# I have a question in regards to Transformers

Discussion in 'General Electronics Chat' started by I Want To Learn, Jan 28, 2009.

1. ### I Want To Learn Thread Starter New Member

Jan 28, 2009
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Hello.

I have a question. Hopefully, someone can answer it.

Right!

If one has an audio amplifier that offers two 15-amp fuses (one per transformer), would that mean each Transfomer is 1.5 - 2 kW?

Can a fuse determine the capacity of a Transformer?

2. ### beenthere Retired Moderator

Apr 20, 2004
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No, not from the size of the fuses, although it's an indicator. Power is a product of: Current X Voltage, currrent squared X resistance (the load), or voltage squared/resistance.

But if the speakers (assuming this is an output transformer, rather than the input power transformer) are 4 ohms, the power is 225 X 4, or 900 watts.

3. ### I Want To Learn Thread Starter New Member

Jan 28, 2009
6
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Cheers beenthere!

So, how does one determine the capacity of a power transformer when, there is no indication stating it's capacity?

The amplifier in question offers two transformers using two 15-amp circuit breakers and, offers, an output wattage of 350 (in 8 ohms), 650 (in 4 ohms), & 900 (in 2 ohms) watts per channel.

Is there a calculation you can share with me to find the answer?

4. ### Audioguru New Member

Dec 20, 2007
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If the amplifier circuit is linear, not switching and if the power numbers are truthful RMS power at clipping, then 350W x 2= 700W in output plus about 500W in heat which is a total power of 1200W.

Amplifier have not used output transformers for about 50 years.

5. ### I Want To Learn Thread Starter New Member

Jan 28, 2009
6
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Cheers Audioguru!

The amplifier in question is a Crest Professional Series 5000. It was designed in 1982. Unfortunately, Crest does not offer any information in regards to this amplifier on their website.

The most I have is a spec sheet which states "FTC rated continuous average sine wave over a bandwidth from 10Hz to 20kHz."

It weighs around 80 pounds.

6. ### studiot AAC Fanatic!

Nov 9, 2007
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I thought the amp sounded like an eighties design. They used to put separate power supplies for each channel with hefty reservoir capacitors, in the upmarket models.

Fuses have to be rated for the cold inrush current of these caps. This can be sevral times the normal running current.

If this was a dual voltage job (120v /240v) the fuse may also be double rated for 120v, have you checked this?

7. ### I Want To Learn Thread Starter New Member

Jan 28, 2009
6
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My apologies.

The amplifier uses two 15-amp circuit breakers no fuses. It cannot be switched to 240 volts externally.

Each Transfomer house two 17,500 MFD 100 volt capacitors. Does that help?

Could each Transformer be 1 kW?

Cheers for the help!

Nov 9, 2007
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9. ### I Want To Learn Thread Starter New Member

Jan 28, 2009
6
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Cheers studiot!

The bluebook value won't offer that type of information. It is very simplistic.

I always thought the fuse/circuit breaker before the power transformer, was there to prevent overload to the transformer.

I just want to know if the fuse/circuit breaker rating is based on the continuous duty cycles at the rated amperage or, peaks and, would that equate to how much watts the transformer can dissipate.

Maybe it would be best to just connect a regulator and monitor current drawn at the given load using a sine wave?

10. ### studiot AAC Fanatic!

Nov 9, 2007
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A rule of thumb for power amplifiers voltage rails is:

Vsquared divided by eight times the load impedance

This puts the power rail at about 140 or ±70.

If however the 600watts is derived from a bridge configuration (these were just coming in in the early 80s) you can get four times the power from a given rail.

This would make a 70 volts rail or ±35, which squares with the 100volt capacitor.

Did you say there was only one per channel?

5 amps rms running would be about right for a 600w amp, allowing for a few inefficiencies, so my estimate of a fuse at three times this for inrush currents holds good. 17500μF caps were big in those days.

Your transformers are unlikely to be 1Kw. I would guess the next size above 600w would be 750w, so they will be between 600 and 750.

Oh and if you look at fuses they say 'rms rating' so the peak current is already built in.

11. ### I Want To Learn Thread Starter New Member

Jan 28, 2009
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The voltage rails are 98 volts +/- for each channel. There are two total.

One transformer is attached to each voltage rail.

Two capacitors are connected into series creating 200 volts 8750 mfd. There are four capacitors total, two per transformer.

I do not see any fuses except two 15-amp circuit breakers. One for each transformer. So, would the same method apply as fuses in reference to the peak value already built in?

Here is a question that crossed my mind.

Would a transformer's secondary voltage relate to it's current capacity?

Thank you for the help. As you can see, I am not an EE. I am just the average bloke with no schooling on electronics with a schematic in my hand.

Last edited: Jan 29, 2009