Electronics > Beginners
Secondary current rating of a center tap transformer?
techguru:
I have to design a regulated power supply with following specification;
DC output voltage: 0 to 30 V adjustable.
DC current:0 to 3A adjustable.
I am a second year under graduate student, I cant understand difference between variable current function, current limiting function and fold back current limiting?
Kindly Help me in designing the circuit and Explain the above question.
drussell:
--- Quote from: not1xor1 on November 07, 2018, 07:24:48 am ---as other people have already stated you can get no more than about 3A DC from the dual rails
--- End quote ---
Well you certainly can intermittently. It is all about temperature rise. If you can tolerate extra temperature rise and the transformer is beefy enough, you can intermittently overload it far beyond that or with appropriate cooling (like forced air) can run over the nameplate rating of most transformers even continuously.
Good companies like Hammond and Tirad talk about this in their specifications and make quality, conservatively rated transformers. You need to evaluate the actual unit you have to be sure what you can get away with. If in doubt, monitor the temperature rise over the course of several hours of continuous loading at your desired level and watch what it does. A transformer rated for intermittent duty won't be able to blast out full power continuously while something designed for continuous maximum-blast will hold up well in most uses.
Rectifier topology obviously plays a role in this as it will dictate "how" the power is delivered from the transformer, what part of the cycle is actually conducting, etc.
Do you know where the transformer came from originally, what it's intended purpose was?
Additionally, what is your intended use?
mvs:
--- Quote from: techguru on November 09, 2018, 05:51:26 pm ---I am a second year under graduate student, I cant understand difference between variable current function, current limiting function and fold back current limiting?
--- End quote ---
I would assume that current limiting function is fixed. It is used to protect the power supply from getting damaged through excessive current draw.
Variable current limiting can be additionaly adjusted to some lower value, so that power supply would act as a programmable current source.
Foldback current limiting acts as a fixed or variable current limiting with additional reduction of current and voltage to some very low value once it get triggered. It was used quite often in old linear power supplies to reduce power disipation and risk of fire during output short circuit condition.
Ian.M:
In response to drussell's comments:
OTOH if you simulate the rectifier circuit, you'll find that Hammond's derating factors for rectifier use are the ratio of the avg. DC current out of the rectifier to the RMS AC current into it, so are generically applicable to any competently designed line frequency transformer, even cheap ones.
Therefore, if your transformer is rated for continuous operation at a particular max. RMS secondary current at a specified max. ambient temperature, Hammond's derating factors *are* applicable to calculate the max continuous DC load current at that max. ambient temperature.
not1xor1:
--- Quote from: techguru on November 09, 2018, 05:51:26 pm ---I have to design a regulated power supply with following specification;
DC output voltage: 0 to 30 V adjustable.
DC current:0 to 3A adjustable.
I am a second year under graduate student, I cant understand difference between variable current function, current limiting function and fold back current limiting?
Kindly Help me in designing the circuit and Explain the above question.
--- End quote ---
You can get 5A out of your transformer if you limit the output to half voltage. You can use a switch (acting on a relay or power mosfet, scr, etc) to change between 0-12V 5A | 0-24V 3A as various bench PSU manufacturers do.
If you want to get 30V out of your transformer, then you have to use a large capacitor to keep the ripple at minimum and limit the maximum output current to 1.5-2A to reduce the power dissipated by the transformer.
Regarding the various kind of current protection/limit/regulation, you can have (even more of this in the same circuit):
-1) overcurrent protection
a comparator wich switches-off the PSU output when the output current gets above a set limit (e.g. for battery protection or electric motor PSUs).
-2) current limit
you can use a sense resistor and the Vbe threashold of a BJT to "steal" current from the base of the pass BJT when the voltage drop through the sense resistor gets above the trhreshold. The regulation is quite coarse as the current increases when the output voltage gets down. This is a problem as a pass BJT has to dissipate a lot of power in case of shortcircuit.
-3) foldback current limit
a foldback circuit uses a trick, adding a 2 resistors voltage divider to the base of the current sense BJT of the previously described circuit, in order to reduce the current as soon as the output voltage decreases.
-4) proper current regulation
that is done by using an operational amplifier (or a differential amplifier made by discrete BJTs) and allows the PSU to work within the limits of a set voltage and current, automatically switching between constant voltage and constant current mode according to the load.
For more details I suggest you to carefully read the Agilent "DC power supply handbook"
http://literature.cdn.keysight.com/litweb/pdf/5952-4020.pdf
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