I have to replace a KBPC602 bridge rectifier. It is rated 200VRMS at
6.0 A. It is wired into the circuit using only the + and - terminals. The AC connections are not used. If I were to build a bridge out of four 6.0 amp 100V diodes, would I be able to directly replace this device? Does the circuit basically require a 12.0A 200V device? Conversely if I were to place four 3.0 amp 200V diodes in parallel would that arrangement work also? Thanks. Lenny Stein, Barlen Electronics.
Lenny: How is the bridge wired into the circuit? ..... as a half wave rectifier? ... or ?? Might I suggest that hooking up diodes in parallel without low ohm (equalizing) series resistors is not a good practice.... in fact, it is not a good practice to hook up diodes in parallel even with series resistors. Also if diodes are hooked up in series to increase voltage rating... fairly high ohm parallel equalizing resistors should be used. All of this is needed because no two semiconductor junctions are the same and one or the other diode will hog the current or voltage. Most good designs AVOID these kind of hook ups altogether and go with a single diode device. You should look at the circuit and determine what the rough current draw is and what voltage it should handle...... if the circuit current permits a single 6 amp "barrel" diode may suffice or a heat sinkable diode in a To-220 or Stud-mount case with a higher current rating could be used. Daniel Sofie Electronics Supply & Repair - - - - - - - - - -
Captain Video, This is a strange way to connect a rectifier diode, unless the design called for a high current diode and only a bridge was handy. Is this a "commercial" piece of equipment or is is someone's homebrew design. . Take a close look at the circuitry and components and fairly easily you should be able to determine the approximate voltage and current needed for the replacement (single) diode if in fact it is used as a rectifier in a half wave configuration.
If there is something else going on here with the circuitry and it is a commericial design and it has worked that way for years then why not just replace it with an "original" 6A, 200V bridge or even a higher voltage and current rating as these bridges are commonly available, even from scrapped televisions and other devices.
In either case, you should consider Sofie's textbook argument regarding parallel and series diode connections, he is absolutely correct with what he stated.
Yeah, I was thinking that too, but couldn't quite get my head around it. I'm not sure how they rate bridges. When it's being used normally, two diodes are in conduction at any one time, in what amounts to a series circuit, so the quoted current rating must only be the rating of any one diode. However, as there are two diodes series'd into the circuit, the voltage rating should be twice that of any one diode, so when a bridge is quoted at 200v, is each individual diode 100v ? That would make the arrangement that's being enquired about, still 200v, but at twice the current, as all four diodes will be in conduction. Either way, should be easy enough to pick up either a direct replacement bridge, or 200v 3A diodes, which should be plenty enough.
The application is an Elmo school overhead projector. The unit uses an
82v 360W bulb. It looks like the bridge, that is, two sets of diodes in series with those two sets then in parallel, is in series with the lamp. When these bridges short, (usually just one diode in the bridge shorts) the bulbs burn out. In this application it is used as a half wave rectifier. I thought that perhaps by using a bridge in this manner they were attempting to get a higher voltage drop across the unit and less voltage to the bulb as there would be two junction drops instead of one. However with those junctions then paralleled with the other diodes maybe it would be a wash. Lenny.
So is this arrangement hooked directly to the incoming line voltage ( I guess 110v in your neck of the woods ? ) The drops across the diodes will be insignificant in this application, I feel, and it will be all about 'area under the curve' from the half wave rectification that the diode is performing. A bulb rated at 82v ( RMS ?? ) 360W will pull about four and a half amps RMS when hot, more when it's cold. I guess that this is what this is all about. The designer figured that it was a good way to get a high voltage high current diode in a compact package with enough surface area to self-cool. As it's a schools unit and probably subject to all sorts of health and safety regulations, I think that I would feel inclined to stick with the way it was designed originally, possibly uprating the specs of the bridge, if they are a regular failing point.
It may be the other way around. I have seen a small bridge used as protecion. When the big projector bulb fails, there's a big current spike, blowing the diode.
FWIW on ESP website (Rod Elliot), the 100W guitar amp project, power supply schematic shows a "protector" bridge.
Funny that the back EMF theory as a cause of destroyed bridges had occurred too. Maybe I should install a small snubber, (perhaps a 1.0 ohm resistor in series with say a .10uf capacitor across the bulb. BTW the bulb is very common and about 9.00 dollars. Lenny.
Using devices in series parallel is an effective way to fail-safe things. Any one of those diodes can short or go o/c and the rectifier still works as a rectifier.
And you say its failed before - sounds like the designer was trying to reduce the failure rate instead of getting to the bottom of the cause.
I dont know what type of bulb it uses, or why the BR is failing, but if its a filament bulb, arc over will draw huge currents at fail time. Fitting an mcb in the line to the bulb may save the diode sometimes, maybe. This must be a magnetic one, not a thermal one.
Of course you could eliminate the problem entirely by using a mains voltage bulb and losing the BR.
I'm not sure a CR across the bulb would help, as its current surge thats killing the BR. But if you do that then 100 ohms and 0.1u in series is typical.
Another solution would be to lose the BR and fit a capacitor dropper with a fixed R to drop a bit of the power. Now the C&R will prevent any monster currents when the bulb dies. Just a small R, not much Pdiss and an X rated cap. The R helps limit peak i, though with the lightbulb's R you should be fine with just the C alone.
For some reason a lot of projectors seem to use oddball voltage bulbs. If the bridge is failing a lot, I'd try installing a buck transformer if there's room. You'd probably have to series a couple of secondary windings to get the odd 38V drop though.
Lenny: Now that you have finally given us more information I would be inclined to suggest that you replace the bridge with a "like and kind" exact OEM part or a possibly upgraded voltage and current version. Since this is in a commercially designed overhead projector and used in a school, you would be best advised NOT to CHANGE the original design or use anything other than a "factory" part. Bridges of this nature and rating are readily available so stop wasting time posting to the newsgroup and get online and order the proper part.... get the proper and safe part that was originally installed.
The one word that comes to my mind is "liability".
I talked to a tech at Elmo this morning and asked about the use of the bridge as opposed to using a large single diode. He said that the engineers found that the DC when looked at with a scope from the bridge was cleaner than the output of just a single diode. Does that sound possible? As far as wasting anyones time, if I was I appoligize. I thought we were having an interesting discussion. Lenny
Lenny: We were and are having an interesting discussion and thread and no, you are NOT wasting anyone's time. Instead of trying to find ways to jury rig a substitute for a common and readily available bridge rectifier you should have just procured one and installed it and be done with it.... saying that, perhaps the only person's time "wasted" was your own. Until your later posting that revealed it was for a commercially manufactured overhead projector being used in a school environment you had everyone guessing about how the bridge was being used in the circuit, in fact, you never revealed the circuit in any kind of detail as it related to equipment, configuration, voltage, amps, etc... even after several reply posts asked you for details about it. As you know, none of us can read minds and no one has a working crystal ball on the newsgroup so the more information an OP can give, the better and more straight forward will be the answers. The best answer for fixing the overhead projector is to replace the bridge with an OEM part..... obviously other parts and configurations will work, BUT if something goes wrong and the projector malfunctions and causes smoke and fire damage... particuarly in a school environment.... your LIABILITY will be much more exposed if you DIDN'T use replacement parts that were used in the original manufacture. Enough said. Lenny, please feel free to start up more "interesting" conversations... it sure beats the spammers and the trolls and their filthy subject matter. Electronics discussions are much more enjoyable than that crap. Best Regards, Daniel Sofie Electronics Supply & Repair
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