500E Posted January 3, 2012 Report Share Posted January 3, 2012 Had a 300 in for service on checking through on initial inspection No clutch light.Pulled that little bulb, It was O\C easy repair, replace bulb, check operation all working as required.Do rest of service, pull out,start eng, engage blades, POP goes the clutch trip, stop eng re set trip try clutch engage pop ammeter goes momentary 50+ not good look at schematic for clutch actuator circuit, bulb fed via 2 diodes 1N599 on board 4 (on this ship left side of instrument consul half way up ) meter both they are short circuit.Have deep think all we had done was change bulb, remove new bulb & check resistance against 2 others the bulb was SHORT circuit .2 Ohms, it had taken both diodes before the trip went out.Tried a new Avitar to day can see why you used a drawing rather than a photo,the new software thing sucks on photos just want my old av back.The pick was so poor i would rather nothing Quote Link to comment Share on other sites More sharing options...
iChris Posted January 7, 2012 Report Share Posted January 7, 2012 (edited) Had a 300 in for service on checking through on initial inspection No clutch light.Pulled that little bulb, It was O\C easy repair, replace bulb, check operation all working as required. Do rest of service, pull out,start eng, engage blades, POP goes the clutch trip, stop eng re set trip try clutch engage pop ammeter goes momentary 50+ not good look at schematic for clutch actuator circuit, bulb fed via 2 diodes 1N599 on board 4 (on this ship left side of instrument consul half way up ) meter both they are short circuit. Have deep think all we had done was change bulb, remove new bulb & check resistance against 2 others the bulb was SHORT circuit .2 Ohms, it had taken both diodes before the trip went out. I assume after you replaced the bulb everything worked fine, including the operational check with the clutch light working as normal. Even through the whole start-up and just before engaging the clutch switch, was the clutch light still on and everything normal? If prior to switching the clutch switch to engage, everything was ok, and at the moment of switching the bulb shorted, I could see the right diode shorting due to the excess current flow and popping the breaker because of the bulb short. However, I don’t see the left diode shorting at that point because once you switch to engage, points 1 & 3 of the clutch switch places one side of the left diode (anode) at ground potential and the shorted bulb places the other end (cathode) at ground potential. Therefore, there’s no difference of potential across that left diode and no current flow. Looking at it with the switch in the release position, a shorted clutch light would take out the left diode and pop the breaker, but the right diode would have both ends at ground and zero current flow. Interesting that both diodes shorted at the same time and the same for the shorted bulb (.2 ohm) that seemed to work at first. Do you remember the resistance of the good bulbs? Edited January 7, 2012 by iChris Quote Link to comment Share on other sites More sharing options...
500E Posted January 11, 2012 Author Report Share Posted January 11, 2012 Hi iC The flow wasNo warning lightCh bulb (o\c)Replace bulbClutch was partially engaged prior to replacing.Cycled clutch full on, full off, Twice.3rd cycle on pre flight test for full travel, light on for split second ammeter flashed to 5\60 Amps trip pops, re set trip try to cycle to full off, tripp pops.Remove new bulb try again still pops both on \ off, Now worried what have we done ??.Check live side bulb connection shorted to ground, examine diagram test diodes both SC.Replace & test without bulb, worked OK, refit bulb trip popped. (bu***r)Remove bulb check .2 Ohms with lead resistance!!.Replace bulb all OK.Would suggest that the age of diodes may be a factor, the 1N599s no longer available, remember we tried to cycle without bulb this would have put full volts on other diode. Quote Link to comment Share on other sites More sharing options...
iChris Posted January 12, 2012 Report Share Posted January 12, 2012 (edited) Hi iC Have deep think all we had done was change bulb 3rd cycle on pre flight test for full travel, light on for split second ammeter flashed to 5\60 Amps trip pops, re set trip try to cycle to full off, tripp pops. Remove new bulb try again still pops both on \ off. Now worried what have we done ??. Check live side bulb connection shorted to ground, examine diagram test diodes both SC. Replace & test without bulb, worked OK, refit bulb trip popped. (bu***r) Remove bulb check .2 Ohms with lead resistance!!. Replace bulb all OK. Would suggest that the age of diodes may be a factor, the 1N599s no longer available, remember we tried to cycle without bulb this would have put full volts on other diode. Once either diode shorts the bulb being in or out doesn’t matter, you’re going to have a popped breaker when the switch is in the engage or disengage position with the possibility of taking out the remaining diode. A shorted bulb would only affect one diode. Those diodes were rated at 50V peak reverse voltage and the full battery voltage shouldn’t have been a problem; however, I do think a diode was the main problem. That light bulb my have been OK. Here is the scenario as an academic exercise: When you switch to the engage position, contacts #4 & #6 of the clutch switch connect allowing 12vdc to flow to contact #2. At the same time, via contact #2 and the normally closed contacts of the engage limit switch, 12vdc also flows to the engage side of the motor and the anode of the right diode. Since the right diode is now forward biased, it allows current to flow to the clutch light and turn it on. At the bottom of the clutch switch, contacts #1 & #3 connect and place a ground on the anode of the left diode. At this point, the right diode is forward biased allowing current to flow to the clutch light and the left diode is reversed biased and blocking all current flow. What may have happened, when you switched to engage, was the left diode failed. Once the left diode failed (shorted) the current flowing through the right diode now has a direct path to ground via the clutch switch, contacts #1 & #3. The direct short tries to draw infinite current through both diodes, which it can’t do, so the breaker pops. However, before the breaker popped the right diode may have shorted due to the excessive current flow. The 1N599 maximum forward current rating is .3 amps, far below the breaker rating. Those are small signal diodes just to accommodate that clutch light. Once the diode failed the bulb being in or out doesn’t matter. That bulb my have even been OK. Determining the resistance of a bulb with a meter is inaccurate. The filament has a positive temperature Coefficient. This means its resistance rises with temperature. When cold you'll get a few ohms of resistance, but at operating temperature it raises dramatically. Bulbs fail 99% of the time when the filament burns-out and the circuit opens, infinite resistance on the meter. On the rare occasion you’ll get a resistance reading of zero, then the filament inside the bulb probably burned-out, fragmented, and shorted across both sides at the filament supports. A typical light bulb has around 18 Ohms, but this amount varies among different size, brightness, and brands. Your normal MAG-LITE flashlight bulb (non-LED type) reads in the range of 0.2 - 1.2 ohms on some digital meters and the bulb works fine. Why is it not recommended to measure the resistance of a filament directly with the multimeter? Why does a lamp change resistance? That 1N599 diode crosses to a NTE 117. Fry's Electronics has a line of NTE parts. Both the NTE116 and NTE125 will also work for this application and have ratings at or above the 1N599. They are available online at Sold State Electronics at link below. The 1N599 Cross References to an NTE 117NTE 117 in-stock at Sold State Electronics Edited January 12, 2012 by iChris 1 Quote Link to comment Share on other sites More sharing options...
aeroscout Posted January 12, 2012 Report Share Posted January 12, 2012 Dang, a quite extensive well supported well referenced post. Quote Link to comment Share on other sites More sharing options...
500E Posted January 13, 2012 Author Report Share Posted January 13, 2012 Hi iCBulb def US found the darn thing in scrap bin, wired to a batt with 20 amp trip bang goes the trip.Understand the variable resistance is inaccurate was looking for SC there was no resistance without leads bulb across meter direct, My feeling the second diode was taken out with spike they are old. Quote Link to comment Share on other sites More sharing options...
iChris Posted January 13, 2012 Report Share Posted January 13, 2012 (edited) Hi iCBulb def US found the darn thing in scrap bin, wired to a batt with 20 amp trip bang goes the trip.Understand the variable resistance is inaccurate was looking for SC there was no resistance without leads bulb across meter direct, My feeling the second diode was taken out with spike they are old. Case Closed... That was a rare, but possible, chain of events. The corrective action still would have been to replace both diodes, as you did. Both are right next to each other and the cost is negligible, about $2.00 each. Excellent troubleshooting. Edited July 16, 2016 by iChris Quote Link to comment Share on other sites More sharing options...
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