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How to determine max output current of a power transformer?

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neazoi

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Hi,
My Linear PSU failed (burns the mains fuse). I was sold this second hand to be a 40A 13.8V PSU. It has 4 transistors 2n3055 on a left heatsink, another 4 2n3055 on the right heatsink and one more 2n3055 at the back of the PSU. The design is not regulated by a chip, unless there is a 3-pin chip that looks like a transistor. Anyway that's the info.

I operated it for 6 months or more with a 200W HF transceiver on SSB, which required 41A (by the manual). Even on full power out of the transceiver, I did not notice any significant heating of the heatsinks (cool when I added a fan myself). The PSU is an old one made by RMS (made in Italy) model K1540, which might mean 15v max 40A, or maybe not. At the time it was burnt out, I had not previously operated it. I switched this on, I drawn the power and it failed. So I had not previously heated it.


Now here is the question.

1. Was that PSU ever capable of giving out 40A or was I pushed it? (I have to say that when it worked at peak powers I could hear a bit more hum out of the transformer, as peak current was drawn. But I believe this was normal.)

2. I would like to build another PSU out of the good parts of this old one, so I wonder if the transformer alone can give me 40-45A?

Some info I can give which might help:
The transformer measures 11 x 15 x 13 centimetres in size (including the plastic bobbin of the winding) and the secondary enamel wiring into it is 4.14mm in diameter. The iron core alone is 6 x 15 x 13cm.
The 220v mains fuse was rated for 4A.

I have no other way to measure the transformer current, so I wonder if it worth's making a 40A PSU out of it (maybe with a few 2n3771) or if the transformer is not capable of this power.
 
Last edited:

Hi,

From nowadays view it makes not much sense to build a 15V 40A linear power supply.
It means 600W but needs at least a 1000VA transformer, usually a lot more.

The mains current is far away from being sinusoidal, thus it causes overtones.
The linear regulation will cause a lot of heat = wasted energy.
The switch ON current if a toroidal transformer may easily be 20 times of nominal current.
It needs big electrolytics capacitors that will age (dry out) with time.
It needs big heatsinks, you even used a fan.

Maybe even the heatsinks and the fan cost as much as a cheap switch mode power supply.

But you are free to do whatever you want.

Klaus
 

    neazoi

    Points: 2
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Was that PSU ever capable of giving out 40A or was I pushed it?

These regulators are very rugged and practically indestructible. But did you take a look at the filter caps?

At 40A, you should have both capacitor and inductor in the pi configuration at the output.

The efficiency can be high: around 50-60% is not uncommon (600W output but 1000W input).

When turned on, the PSU appears like a dead short the the mains. It may appear to be a 5kW load (just a guess: it may be 10kW for the first 5-10 ms)

What was the mains fuse rating? Replace it and try again!:)
 

    neazoi

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What was the mains fuse rating? Replace it and try again!:)
I did that of course and it burned it instantly again.
The fuse is 220v 4A.
Taking into account the efficiency if a linear PSU, is this fuse rating too low for the PSU to have an output of 40A?
--- Updated ---

I understand all these. It is just in HF radio, noise from a SMPSU is unacceptable. Even these nice commercial SMPSUs sold for HAM radio have a knob to shift the PSU frequency so that it does not interfere with the station you are trying to listen. Now this in theory is good but has 2 problems:

1. The stability of the SMPSU frequency is not good, so the frequency and harmonics wander around and you need to readjust the think from time to time.
2. As you scan the band you hear all these garbage, every 60KHz apart (harmonics of the PSU)

And it is not just the frequencies transfered through the cables (which could be filtered I believe with a good LPF). Even the cabling out of the PSU and up to the transceiver radiate all these harmonics like an antenna, so shielding is really a nightmare if impossible.

On HF radio, all these are unacceptable.
So yes, as a general purpose PSU a SMPSU, with all it's advantages, is good, bot not near an HF transceiver!

Over the years, nothing can convince me yet for the opposite, I guess this is why Astron is still making them even on high powers. They are not portable but they are good for lower frequency radios. On VHF/UHF I bet it does not matter a lot.
 
Last edited:

The fuse is 220v 4A.
Taking into account the efficiency if a linear PSU, is this fuse rating too low for the PSU to have an output of 40A?

I do not understand how it worked for six months without the fuse blowing!

Mains fuses (even HRC fuses) can be often overrated; I mean that a fuse marked 4A can happily take 8A without blowing!

But they have become better (and more expensive) with continuous development (phrase borrowed from the manufacturers).

But you need to check the capacitors in the output filter state first.
 


    neazoi

    Points: 2
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Hi,

A possible explanation for fail
* dried out electrolytic capacitors
* regulation interferance with HF
Both may result in oscillations with uncontrolled high current peaks that cause damage of additional components.

I know that the HF guys like to use linear power supplies. This is nothing unusual.
And I agree that there are SMPS with noisy output.
Each has it's benefits and drawbacks.

Due to the low frequency the conventional supplies need big capacitors to store energy.
If one wants to keep the low frequency voltage ripple low (example: below 1V) you need about 200,000uF
If you want less ripple then you need a linear active stage. Simplest case: a gyrator stage and capacitor..
But for a more accurate output voltage you need some kind of reference and a feedbacked regulation loop.

An SMPS already has a feedbacked regulation loop.
The output voltage is known and rather stable.
It usually has much less output ripple than a simple conventional supply, but ripple frequency is higher.

So adding a simple gyrator stage with a capacitor should be more simple and more effective in ripple suppression than at a conventional supply due to the higher frequency. Even a damped LC should drop the HF noise rather good.
Simple calculations, small parts, but one still needs to take care about wiring/ PCB layout.

Mind: For each 1V of headroom to regulate a 40A suply you need to accept 40W of heat. An SMPS surely needs way less headroom.

Thus my personal preference is an SMPS with some kind of post filtering. But there's no need to do like I do.
I just tell my opinion.

Klaus
 

    neazoi

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Thus my personal preference is an SMPS with some kind of post filtering. But there's no need to do like I do.
I just tell my opinion.
No I understand you got a strong point here no doubt.
I have not yet used a SMPSU designed for Ham radio usage, the only experience I have is these LED lights modules and they ARE VERY noisy, you can hear the 60KHz spaced harmonics all the way from 120KHz to 30MHz and more. However SMPSUs that are designed specifically for HAM usage, they do have a frequency shift knob. Now this tells me that they DO produce noise, even these designed for a purpose where noise is unacceptable. It seems that they can't make one without noise escaping (either out of the cables or just leaking) and the only countermeasure is to shift the noise. This is why I am suspicious about them. Else their current capability, efficiency, size and price are just amazing.
--- Updated ---

I do not understand how it worked for six months without the fuse blowing!

Mains fuses (even HRC fuses) can be often overrated; I mean that a fuse marked 4A can happily take 8A without blowing!

But they have become better (and more expensive) with continuous development (phrase borrowed from the manufacturers).

But you need to check the capacitors in the output filter state first.

What amperage fuse would you expect, for 40A output (220v input) given a 50-60% PSU efficiency?
 

The transformer can be identified as EI150c, rated power with fully utilized winding window 600 to 650 VA. DC power supply with bridge rectifier has a form factor of about 1.6 (ratio of transformer rms output current to DC output current) respectively useable output power is maximal 400 W. Assuming 15 V transformer output voltage, which is already marginal, you won't get more than 27A continuous DC output.

4A mains fuse is a bit low but can work, must be however slow blow type. Fast fuse will immediately burn due to inrush current. If a slow blow 4A fuse blows without load current, you have some defect in your power supply, e.g. transformer winding short, defective rectifier or capacitor, defect in linear regulator.

I presume you know how to locate the defective part by disconnecting parts of the circuit, or measurements with multimeter.

Linear power supplies have been used for decades, it has clear advantages regarding EMI.
 

The transformer can be identified as EI150c, rated power with fully utilized winding window 600 to 650 VA. DC power supply with bridge rectifier has a form factor of about 1.6 (ratio of transformer rms output current to DC output current) respectively useable output power is maximal 400 W. Assuming 15 V transformer output voltage, which is already marginal, you won't get more than 27A continuous DC output.

4A mains fuse is a bit low but can work, must be however slow blow type. Fast fuse will immediately burn due to inrush current. If a slow blow 4A fuse blows without load current, you have some defect in your power supply, e.g. transformer winding short, defective rectifier or capacitor, defect in linear regulator.

I presume you know how to locate the defective part by disconnecting parts of the circuit, or measurements with multimeter.

Linear power supplies have been used for decades, it has clear advantages regarding EMI.
Thanks FvM!
All understood.
The 600-650VA you gave for the transformer is based on the information I supplied in the post right?
 

That's information given in transformer handbooks for EI150c core, which corresponds to the reported core size.
 

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