Any electronic experts still awake?

I recently converted my SM from a generator to an alternator using a diode instead of a manual switch. The one I used came off another tractor I have. It's about the size of a pencil eraser. I'm looking to do another one but the new diodes I see for sale (4n?001) are tiny, about the size of a pencil lead. Has technology changed that much or am I missing something?

It should handle 1-2 amp, look up the part number on the web.

I think a 1N is to small, would go with a 4N

Any of those "tiny" 1N4001 thru 1N4004 series diodes will work fine! Or substitute a #168 or #194 instrument panel bulb for the diode.

You can also rip one out of the diode bridge from a bad alternator. Be sure to use the one allowing volts to go to the alt, but blocks return. Jim

I use 1N5408's. @ 3 Amps and 1000 PRV, they are overkill for this application, but are CHEAP and don't fail.

Thanks folks. I can move on with more confidence than when I started for sure.

The amp draw of the 10SI alternator rotor is in the 2.3 - 2.6 amp range. You need a diode that can handle that load.

True internal load, but I believe the Excite circuit will energize through a small light bulb that would never pass 3 amps. Jim

(quoted from post at 07:00:45 12/16/14) True internal load, but I believe the Excite circuit will energize through a small light bulb that would never pass 3 amps. Jim

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I use 400 mA if I remember correctly. Bought at Radio Shack.

The point is, though, with no lamp or resistor in the "excite" circuit, just a diode, the diode likely DOES pass 2 or 3 Amps.

A quick 'net search shows the 1N5408's I use to be available for between $0.18 and $2.00 each.

They work, they last, and they are CHEAP, so there's NO reason to use wimpy diodes.

(quoted from post at 22:00:45 12/15/14) True internal load, but I believe the Excite circuit will energize through a small light bulb that would never pass 3 amps. Jim

Click to expand...

A light bulb is designed to operate with a 12 volt drop across it. The amperage will never be more than the bulb is designed for. Adding the additional resistance of the field coil drops the amperage passing through it even more. In this case the bulb is the main factor limiting the amperage draw. With a diode the field coil is the limiting factor.

The voltage applied to the excite terminal will determine the speed at which the alternator starts to charge. The more resistance in the bulb the higher the rpms needed for the alternator to start to charge. If the resistance through the bulb is such that you have 4 - 6 volts at the excite terminal the alternator will start to charge in the 1000 - 1400 rpm range. If using a diode with low resistance you will have very nearly battery voltage on the excite terminal and the alternator will turn on at around 600 rpm. If you use only a diode that has very little resistance it should be large enough to handle the rotor amperage draw at battery voltage. A three amp diode will have sufficient capacity for this. Since most diodes will handle larger amperage loads than for which they are designed, they will usually carry the overload for at least a bit of time. A one amp diode may work as long as you turn on the switch and then start the engine. The same diode may fail if the switch is turned on and the alternator does not shortly begin to charge. A failure of the diode trio could also overload the diode even though the alternator could continue to charge.

An alternator excited by a diode could put out full amperage with a failed diode trio. If the same alternator was excited through a light bulb the maximum output would be considerably less due to lower voltage available to power the field coil in the rotor. In this case the light would remain on since there would be no voltage coming from the diode trio to turn it off.

OK good explanation. Jim