Choosing Best Rectifier/Diodes for 12V 50A Transformer

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Discussion Overview

The discussion revolves around the selection of rectifiers and diodes for a linear power supply using a 12V transformer. Participants explore the implications of voltage drop across diodes, particularly in high-current applications, and the effects of load on output voltage.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant notes that using common diodes results in a significant voltage drop, leading to an output of 10.5V without load.
  • Another participant mentions that Schottky rectifier diodes, while having a lower forward drop, still exhibit about 0.7V to 0.8V drop at high currents.
  • A participant explains that a bridge rectifier will yield a higher DC voltage than the AC output, suggesting that the unloaded voltage will be near the peak AC voltage minus diode drops.
  • It is proposed that using larger diodes in parallel can help reduce voltage drop, although variations in diode characteristics may lead to uneven current sharing.
  • The original poster clarifies a misunderstanding about the current requirement, stating it is 5A instead of 50A, and expresses intent to use the supply for an audio amplifier or halogen lamp.
  • Another participant emphasizes that the unloaded voltage will be near the peak AC voltage and that ripple will affect the average voltage under load, depending on the load, capacitor value, and transformer resistance.
  • There is a suggestion that audio amplifier supplies typically incorporate a series regulator to manage ripple and voltage sag.

Areas of Agreement / Disagreement

Participants express varying views on the best approach to minimize voltage drop and manage load effects, with no consensus reached on a single solution. The discussion remains unresolved regarding the optimal configuration and components for the power supply.

Contextual Notes

Participants highlight the importance of understanding the relationship between AC voltage, diode drops, and load effects, but do not fully resolve the implications of these factors on the design of the power supply.

derek10
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Hi

I want to make an simple linear power supply, using an 12V 50A transformer rectifier and a capacitor.

What kind of rectifier/diodes are best to avoid an excessive voltage drop from the transformer? Because using common diodes the voltage drops to 10,5V without load.

Many thanks.
 
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derek10 said:
Hi

I want to make an simple linear power supply, using an 12V 50A transformer rectifier and a capacitor.

What kind of rectifier/diodes are best to avoid an excessive voltage drop from the transformer? Because using common diodes the voltage drops to 10,5V without load.

Many thanks.

At those kinds of currents, even Schottky rectifier diodes will still show about 0.7V to 0.8V forward drop:

http://www.digikey.com/scripts/dkse...&newproducts=0&ptm=0&fid=0&quantity=0&PV69=80

Why do you need 50A?
 
You know if you use bridge rectifier on a transformer that has 12V ac output, you get higher DC voltage. So it is not 12V-2 diode drop= 10.5V! For no load, you get 12X1.4-1.5V≈15V! This is because you get the peak voltage, not the RMS voltage. Now the trick is "NO LOAD". Under load, voltage drop depending on the internal resistance of the transformer and the size of the cap.

There is no way to avoid voltage drop across the diode. As Berkman suggested, Schottky has less voltage drop but still significant. To minimize the drop, not only use the biggest diode, use more in parallel so they share the current and reduce the voltage drop a little. It is not that efficient as the Vbe drop of each diode is a little different and end up one carry more than the other. But that still will help.
 
Many thanks

Sorry I wanted to say 5A not 50.

It may be used for an audio amp but it might be used for powering a halogen lamp too it doesn't have a defined use yet.

Thanks yungman, I didn't knew that rectified and filtered voltage was higher than the AC output! (I have little knowledge about AC electricity) I will try the parallel Schottky diode setup and see if voltage under load is ~12V. I will update after finushing building it.
 
derek10 said:
Many thanks

Sorry I wanted to say 5A not 50.

It may be used for an audio amp but it might be used for powering a halogen lamp too it doesn't have a defined use yet.

Thanks yungman, I didn't knew that rectified and filtered voltage was higher than the AC output! (I have little knowledge about AC electricity) I will try the parallel Schottky diode setup and see if voltage under load is ~12V. I will update after finushing building it.

5A is a different world from 50A!, You can find one single diode to do the job. Just get the diodes and make sure you use BRIDGE rectifier configuration. Put a load and draw 5A and measure the voltage. I don't know your transformer, it might drop voltage under full load. There is only one way to find out...try it.
 
derek10 said:
Many thanks

Sorry I wanted to say 5A not 50.

It may be used for an audio amp but it might be used for powering a halogen lamp too it doesn't have a defined use yet.

Thanks yungman, I didn't knew that rectified and filtered voltage was higher than the AC output! (I have little knowledge about AC electricity) I will try the parallel Schottky diode setup and see if voltage under load is ~12V. I will update after finushing building it.

This can be quite a problem once you start to take current from your supply. The unloaded voltage will be near the peak AC volts (less some diode drop) and will stay there if there is a smoothing capacitor. Once you start taking current, there will be ripple and the average voltage will drop noticeably. The actual amount will depend upon the load, the value of the capacitor and the source resistance of the transformer.
Audio amp supplies will usually have a series regulator to take out the ripple and voltage 'sag' and halogen lamps do not need DC (so why bother?).

A bridge rectifier will reduce the ripple significantly but, of course, there will be two diode voltage drops involved. Most PSUs use a bridge, these days.
This wiki article is worth reading.
 

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