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kwame

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Hi
folks ,i have just bought a 240VAC -12V 8 AMP transformer.I need a bridge rectifier for the transformer.I am a little confused about the the exact of capacitor i need to use. I bought a 2200, 35v electrolyte capacitor.Can this capacitor do the job or it will be ruined.
 
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The capacitor should be okay as long as you put it in the right place. However, if you are going to draw anywhere near the transformer's rated current, it's not big enough. The loose "rule of thumb" is around 1000-2000uF per Ampere.

When it comes to bridge rectifiers, I always like to over-spec. I would probably go with 400-800 PIV and 25 or 35 Amps. They don't cost that much.
 
Here is the ripple formula:

Vr = (Idc)/FC

Vr = p-p ripple voltage
Idc = dc load current
F = ripple frequency (half wave = 60, full wave = 120)
C = filter capacitance
 
The size of the capacitor determines the amount of ripple voltage for a given load. Here's the calculation to determine that.

So you select the capacitor size based upon the load and the amount of ripple you can tolerate in your design.
 
Allowable Ripple

You only need to worry about the ripple if your load can handle much ripple. For instance if you are powering up a light then the ripple wouldn't matter. If you are powering up a motor, the ripple probably doesn't matter. If you are powering up a controller then the ripple will probably matter.
Dale
 
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one thought

I will need a current of at leas 6AMP so i need to replace the capacitor.Does the bridge rectifier cause any decline in the current in anyway.This is because the bridge rectifier shoots up output voltage a little bit by 3V(from 12-15vDC).Does the current also shoot up,decline or remains
 
jAGUAR JOE CRUTSCHOW,
i am more interested in the output current.The ripple voltage is not a bother.
 
The output current is determined by the load. The diode drop reduces the voltage by about 1.2 Volts per diode and the circuit goes through two diodes at a time for about a 2.4 Volt drop. This varies from different diodes used in different bridges and varies a little bit with current, but expect about a 2.4 Volt drop due to the bridge. The current is solely a function of the load. Ohms law is V = IR.
Dale
 
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Then don't worry about the capacitor at all (assuming you don't care about the ripple at all). The more you care about ripple, the more you need a capacitor (and the bigger the capacitor you need the bigger the load current is).

Hell, if you can tolerate 100% ripple, just leave the capacitor out altogether. What are you powering here, anyway?
 
When you have a very large filter capacitor connected to a rectifier's output, there will be a large surge of current into the capacitor when power is first applied. So large, that small rectifier diodes could perish. Heftier diodes and sometimes a current limiting resistor might be needed.

I don't see how you can have a 3v overshoot when power is applied especially in view of my first comment. Is the load resistive?

If the rectifier is rated for the load current and the associated wiring is also, then they shouldn't impede that current.
 
Load

The load is a permanent magnet 12VDC motor.
This is the best technology forum on the planet!
 
.Carbonit thanks for your latter contribution;your comments are well noted .Jaguar Joe, The capacitor is 35v so naturally there might be some overshoot.You try this experiment and then measure the voltage with using a multimeter .You would realise a little overshoot in the voltage and most authorities would concur with me.
 

What does a 35v capacitor rating have to do with overshoot, naturally or otherwise?
What experiment?
If you measure the voltage of the power supply under load then suddenly remove that load, the voltage will rise up to the open circuit voltage appearing like overshoot.

If all you are doing is running a motor, you do not need a capacitor. They will run fine on pulsating DC.
 
Pulsating dc

Can the pulsating DC also be used to power a Dc linear actuator moving a load with damage to the actuator?
 
Chances are (extremely high) it won't damage it. How it will actually function is another thing; unless someone actually has experience with this, probably the only way to know is to try it. If it vibrates, you need a filter capacitor. Depends on a lot of factors.
 
More than meets the eye?

I think this may be one of those questions that may seem trivially simple at first glance, but are actually deeper than they appear.

Take the matter of the output voltage of a simple transformer power supply (transformer, bridge rectifier). I made a simulation in LTspice (attached below), and this is what I found.

With a 2200µF filter capacitor, the output (according to LTspice) was 9.5V average, 9.56V RMS:

**broken link removed**

Without the filter capacitor, the output was only 6.46V average, 7.42V RMS. (Both were with a 10 Ω load; Kwame's load is actually bigger than this, as he said 6 amps, I think.)

**broken link removed**

So the filter capacitor actually increases both the average and RMS voltages. Just looking at the waveforms, this seems obvious (the area below the curve is larger with a capacitor). Who knew? (OK, maybe you did, but I didn't.)

By the way, this is also a very good demo of our good friend (not!) ripple.

Discuss amongst yourselves.
 

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