diodes, choosing the right voltage and size?

[dnmun]

i have a small 3A ping charger which has a small diode in the back end that supplies the current for the fan and the opamps and led from a winding on the transformer separate from the one that supplies the schottky diode and the current to the output.

the diode has burned up and so there is no circuit current for the back end so the charger is dead.

i am trying to mentally determine how to size the diode in order to buy a replacement.

the fan is 12V, the capacitor is 50V so i know it is in this range, and the opamps cannot handle more than 20V i assume.

if i choose a 20V diode, i assume i will miss getting as much power from each cycle as i would with a 15V diode. but i do not know how much current is needed to drive the fan and the op amps along with the led. i doubt if a few volts is gonna affect the led, or the opamps but the fan will need a lot of juice to run so that is where i am mentally now, figuring the load part for the diode.

i am looking at 15V 5W diodes now off ebay since the shipping is so cheap from hong kong. but i don't know if 250mA will drive the fan.

i am gonna check the voltage on the output of another charger to be certain of what i need, but i wondered if there is a procedure to follow to determine how to size the diode i need capable of handling the power.

and if someone has already had to go through this exercise or knows the part, i would appreciate that input too.

 

[Jeremy Harris]

Is this a rectifier or a zener regulator diode?

If it's the former, then a 1N4001 would work fine, if it's the latter then we probably need a bit more info about the circuit. From your description it sounds like it's just a rectifier, so any general purpose rectifier diode, like the 50 V rated 1N4001 may do just fine.

If it's running at a high frequency (which maybe the case here) then you might be better with a fast recovery rectifier, something like a 1N4933.

 

[dnmun]

yes, it must be a high speed diode because the transformer is switch mode. i assume 20-30k at least.

so a regular zener or 'blocking' diode would not have enuff recovery time?

so i should be looking at schottky diodes, but the 40V seems too high for the op amps so that is why i was thinking 15V. but this is under the assumption that i need to capture as much of the forward bias as possible to get the most power from each cycle to keep enuff current flowing to drive the fan.

or maybe even as low as the fan, 12V. which seems really low for high speed diodes.

mostly i have looked at power diodes, this is low power by comparison.

gotta google 1N4933

 

[Jeremy Harris]

You don't need to worry about Vr, as long as it's high enough (and 50V should be fine if the fan is running down at 12V or so). The rating doesn't affect the way it works, at least not directly. My guess is that any 1A, 50V fast recovery diode should be OK, doesn't really need to be a Schottky, I think.

 

[dnmun]

ok, i was doofus thinking.

the breakdown voltage means nothing except that i do not wanna have a diode in the circuit with a breakdown voltage higher than the capacitor or the opamps. the voltage that is sustained on the cap for the circuit to use is gonna be regulated by another circuit element, and the current capacity of the diode has to be adequate for the needs of the fan and led and opamps.

learned a lot in a few minutes, all the axial diodes are not alike, the plastic ones are thick or thin depending on the current capacity, so DO-35, DO-41 for lower current or the other fat one R-6 and P600 are for higher current diodes. so that is why they look different.

so i agree it has to be the 1N4933, 5.7 cents each in my old mouser catalog for 25ct order, but shipping is gonna make it easier to just go to hong kong. you can't buy just one, or ten or even 20!!!!!

http://www.ebay.com/itm/50-PCS-1N4933-1A-50V-Rectifier-Fast-Recovery-Diode-New-/260847920071?pt=LH_DefaultDomain_0&hash=item3cbbbf63c7#ht_1535wt_1200

 

[Jeremy Harris]

It's a pretty crazy world when you end up getting stuff like this cheaper from China. Really makes the whole economy thing hit home.

 

[amberwolf]

You could probably find an equivalent part in some other power supply that has suffered the curse of the fatal death.

(as opposed to the temporary death of this one)

 

[etriker]

You could probably find an equivalent part in some other power supply that has suffered the curse of the fatal death.

(as opposed to the temporary death of this one)

Junk tvs are loaded with them.

cool tool

http://www.ebay.com/itm/Transistor-Semiconductor-Parameter-Tester-Meter-DY294-/330781983383?pt=LH_DefaultDomain_0&hash=item4d04248297

 

[dnmun]

You could probably find an equivalent part in some other power supply that has suffered the curse of the fatal death.

(as opposed to the temporary death of this one)

i collect stuff like AW too. so i spent like 4 hours reading about how diodes are labeled and all the hong kong sources. learned a lot about diodes today.

so i looked in my little box of resistors and other transistors and stuff i had cut out of old nicad chargers and tv and radios and found two diodes labeled 1N4001. bingo!

it is a vishay diode, basically identical to the 1N9433 so since i had just enuff leg left to make the bend and get it down through the hole, soldered it in place, plugged the charger in, still dead. even worse when i put my watt meter on the end the wattmeter started fading out so i could not pull it off fast enuff. man i hate breaking stuff while fixing other stuff.

but my wattmeter is ok, but that charger is now officially dead. parts for other ping chargers. it was the 3A60V little ping charger and nick had used it forever, the case had even melted partially from such abuse for so long. so now it will be part of THE PILE.

but in searching around i found this place where they sell electronic hobbiest stuff. and they had this diode which i realized is the exact thing needed for people to use when they decide to serial their 36V batteries to make a 72V pack.

the diodes are added in parallel to each pack so that if there is a short on the output of the two packs, the one pack does not short itself on the other pack. i think everybody knows how richard tells everybody to include them.. anyway this is the diode i found and if your pack has a BMS on it to shut off the output after the initial current surge, this diode should be big enuff to handle the short, maybe:

http://www.ebay.com/itm/10PCS-10SQ045-10A-45V-Schottky-Diodes-Solar-Panel-Wind-Rectifier-10AMP-/400295961956?pt=LH_DefaultDomain_0&hash=item5d337fa564#ht_4899wt_982

but your pack has to stay under the 45V so no overcharging above 45V when this diode is in parallel.

these guys also have a huge schottky for combining power supplies at their output:

http://www.ebay.com/itm/100-Amp-Schottky-Rectifier-Diode-IR-100BGQ015-x10PCS-/400244740084?pt=LH_DefaultDomain_0&hash=item5d30720ff4#ht_3043wt_982

is that a monster or WHAT?

edit, and one other thing i found while digging through ebay, some big 60V schottky diodes that will handle 10A. i wanna hack up one of these kingpan chargers from 4A to 8A if i can so this would help convert a 48V kingpan charger from the 3-4A stock to where maybe get 6-8A output with a hack on the shunt and also change the NPN switching transistors in the front end to the big TO3P package that can handle the heat. the J13009 TO3P transistors are on their way now so i may order some of these schottky diodes to do the hack yet:

http://www.ebay.com/itm/5-X-MBRF1060CT-Dual-Schottky-Rectifier-10A-60V-ITO-220AB-/180945846443?pt=LH_DefaultDomain_0&hash=item2a213680ab#ht_1646wt_699