power available is 10 watts. When you connect the second transformer, 24 volts is stepped up to 115 volts but the max power available is still 10 watts. The max current available is (24/115) X 417ma or about 87 ma. These numbers assume 100% efficient transformers. Real life isn't that efficient and losses lower the power available at the output. How much depends on winding resistances, coupling coefficients, core losses and the phase of the moon (not really but we need to have some fun here).
:
:
::Almost. 10 volt-amperes at 24 volts is about 417 ma. Keep in mind that this is the maximum current at 24 volts. The max current at 115 would be 24/115 X 417 or about 87 ma. using them back to back will give less due to losses. So expect 110 volts or so at 80 ma or so (both numbers are swags).
::
:::I have two class 2 transformmers. The primary is 115 volts and secondary is 24 volts @ 10 volt amps. Would this equate to be 440 milliamps in current and could I connect two of these transformers back to back for use in a B power supply, provided the load doesn't exceed 440 milliamps?
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6/21/2010 11:22:36 AMLewis

:Hi
:
: I took it as 10 amps. If really 10 volt/amps small transformer, less than half amp. Current will be much less. More like 80 ma as others said.
:
:Norm
:
::Remenber that watts is watts. Watts out can never be more than watts in. The secondary (24 volt) side max power would be 0.417 X 24 = 10 watts into a resistive load (the volt-ampere rating is for all loads, regardless of power factor). Therefore the max power available is 10 watts. When you connect the second transformer, 24 volts is stepped up to 115 volts but the max power available is still 10 watts. The max current available is (24/115) X 417ma or about 87 ma. These numbers assume 100% efficient transformers. Real life isn't that efficient and losses lower the power available at the output. How much depends on winding resistances, coupling coefficients, core losses and the phase of the moon (not really but we need to have some fun here).
::
::
:::Almost. 10 volt-amperes at 24 volts is about 417 ma. Keep in mind that this is the maximum current at 24 volts. The max current at 115 would be 24/115 X 417 or about 87 ma. using them back to back will give less due to losses. So expect 110 volts or so at 80 ma or so (both numbers are swags).
:::
::::I have two class 2 transformmers. The primary is 115 volts and secondary is 24 volts @ 10 volt amps. Would this equate to be 440 milliamps in current and could I connect two of these transformers back to back for use in a B power supply, provided the load doesn't exceed 440 milliamps?
With a resistive load like a light bulb, ten VA = ten Watts. Since the manufacturer can't control what a person is connecting to this transformer, you can burn it upp by connecting it to a inductive or capacitive load a Wattmeter will indicate zero Watts while the transformer burns up. Siince the Voltage and Amperage are 90 degrees out of phase in a completely reactive circuit, you are putting out all sorts of Volts and Amperes, but no real power measured in Watts. A Wattmeter has it's field measuring Volts, and its armature measuring Amperes, so therefore, when you have max amps, there are min Volts, and vice versa, so your meter reads zero, even thoo you are burning up the transformer by exceediing it's Volt-Ampere rating.
The isolation transformer I built, I fished out of the trash two 115/6 Volt transformers @ 20 Amps. This is equal to 120 VA, or 120 Watts of resistive load, Actually, I can run two sixty Watt bulbs all day and the transformers only get comfortably warm. Using this hookup, the oh, say thirty watts that a AA5 draws is nothing to this rig. The transformers also have three taps on the primary, well one primary is now a secondary, so have a rotary switch (trash can, again) so, I have OFF, 105, 115, and 125 Volts output. Each transformer says "1500 Volts insulation rating" So with two transformers, I am 3000 Volts isolated from Georgia Power. I have used it more for using trouble lamps outside in the damp grass while working on things like pickup truck and lawnmower, it (they?) give me a sense of security by being isolated from 120 Volts. But, this 10 VA business, that's ten Watts of resistive load. It sounds like, to me, that these guys are going to get a little warm on any load that you might be working on. Anyhow, hook 'em up and see what happens. By the fact that mine only get warm at full load tends to make me think that there might be an error in my number crunching.
Lewis

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6/21/2010 12:55:03 PMBrian

Thank you all. The back to back hook-up appears to be handling my 1938 4 tube battery radio OK.

::Hi
::
:: I took it as 10 amps. If really 10 volt/amps small transformer, less than half amp. Current will be much less. More like 80 ma as others said.
::
::Norm
::
:::Remenber that watts is watts. Watts out can never be more than watts in. The secondary (24 volt) side max power would be 0.417 X 24 = 10 watts into a resistive load (the volt-ampere rating is for all loads, regardless of power factor). Therefore the max power available is 10 watts. When you connect the second transformer, 24 volts is stepped up to 115 volts but the max power available is still 10 watts. The max current available is (24/115) X 417ma or about 87 ma. These numbers assume 100% efficient transformers. Real life isn't that efficient and losses lower the power available at the output. How much depends on winding resistances, coupling coefficients, core losses and the phase of the moon (not really but we need to have some fun here).
:::
:::
::::Almost. 10 volt-amperes at 24 volts is about 417 ma. Keep in mind that this is the maximum current at 24 volts. The max current at 115 would be 24/115 X 417 or about 87 ma. using them back to back will give less due to losses. So expect 110 volts or so at 80 ma or so (both numbers are swags).
::::
:::::I have two class 2 transformmers. The primary is 115 volts and secondary is 24 volts @ 10 volt amps. Would this equate to be 440 milliamps in current and could I connect two of these transformers back to back for use in a B power supply, provided the load doesn't exceed 440 milliamps?
:With a resistive load like a light bulb, ten VA = ten Watts. Since the manufacturer can't control what a person is connecting to this transformer, you can burn it upp by connecting it to a inductive or capacitive load a Wattmeter will indicate zero Watts while the transformer burns up. Siince the Voltage and Amperage are 90 degrees out of phase in a completely reactive circuit, you are putting out all sorts of Volts and Amperes, but no real power measured in Watts. A Wattmeter has it's field measuring Volts, and its armature measuring Amperes, so therefore, when you have max amps, there are min Volts, and vice versa, so your meter reads zero, even thoo you are burning up the transformer by exceediing it's Volt-Ampere rating.
: The isolation transformer I built, I fished out of the trash two 115/6 Volt transformers @ 20 Amps. This is equal to 120 VA, or 120 Watts of resistive load, Actually, I can run two sixty Watt bulbs all day and the transformers only get comfortably warm. Using this hookup, the oh, say thirty watts that a AA5 draws is nothing to this rig. The transformers also have three taps on the primary, well one primary is now a secondary, so have a rotary switch (trash can, again) so, I have OFF, 105, 115, and 125 Volts output. Each transformer says "1500 Volts insulation rating" So with two transformers, I am 3000 Volts isolated from Georgia Power. I have used it more for using trouble lamps outside in the damp grass while working on things like pickup truck and lawnmower, it (they?) give me a sense of security by being isolated from 120 Volts. But, this 10 VA business, that's ten Watts of resistive load. It sounds like, to me, that these guys are going to get a little warm on any load that you might be working on. Anyhow, hook 'em up and see what happens. By the fact that mine only get warm at full load tends to make me think that there might be an error in my number crunching.
:Lewis
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6/21/2010 2:09:50 PMLewis

:Thank you all. The back to back hook-up appears to be handling my 1938 4 tube battery radio OK.
:
:
:::Hi
:::
::: I took it as 10 amps. If really 10 volt/amps small transformer, less than half amp. Current will be much less. More like 80 ma as others said.
:::
:::Norm
:::
::::Remenber that watts is watts. Watts out can never be more than watts in. The secondary (24 volt) side max power would be 0.417 X 24 = 10 watts into a resistive load (the volt-ampere rating is for all loads, regardless of power factor). Therefore the max power available is 10 watts. When you connect the second transformer, 24 volts is stepped up to 115 volts but the max power available is still 10 watts. The max current available is (24/115) X 417ma or about 87 ma. These numbers assume 100% efficient transformers. Real life isn't that efficient and losses lower the power available at the output. How much depends on winding resistances, coupling coefficients, core losses and the phase of the moon (not really but we need to have some fun here).
::::
::::
:::::Almost. 10 volt-amperes at 24 volts is about 417 ma. Keep in mind that this is the maximum current at 24 volts. The max current at 115 would be 24/115 X 417 or about 87 ma. using them back to back will give less due to losses. So expect 110 volts or so at 80 ma or so (both numbers are swags).
:::::
::::::I have two class 2 transformmers. The primary is 115 volts and secondary is 24 volts @ 10 volt amps. Would this equate to be 440 milliamps in current and could I connect two of these transformers back to back for use in a B power supply, provided the load doesn't exceed 440 milliamps?
::With a resistive load like a light bulb, ten VA = ten Watts. Since the manufacturer can't control what a person is connecting to this transformer, you can burn it upp by connecting it to a inductive or capacitive load a Wattmeter will indicate zero Watts while the transformer burns up. Siince the Voltage and Amperage are 90 degrees out of phase in a completely reactive circuit, you are putting out all sorts of Volts and Amperes, but no real power measured in Watts. A Wattmeter has it's field measuring Volts, and its armature measuring Amperes, so therefore, when you have max amps, there are min Volts, and vice versa, so your meter reads zero, even thoo you are burning up the transformer by exceediing it's Volt-Ampere rating.
:: The isolation transformer I built, I fished out of the trash two 115/6 Volt transformers @ 20 Amps. This is equal to 120 VA, or 120 Watts of resistive load, Actually, I can run two sixty Watt bulbs all day and the transformers only get comfortably warm. Using this hookup, the oh, say thirty watts that a AA5 draws is nothing to this rig. The transformers also have three taps on the primary, well one primary is now a secondary, so have a rotary switch (trash can, again) so, I have OFF, 105, 115, and 125 Volts output. Each transformer says "1500 Volts insulation rating" So with two transformers, I am 3000 Volts isolated from Georgia Power. I have used it more for using trouble lamps outside in the damp grass while working on things like pickup truck and lawnmower, it (they?) give me a sense of security by being isolated from 120 Volts. But, this 10 VA business, that's ten Watts of resistive load. It sounds like, to me, that these guys are going to get a little warm on any load that you might be working on. Anyhow, hook 'em up and see what happens. By the fact that mine only get warm at full load tends to make me think that there might be an error in my number crunching.
::Lewis

Good, good, good!
As I said, watch the temperature of the transformers. Mine don't get very warm at all when I run them at (by my figgering) the max kVA. Hey, fine by me. Hope this might help.
Lewis

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