Bridge Rectifier Current Rating -- How much is enough?

[hamfiles]

This is for a regulated power supply that I am building.
Instead of single diodes, I intend to use a bridge rectifier to convert the transformer voltage to dc.

My mains is 120 v ac rms, and my transformer is
115 v // 20 - 0 - 20 volts. I intend to limit the current to about 3.5 amps.
The transformer is very rugged, and guaranteed for up to 4 amperes of current.

For a 20 volt ac rms input voltage, and a maximum dc load of about 3.5 amps, how big of a diode bridge would be needed?

I'm using about 4,700 mfds of capacitance as filters, immediately after the rectifier. Since current leads in a capacitor, I am thinking that the moment power is applied, the capacitors will act as a short, and the current will max out, at least for the first time constant, until the capacitors start to charge. During this brief period of time, won't the current be alot higher than the 3.5 amps that the circuit will see when loaded down while in use?

Because of this, should I use a bridge that is rated much higher than 4 amperes?

Or is such a brief surge in current not going to really matter to the diodes in the rectifier?


Thanks in advance for any replies.

 

[Mosfet]

The surge is the main cause of diode failure.
Many '4 amp' bridges have a surge rating of 200 amps.
You might try to find the manufacturers data for yours.
When I use 10k uf or more I also include a soft start circuit.

 

[hamfiles]

That's an excellent idea, thanks Mosfet. I'll either add a softstart circuit, or go killer big.

 

[vince]

A thermistor may be an approach to soft starting (limiting current) while charging up the caps. NTC thermistors have a high resistance at initial closing of a circuit. As the thermistor heats up, its resistance drops and more current passes. Posting a chat topic for specifics is recomended if you think this approach is viable for your project.

NTC=(negative temperature coefficient)

and there is a PTC thermistor (positive temperature coefficient), as it gets warm(passing a thermal threshold ) resistance increases.

Here's a site with a simple article on thermistor basics, if yor not familiar with em...


https://www.ecmweb.com/ar/electric_basics_solid_state_3/


Vince

 

[Sebi]

You build a double (+ and - outputs) supply or a single one?
Because for center-tapped trafo with one DC output need only two 2A rectifier diodes...

 

[seeker]

Hi hamfiles,
if the circuit is not going to be switched on and off a lot then a 5 amp bridge should be ok.

If the fate of the human race depends on it then I would go with a 10 amp rating :lol:

 

[hamfiles]

Thanks for all of your replies.

Vince, thanks for the link, I know about thermisters, although I have not yet used one in a circuit. I'll consider it, but I have already bought an 8 amp bridge rectifier that will more than handle any surge current. Maybe for another job, though...

Sebi, I'm making a dual polarity supply. I like the bridge bridges that are made for this because the diodes are matched and thermally equal.

Seeker, Hah! that's funny! Thanks for the tip. I got a good deal on an 8 amp one, I think I paid $2.50.