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Magneciter diode specs


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  #1  
Old 11-14-2015, 09:39:37 PM
Boydster Boydster is offline
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Default Magneciter diode specs

I need to replace at least one diode on my 7.5kw MDJE magneciter (04sx1n3b).
The one that is definitely dead is the Rectifier Feild Negative and is listed as part number 305P238. I have seen post mount diodes online but I have no idea what kind of specifications I should be looking for...any advice? - Boyd
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Old 11-15-2015, 01:01:29 PM
Leon N. Leon N. is offline
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Default Re: Magneciter diode specs

Hello Boyd: I have the same magneciter that you are talking about. I assume from your information you are aware of the information found in the Onan Owners Manual and parts catalog, 967-320, page 45 and the Onan JB/JC Repair Manual, pages 69 & 75.

I would suggest that short of finding someone on the Stak who would be willing to sell you the Rectifier Field Negative diode, 305P238 you may be able to find a functionally equivalent diode but may have a form/fit problem, but you should be aware of the following technical requirement. That is the diode in question must be able to pass 10 amperes as well as a transient overload of 35 amperes.

Reason I say this is because this diode is labeled as the field power negative diode and sits across a 120 VAC source which in part drives the 400 watt exciter. See the wiring diagram, 625A429 in 967-500, page 74.The exciter provides 400 watts @ 40 volts dc with a transient capability of 1400 watts which translates to 35 amperes DC to the rotor revolving field excitation. That diode provides 1/2 of the 120 VAC rectification in developing the 40VDC to power the rotor via the slip rings.

As for the diode PIV (peak inverse voltage) I would have to guess it is in the neighborhood of 200-300 volts.

Send me a PM and I can send you a technical description of how the exciter works.

Leon
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Old 11-15-2015, 01:34:11 PM
len k len k is offline
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Default Re: Magneciter diode specs

I'ld push PIV higher if you can find it, I think 400 and 600 PIV are common and cost only pennys more, increases reliability.

Diode is called a stud mounted diode, because it has a threaded stud on one end.
They do come in normal and reverse polarity ( whether the stud is the anode or cathode), that's likely why they call it a negative diode.
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Old 11-15-2015, 02:48:31 PM
Leon N. Leon N. is offline
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Default Re: Magneciter diode specs

The diode polarity is based, I believe on their functionality, not their physical features.
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Old 11-15-2015, 02:48:40 PM
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Default Re: Magneciter diode specs

I have a box of mag parts. Used but if I have one I will let you know.
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Old 11-15-2015, 03:01:37 PM
Leon N. Leon N. is offline
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Default Re: Magneciter diode specs

Billy, the negative power diode he wants requires a quantity of two. Strange the user did not mention that. Maybe he has determined that only one is bad?
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Old 11-15-2015, 03:03:10 PM
Vanman Vanman is offline
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Default Re: Magneciter diode specs

Not that it really matters, but I come up with only about 18-3/4 amperes and 75 volts across the 4 ohm field when surging to 1400 watts. Of course I may have made a mistake

Calculating diode capacity in an alternating current circuit is a bit tricky since their voltage drop is practically constant and independent of current flow, completely unlike a resistor. Their peak capacity is typically dramatically greater than their continuous capacity for this reason.

Due to the highly inductive nature of the circuit in question, I'd guess that each diode carries a fairly constant current (that is the whole of the field current) for each half of the cycle that it is on? So a diode rated for half as much should suffice, in theory.

Personally I like oversizing this sort of thing since they're so cheap anyway. And agreed on the PIV, higher only costs pennies more and so no reason not to.

I wonder if the original diodes would have part numbers on them that could be cross referenced to a replacement?

Keith
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Old 11-15-2015, 03:09:15 PM
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Default Re: Magneciter diode specs

If the OP posts a good picture, I'm pretty sure we can find a suitable replacement. Main thing is to know the physical parameters. Stud mount diodes (which I am assuming this is) come in two common sizes; one having a 10-32 thread, and the other a 1/4" diameter thread. After we know that, just buy the diodes with the highest practical rating...
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Old 11-15-2015, 03:32:57 PM
len k len k is offline
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Default Re: Magneciter diode specs

Quote:
Originally Posted by Leon N. View Post
The diode polarity is based, I believe on their functionality, not their physical features.
OPPS......I assumed they were the power diodes on the rotating part of a YD (not magnaciter) genhead.

I haven't heard of a negative diode other than polarity relative to the stud, but I'll back off on my recommendation since I'm not that familiar with magnaciter design.
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Old 11-15-2015, 04:21:20 PM
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Default Re: Magneciter diode specs

Usually there are reverse polarity diodes available , quite often they just have a "R" after or in the same part #. Some examples here of 25 A diodes

https://www.digikey.ca/product-searc...?k=diode%20DO5


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Old 11-15-2015, 04:40:44 PM
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Default Re: Magneciter diode specs

generally diodes with higher PIV and higher current ratings that will mount in the same method are preferable

stud mounted diodes in reverse polarity are used to avoid electrical insulation from the heat sink where several components are mounted.

it is just cheaper to make a modern VR than the Fe & Cu Magneciter
not better
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Old 11-15-2015, 05:31:33 PM
Leon N. Leon N. is offline
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Default Re: Magneciter diode specs

Vanman I hate to disagree with you but take a look at the 7.5 KW JB spec sheet, page 2.

Onan describes the exciter performance by indicating it draws 400 watts at 40 volts. This rectified current is drawn from a separate 120 VAC winding on the stator. So just ratio volts into watts and wha la 10 amperes. You can go further and determine from these numbers the rotor resistance if interested.

Looking at the wiring diagram I mentioned earlier, it looks like there are two power diodes on each side of the AC sine wave so maybe that way Onan builds in more reliability. The DC voltage produced is 0.636 of the peak 120 VAC and further reduced by the action of the resistors and the toroids. Net result as Onan specifies is 40 Volts@400 watts with a transient response of 1400 watts!

I'll wager the YD design cannot match this.
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Old 11-15-2015, 06:18:29 PM
Vanman Vanman is offline
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Default Re: Magneciter diode specs

Hey Leon, I did say that I may have been in error

But, for what it's worth, here is the logic that I used:

Given that the field normally runs at 40 volts and dissipates 400 watts, then it must be flowing 10 amperes and must be 4 ohms.

I then derived current flow for 1400 watts dissipation by taking the square root of (1400 watts / 4 ohms). My keyboard doesn't have the correct symbols

This computes to 18.71 amperes, based on the formula of current squared times resistance equals power.

Voltage at 1400 watts dissipation could be calculated by 1400 watts /18.71 amperes = 74.8 volts

Or by taking the square root of (1400 watts x 4 ohms) = 74.8 volts

I'm still not ruling out a mistake, but I'm pretty sure I figured this correctly

Keith
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Old 11-15-2015, 07:17:41 PM
Leon N. Leon N. is offline
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Default Re: Magneciter diode specs

Keith, I like your reasoning. You could be right! Reason being I most likely incorrectly assumed that the rectified voltage of 40 VDC stays constant during a transient or over load response. All I do know is that Onan provided only 3 numbers, namely, 40 volts, 400 watts and 1400 watts.
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Old 11-16-2015, 02:01:47 AM
len k len k is offline
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Default Re: Magneciter diode specs

There are likely parallels between how a magneciter JB and NHE/NHM operate as far as how rotor voltages vary with load. I believe both use the same type brush system to power the rotor.

For what it's worth Onan NHE/NHM types state that rotor voltage is 18-60 VDC, ( rotor is 22.5 ohms nominal). From my testing I find at no-load it takes ~ 35 volts to make ~120VAC. As the load is increase the rotor voltage has to be increase to maintain 120VAC output, steady state type load increases. These gens use a brush system to energize the rotor, which I think is similar to the magnaciter genhead system.

I'm guessing the 18V occurs just after a full load dump. And 60VDC occurs just as a full load is suddenly dropped on the gen.

Last edited by len k; 11-16-2015 at 02:11:50 PM.
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Old 11-16-2015, 08:21:56 AM
Leon N. Leon N. is offline
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Default Re: Magneciter diode specs

Yes, I think Keith is correct. The only thing that can be deduced from the three Onan magneciter numbers, that is, the 40 volts, the 400 watts and the 1400 watts is that the 7.5 KW JB Magneciter rotor has a DC resistance of 4 ohms. I was wrong in assuming the 40 volts is a constant over the load range. I presume the 40 volt figure is just one data point @ full load.
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Old 11-16-2015, 01:48:55 PM
Vanman Vanman is offline
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Default Re: Magneciter diode specs

Interesting to note that the NHM / NHE field circuit dissipates dramatically less power, based on the figures posted by Len K. All things being equal (and I realize they're not) that would indicate more copper dedicated to the field.

Keith
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Old 11-16-2015, 02:03:13 PM
len k len k is offline
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Default Re: Magneciter diode specs

Possibly more copper length for more amp-turns (stronger mag field).
But I believe JB rotor has thicker copper, and more of it, I think the rotor's bigger.

It's also interesting that both NHE and JB have brushes , yet NHE types seem to develop high slip ring resistance and resulting voltage reg failure. Maybe the higher brush amps of a JB magneciter system have a cleaning effect on the slip rings and brushes? Although a magneciter is pretty bulletproof compared to semiconductors.

Last edited by len k; 11-16-2015 at 02:13:33 PM.
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Old 11-23-2015, 12:16:45 AM
Boydster Boydster is offline
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Default Re: Magneciter diode specs

Firstly I want to say a big thank-you to all that have contributed to this thread! I also want to apologize for not responding sooner but I've had a lot on my plate lately and work has been absolutely bonkers.
I ended up re-soldering a broken connection on one of the diodes and my old mdje is back to running just as she should. I'm still planning on replacing at least a couple of the stud-mount diodes as preventative maintenance as I noticed a substantial amount of corrosion on two of them.
I'll post photos soon in the hope that the gurus here can point me in the right direction when looking for replacements. As I stated in a previous post, I'm pretty comfortable with the mechanical aspect of these generators but the electronics are way over my head and I really do appreciate your help and patience! - Boyd
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