transformer/rectifier question

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Mike
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transformer/rectifier question

Post by Mike » Fri Sep 03, 2004 4:05 pm

I found a 24VCT 10A transformer from www.mpja.com for only $17.99.<p>I always forget, how much power will this produce after rectifier (+/- what?)<p>Also, this is going to be used to power an audio amp, how much wattage of amps can this power? They also have a 4A. How much could that power?

Dean Huster
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Re: transformer/rectifier question

Post by Dean Huster » Fri Sep 03, 2004 5:12 pm

Mike, in general, you can just multiply the current and voltage rating of the transformer to get a general idea of what the maximum ideal power would be, in this case 240 watts for one channel, 120 watts per channel for two, etc. Of course, there will be losses in the transformer as it's loaded down, in the rectifiers, etc. and the amp will probably clip at a voltage a lot less than the supply, so you could probably derate everything by at least 10 to 20% for a more reasonable figure.<p>I had a PAIR of new 60v, 10a transformers that I somehow lost in one of many moves. How I lost that much iron is beyond me, but I did. Must have been in a box hidden in a dark corner of the attic or something. Boy, did I ever have plans for those puppies!<p>Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

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ian
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Re: transformer/rectifier question

Post by ian » Tue Sep 07, 2004 9:23 am

Hey, I know this isn't a advertisment site but I have a better deal. I'm unloading 12VDC 600mA wall warts for $1 and 5VDC 2A wall warts for $2. <p>
"Windmills are the future of power generation."

Mike
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Re: transformer/rectifier question

Post by Mike » Tue Sep 07, 2004 4:49 pm

Thanks, but 600ma is way too low, and so is 5V<p>plus i want internal, not wall warts<p>thanks anyway

k7elp60
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Re: transformer/rectifier question

Post by k7elp60 » Wed Sep 08, 2004 5:48 am

When using a transformer to supply a DC voltage and using brute force filtering one can only get approximately 60% of the rated current from the transformer, without overheating the transformer.
This is because the peak charging current is greater than the average current and puts an extra load on the transformer.

toejam
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Re: transformer/rectifier question

Post by toejam » Wed Sep 08, 2004 6:42 am

from whai i know about transformers, A 24v 10 a transformer can deliver that much power (240 watts) indefenitally.As far as the efficiency of diode rectifiers, i think it is higher than 60%. Brute force filtering is nothing more than high value caps. They may use a lot of current initally charging, but things coast alond smoothly after that.

k7elp60
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Re: transformer/rectifier question

Post by k7elp60 » Thu Sep 09, 2004 6:09 am

In response to tojams post.
I have built numerous linear power supplies that have used brute force filtering. Several years ago I was haveing trouble with a specific power supply transformer overheating. Consulting with a engineer at Triad Transformer Corp., I was told to figure about 0.6 times the rated current for load current with this type of filtering.
On some recent experiments with a fullwave CT configuration and and a fullwave bridge circuit on a transformer that was rated at 12.6V CT @ 4A.
I put a 4A DC load on the circuit and used a 35A bridge rectifier. The peak charging current was 13.2A for the fullwave CT and 9.6A for the full wave bridge. The filter cap was 10,000uF or slightly under the rule of thumb of 3000uF per amp of load current.
These experiments only took about 1/2 hour and the transformer was hot to the touch. The peak charging current was a continous amount.<p>[ September 09, 2004: Message edited by: k7elp60 ]</p>

toejam
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Re: transformer/rectifier question

Post by toejam » Sat Sep 18, 2004 4:32 am

k7elp60 you say that by putting a large cap in parallel to a fw. rectifier and applying a load that matches it's capacity,that transformer's capacity needs to be upped by 60%? I do remember substituting a higher value cap in an old tube amp and the power transformer did fry, but i never attributed it to the cap size.I learn something new, Every time i read this board.

k7elp60
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Re: transformer/rectifier question

Post by k7elp60 » Sat Sep 18, 2004 8:23 am

Tojam, what I am saying is that it has been my experience if you are using brute force filtering for a continous load on a transformer you need to only use 60% of the transformers rated current capacity. For example if the transformer is rated at 10 amps, then only put a 6 amp load on it.

toejam
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Re: transformer/rectifier question

Post by toejam » Sat Sep 18, 2004 8:34 am

k7elp60;
One thing that comes to my mind is power factor and va of a transformer. my memory ia a bit lax on those facts and maye someone could enlightem me as to this factor in power supply design

k7elp60
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Re: transformer/rectifier question

Post by k7elp60 » Sun Sep 19, 2004 11:44 am

Tojam,
I will try to answer your question. As far as power factor is concerned I think it is not even a factor with the smaller power supplies. Most of what we have been talking about are 250VA or less.
As far as VA it is directly related to the amout of power the transformer will handle, and as a result the physical size of the core. The higher the VA the larger the core, rather it be the metal plates or the toroid. Here again VA means volts time amps. The primary and the secondary are the same. I realize that the transformer is not 100% efficient, but for all practical purposes it is. If you know the VA rating and the secondary voltage it is just a matter of dividing the VA amout by the voltage for the secondary current. For stepdown transformers the primary current will be the secondary current divided by the turns ratio. For example if we have a 120VA transformer with a 12V secondary, and 120V primary, the secondary can supply 10amps RMS to an AC load and the primary current will be 1 amp. So the turns ratio is 10:1 primary to secondary.
Going one step further, if we are using a full wave rectifier with brute force filtering and the rule of thumb of secondary current max times 0.6 the average secondary current is 6 amps and the primary current is 0.6 amps average.
Hope this helps

rshayes
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Re: transformer/rectifier question

Post by rshayes » Sun Sep 19, 2004 8:30 pm

How much power a transformer can supply is very dependent on how it is used. The basic power limit on a transformer is thermal. Somewhere in the transformer is a point where the heat buildup is maximum. This is usually somewhere in the winding area, possibly about 1/3 of the winding depth from the core. This point has to remain cool enough that the insulation does not fail. The heat generated in the transformer has to be carried to the surface and radiated. The heat transfer requires a difference in temperature proportional the the amount of heat being transferred, and the radiation process also requires a temperature difference between the transformer surface and the surrounding environment. The heat that a transformer can tolerate depends on its physical size and the materials used. Depending on the insulation system used, the hot spot temperature can be as low as 90 C (unimpregnated organic materials) to over 220 C (mica, porcelain, glass,quartz, etc.). Most small transformers probably use 90 C or 105 C insulation systems.<p>The two major sources of heat in a transformer are core loss and copper loss. The core loss is the heat generated by the iron core as the magnetic flux changes during the AC cycle. The main factors influencing the core loss are the average voltage and the frequency of the power source. The current drawn from the transformer does not significantly change the core loss.<p>Copper loss is the loss caused by current flowing through the primary and secondary windings. The total copper loss is represented by the RMS curretn squared times the winding resistances. It is not changed by the voltage that the transformer is being operated at.<p>The optimum design for a given transformer usually has the core and copper losses approximately equal at full load.<p>The end result, since you are not changing the voltage that the transformer is being used at, is that the secondary current should be limited to its RMS current rating.<p>The current delivered to the load is the average current, and unfortunately, this is always less than the RMS current in the transformer.<p>If the rectifier uses a choke input filter with a large choke, the transformer current is close to constant over the AC cycle, and the RMS current is only slightly more than the DC load current. The output of a choke input filter is the average voltage from the transformer, This is about 90% of the RMS secondary voltage. The transformer output would be about 216 watts in this case.<p>Now assume a capacitor input filter with a conduction angle of 18 degrees. This means that the rectifiers only conduct 10% of the time. The ripple voltage will be about 1.7 volts peak to peak, so this is an extreme case. As a rough approximation, the rectifier current will flow as a pulse that is 10 times the load current. During this pulse, the transformer will dissipate 100 times the power that it would with the average load current. Overall, this occurs for 10% of the time, so the RMS current is about 10 times the load current in this case. The load current would thus be 1 amp for a 10 amp RMS ripple current. The average output voltage is the peak voltage less 1/2 of the ripple voltage, or about 33 volts. The total output power is thus 33 watts. The filter capacitor would be 4400 microfarad. This is a rough approximation, but you see the idea.<p>If we assume a 36 degree conduction angle, the ripple increases to 6.5 volts. The current flows for 20 % of the time, and the RMS current is now about 5 times the load current. The load current can now be 2 amps, and the average voltage is now about 31 volts. The total output power is 62 watts with the same stress on the transformer. The filter capacitor is now 2000 microfarad.<p>These are very approximate estimates. Usually the series winding resistances in the transformer also broadens the conduction angle and decreases the RMS current. This would allow a little more power to be delivered to the load at a slightly lower voltage.<p>The good news is that transformers, unlike semiconductors, take several minutes to heat up. Brief overloads are not much of a problem. Many loads, such as audio amplifiers, do not need continuous maximum output from the power supply. This may explain why there aren't more blown transformers.

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