Rectifier If rating?

Hi, I'm cobbling up a small DC PSU to run a couple LEDs in a compressor. There's a transformer that is supplying 6.3V AC and it's rated at 2A.

I anticipate using no more than about 100mA for the LEDs total. I want to get a little bridge rectifier from mouser, and I see there's all kinds of forward current (If) ratings. Would I be OK to use anything 2A and above? I mean, couldn't I even get away with 0.5A?

I don't know if there's any kind of surge to consider, etc.

It's not the expense - we're talking pennies difference between the lower current ratings and higher ratings - it's the form factor. The lower current rated ones are smaller which is attractive where space is limited.

I've tried googling this sort of thing, but end up mired in endless graphs and discussions, and the occasional scolding of others, but little practical simple info.

Thanks for your thoughts.

Mike

PS - I know I could probably get by with just a single diode to run the LEDs (plus resistors of course), and I've been told there's no flashing pulse due to the extreme ripple, but I'd just rather make a small little DC supply, as much to learn as to mitigate any possible strobe effect...

Phrazemaster said:

Hi, I'm cobbling up a small DC PSU to run a couple LEDs in a compressor. There's a transformer that is supplying 6.3V AC and it's rated at 2A.

I anticipate using no more than about 100mA for the LEDs total. I want to get a little bridge rectifier from mouser, and I see there's all kinds of forward current (If) ratings. Would I be OK to use anything 2A and above? I mean, couldn't I even get away with 0.5A?

Click to expand...

generally you can always substitute higher current parts....

The failure mode for diodes wrt forward current is over-heating so an average current phenomenon. 

JR

I don't know if there's any kind of surge to consider, etc.

It's not the expense - we're talking pennies difference between the lower current ratings and higher ratings - it's the form factor. The lower current rated ones are smaller which is attractive where space is limited.

I've tried googling this sort of thing, but end up mired in endless graphs and discussions, and the occasional scolding of others, but little practical simple info.

Thanks for your thoughts.

Mike

PS - I know I could probably get by with just a single diode to run the LEDs (plus resistors of course), and I've been told there's no flashing pulse due to the extreme ripple, but I'd just rather make a small little DC supply, as much to learn as to mitigate any possible strobe effect...

Click to expand...

That’s very helpful JR, thank you. I appreciate your time.

you can get 1 Amp at 400 volts for 58 cents nowadays, which is expensive because if you buy a whole tube  the price goes down to 5 micro cents per unit.

https://www.mouser.com/ProductDetail/Vishay-Semiconductors/DF04M-E3-45?qs=sGAEpiMZZMtQ8nqTKtFS%2FMRt2%2F0z7Bctw%252BAW%2FfpzCg0%3D

CJ said:

you can get 1 Amp at 400 volts for 58 cents nowadays, which is expensive because if you buy a whole tube  the price goes down to 5 micro cents per unit.

https://www.mouser.com/ProductDetail/Vishay-Semiconductors/DF04M-E3-45?qs=sGAEpiMZZMtQ8nqTKtFS%2FMRt2%2F0z7Bctw%252BAW%2FfpzCg0%3D

Click to expand...

Mmmm that seems a little spendy...

Most rectifiers have and average and a peak current specification. The peak value is important when using a capacitor input filter because the diode conducts for only part of the cycle to recharge the cap. Hence the current through the diode is a series of short sharp peaks even though the average is much lower. Fortunately the peak current rating of most diodes is at least ten times the average rating so you normally do not need to worry about it.

Cheers

Ian

Thank you Ian. I feel humbled to be schooled by you and the other greats in this thread on such basic components.

You guys are the best!

Thx,

Mike

Not to confuse you with TMI but I just looked up a data sheet for the common 1n4001-3 diode. https://www.diodes.com/assets/Datasheets/ds28002.pdf

They specify a 1A continuous current rating and a 30A peak current, but it is perhaps worth noting that the 30A peak is further qualified as for only one 8.3mSec half sine wave.

I repeat the over current failure mode is heating (the silicon literally melts). Larger physical diodes have more thermal mass so can survive higher transient current.

JR

Oh dear, you most be rubbing off on me a little bit JR but I actually understood...all of that.

Thank you!

Mike

Once running, you don't need a lot more (say 2X) than the DC current.

But the Start-Up Surge takes a little life off the rectifier every start. Early in my career I replaced a lot of rectifiers which were OK steady-state but lasted 50-100 start-ups.

And start-up surge was hard to calculate in 1976.

My rule of thumb is the rectifier should be bigger than the *transformer* AC rating. As JR says there is >10:1 maybe 30:1 single-cycle headroom, while the short-circuit current of small PTs is 5X-15X the long-term rated current. So even a large cap is unlikely to wear-down the rectifier. With that rule I "never" had come-backs.

Yes, a 2A bridge is bigger than a 0.5A bridge. If you have excess voltage to spare, adding a resistor can limit the surge, but that takes space also, and the computation may come down to trial and re-trial.

Fabulous insights.

Thank you PRR.