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Author Topic: 12 VOLT 30 AMP SUPPLY NEEDING 100 AMP RECTIFIER??????????????  (Read 5076 times)
KC9VZB
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« on: December 30, 2012, 08:50:53 AM »

ON DX ZONE THE TOP SUPPLY,IN THE TEXT SAYS THE RECTIFIER DIODES NEED TO HANDLE 100 AMPS PEAK FORWARD CURRENT.WHAT THE HECK? I SUSPECT THIS IS A TRANSLATION FOPAW ,MEANING VOLTS.OTHER DESIGNS DESCRIBE USING A 35 AMP BRIDGE,WHICH IS WHAT I PICKED UNTIL I READ THE TEXT.I HAVE THE BIG RESISTOR FOR TESTING UNDER FULL LOAD.
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AC2EU
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« Reply #1 on: December 30, 2012, 09:38:14 AM »

Don't believe everything you read on  the internet!
Diodes have inrush current specs that are typically 10X the rated current for one thing.
The diode will never "see" a peak because of transient nature of a peak and the energy stored in the cap.
100W rigs might peak at 20amps .
Besides is the 100A coming from if the transformer can't supply it?
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W5FYI
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« Reply #2 on: December 30, 2012, 10:07:22 AM »

Calm down, Little Grasshopper. There's no need to scream in all-caps. So you made a faux pas; no big deal. There are plenty of SPICE models around for you to test your circuit, so why not learn from what is available?
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KA4POL
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« Reply #3 on: December 30, 2012, 10:20:05 AM »

Take a look at how the pros do it. Take a commercial schematic and study it. You'll see there is absolutely no reason to panic  Cool
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AA4HA
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« Reply #4 on: December 30, 2012, 10:21:02 AM »

Don't believe everything you read on  the internet!
Diodes have inrush current specs that are typically 10X the rated current for one thing.
The diode will never "see" a peak because of transient nature of a peak and the energy stored in the cap.
100W rigs might peak at 20amps .
Besides is the 100A coming from if the transformer can't supply it?

The peak current through the diodes will probably happen when the supply is first switched on and the filter capacitors appear as near dead shorts across the rectifier until they charge up. Then, as you said, it is going to be limited by what the transformer core is capable of delivering as a current spike (that nice healthy thunking sound when the supply is switched on).

You then have your peak current capability of the supply. Preferably limited by what the transformer can do and not using the rectifier diodes as magic smoke packet releasers. A real simple linear supply is a transformer and a rectifier. Capacitors are nice to knock down the ripple, some sort of regulator to keep things generally in the ballpark and if you are really clever, some sort of crowbar or current sensing protective circuit.

For bigger supplies they may use a step-start circuit to slowly bring the caps up to a charged state when the supply is switched on. This is some sort of time delay circuit, relay and ballast resistor that is engaged for a few hundred milliseconds while the caps charge up. In my Harris RF-350K's they use step-start to charge up the 60-80,000 uFd filter capacitor. I have a homebrew supply that has 120,000 uFd filter capacitors through a step-start, you can practically arc weld with that supply. The current load they would put across the diodes would kill them if it was not for the step start circuit as the transformer is capable of almost 100 amps.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
KC9VZB
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« Reply #5 on: December 30, 2012, 02:23:08 PM »

 Thanks everyone,I'll go ahead with the 35 amp bridge.Soft start circuit looks pretty simple to add,as I've got some pretty big caps to fill,good Idea.
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KH6DC
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« Reply #6 on: January 02, 2013, 09:02:39 PM »

The Astron RS35M has a 50A bridge rectifier and the specs show the PS has a 35A peak with a 25A continuous.  It's safe to go 2x and use a 60A or better rectifier.  Good luck
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73 and Aloha,
de Delwyn, KH6DC
KA4POL
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« Reply #7 on: January 02, 2013, 09:56:08 PM »

And cost is almost the same.
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G3RZP
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« Reply #8 on: January 03, 2013, 12:29:36 AM »

Try figuring out the peak repetitive rectifier current with a 30 amp load and whatever size filter cap you're going to use. You may well find that a 30 amp rectifier is a bit near the bone. Check too the ripple current rating of the cap - I always work on a ripple current of about 1.5 times the DC load current with a bridge.
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KE3WD
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« Reply #9 on: January 03, 2013, 02:33:15 PM »

Derating parts like rectifier bridge is good sense. 
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N3QE
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« Reply #10 on: January 03, 2013, 04:50:01 PM »

ON DX ZONE THE TOP SUPPLY,IN THE TEXT SAYS THE RECTIFIER DIODES NEED TO HANDLE 100 AMPS PEAK FORWARD CURRENT.WHAT THE HECK? I SUSPECT THIS IS A TRANSLATION FOPAW ,MEANING VOLTS.OTHER DESIGNS DESCRIBE USING A 35 AMP BRIDGE,WHICH IS WHAT I PICKED UNTIL I READ THE TEXT.I HAVE THE BIG RESISTOR FOR TESTING UNDER FULL LOAD.

In real world commercial supplies, the transformer winding resistance will directly relate to the rectifier peak current rating.

In bizarro found on the internet designs, there are gargantuan imbalances in some parts that require uprating other parts and other weird-ass stuff like inrush current limiting/step-start. These are signs of poor design based on specsmanship rather than actual performance.
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KA3NXN
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« Reply #11 on: January 04, 2013, 05:20:23 PM »

I built my own from discrete diodes that I took out of a large 200 amp charger/battery booster. They are quite large but just purr along at 70 amps. running a large 6 meter solid state amp.

Jaime-KA3NXN
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KA1MDA
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« Reply #12 on: January 08, 2013, 09:54:30 AM »

ON DX ZONE THE TOP SUPPLY,IN THE TEXT SAYS THE RECTIFIER DIODES NEED TO HANDLE 100 AMPS PEAK FORWARD CURRENT.WHAT THE HECK? I SUSPECT THIS IS A TRANSLATION FOPAW ,MEANING VOLTS.OTHER DESIGNS DESCRIBE USING A 35 AMP BRIDGE,WHICH IS WHAT I PICKED UNTIL I READ THE TEXT.I HAVE THE BIG RESISTOR FOR TESTING UNDER FULL LOAD.

It could be that the power supply uses a great deal of filter capacitance. On turn on, the filter caps look like a dead short until they charge up. If the transformer is capable of supplying the current, there could be a huge in-rush current flowing through the rectifiers. Also, IIRC, the peak current flowing through the diodes can be larger than the steady state filtered CD current due to rectifier ripple.

The only time I had a commercial P/S fail, the 25A bridge rectifier shorted on turn on (it was a 20A rated supply). I replaced it with a 40A bridge and never had another problem. the price difference between a 200PIV, 25A bridge and an 800 PIV, 40A bridge is peanuts. I use the 800 PIV, 40A bridges exclusively. Cheap insurance.
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WB2WIK
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« Reply #13 on: January 08, 2013, 03:09:57 PM »

Some state "peak" when they really mean "surge."  If it's a surge rating, the pulse width and shape require definition.

Most ordinary rectifier cells are surge rated based on an 8.3 mS pulse (120 Hz), single-cycle; a 35A bridge likely uses cells rated at least 150Apk.
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