Fuses and Ratings — Wattage or Amperage
I have a burning question I'm hoping the group can help me understand. We recently had the vehicle listed above in for an air conditioning complaint that was due to an open in the compressor clutch coil. After we repaired the vehicle I decided to take a closer look to see if I could learn anything more about the failure.
Upon disassembly, this is what I saw:
Taking a closer look, I could see that the clutch is equipped with an internal fuse.
The fuse specifications are as follows:
- 1 AMP
- 187º C
- 250 V AC
Here is link to the data sheet for this fuse:
The circuit on this vehicle is protected by a 10 amp Low Profile Mini Fuse rated at 58v.
Here is the data sheet for this fuse:
Currently, my hypothesis is that the fuses are rated for power and not by simply looking at the amperage. So with the internal 1 amp fuse found in the compressor coil we have a circuit protected to 250 watts and the 10 amp vehicle fuse is rated up to 580 watts. Am I on the right track here?
Thank you for your time.
I see the same 250v 1 amp rating on some fuses in the plugs that go into cigarette lighters. Never understood it. But matched the spec because I didn't know what it meant.
Always safe to follow the "what comes out must go back in" rule !
I believe that while you are correct that using Ohm's law/Watt's law we can calculate the maximum wattage a fuse can withstand that isn't the dynamic that the fuse rating is shooting for- I remember the olde Auto glass fuses used to be marked 32V- so a 10A olde fuse would be capable of withstanding 320W, for instance But a fuse is a member of the Overcurrent Protection Device (OCPD) family-…
It is hard to see on the picture put is the fuse actually connected in serie on the circuit? Could it be part of a loop to prevent induction upon field collapse. Can you clean around to better see how it is connected? Just a thought.
Hi Alex, It's in series and I checked the coil resistance which measured out at 3.2 Ohms.
I believe that the fuse in the picture is actually a thermal fuse as used in a blow drier, heat gun, or a small appliance the design is for it to open when the temperature threshold is exceeded (which it clearly looks like that is the case) & it will stay open I would be interested to see you cut out one end of it & ohm it out & if it has continuity then clearly you just have a…
Thanks Dick, The fuse is open and the coil has 3.2 Ohms of resistance. A another compressor coil I have sitting here measures 3.5 Ohms so I don't believe it's shorted. And yes, I noticed its absence in the factory diagram as well.
Since you're making comparisons between the one clutch coil and the other....you might load the circuit on each and measure current flow and voltage to see if the "true" resistance is the same between the new and the old.
That is a thermal fuse. The design and purpose of a thermal fuse varies from that of a current-limiting fuse. Although they don't list the rating at 12VDC, I think we can safely assume it is higher than the circuit protection rating of 10A, since the rating at 50VDC is 4A. Maybe someone else knows the calculation we're looking for. The purpose of a thermal fuse in electronics is to provide a…
Interesting. I will have to cut one open. My work is almost 100% Navistar. They use Sanden. The body computer monitors current to the coil. If amps go over or under a set of values (don't know what they are) it shuts the coil off and leave a code that just says over/under current. After a period of time or a key cycle the compressor will come back on. This leads to a lot of frustration in…
Is the 1 amp fuse open? 1 amp of current was enough to melt the substrate in the coil. What is the resistance of the coil? Can it be measured? Yes to your hypothesis. I believe all circuits are engineered for power/watts. Those were my questions that come to mind when looking at the failure.
Hi Rick, The fuse is indeed open. Thanks for the input.
Not exactly, if that was true at 580w the fuse would need 69amp to blow (580w÷12v=69.6a) the volt rating means at what voltage the blown fuse will stay blown without possibility of the open fuse arching together and closing the circuit. Like a lightbulb that is open and with movement the filament arcs together and it lights up again temporarily
Thanks info on the max voltage meaning. I believe the amperage you cited should be ~48 rather than 69. Would you agree that based on the time-current chart in the Littelfuse doc that would mean a spike in current to 48amps for less than 30 msec wouldn't blow the fuse?
Scott that's a cool find. Thanks for sharing. I believe as others have stated you're dealing with a thermal fuse. If you look towards the right side of it in the picture you provided, it appears like there's a temperature rating in degrees Celsius. Part of that information is covered up by what I assume is melted insulation. As to the rating itself, I think perhaps you're correct regarding the…
It's not a fuse. It's commonly called a clamping diode and it's there to prevents the inductive spike that occurs every time the clutch is cycled off. The collapse of the magnetic field can produce a voltage surge of hundreds ((if not higher) of volts that can cause electrical interference as well as damage on-board electronic modules. An added benefit is the protection of the A/C relay contacts…
Hi Eli, It's a fuse not a diode.
Hi Scott, Good question. On the clutch coil, that's a thermal fuse. Thermal fuses react to excessive temperature and not to excessive current. That is unless the excessive current is sufficient to cause the thermal fuse itself to heat up to the trigger temperature. In your case 187º C, or 369º F. This in place to prevent a fire due to excessive temperature. I guess you could say the same about…
Hi Scott, So now I know a little more about "Thermal Cut Off Fuses" and how their design is factored into the circuit thanks to the group. The Time/current rating chart in the Littelfuse spec sheet is interesting. If I'm reading that correctly, @ 110% of the rating it would take approximately 360,000 seconds (or more) for that fuse to blow. At 200%, it could take from 150 msec to 5 sec's to…
Hi Scott, That is interesting. So, at 11 amps it would essentially never blow the fuse as it would take 100 hours, or longer, based on that chart. Thanks for posting that, I've seen that before, but I've never looked that closely at it.
250 volts times 1 amp equals 250 watt.....250 watts divide by 12 volts equals 21 amps....250 watts divided by 14 volts equals 18 amp.....so an emergency back up fuse... That is 20 amps???...surprised they would spend the money to put that in the winding.
The bigger question is likely why the thermal fuse opened. Why did the clutch over heat? Slipping due to high pressures or low clutch coil voltage? I would not be too concerned with the current rating, since that rating is fairly irrelevant in this circuit.
Hi Olle, We performance tested the circuit and system pressures. Everything checked out normal. Thanks for your input.
I have seen this before, where the problem was intermittent. In my case it was an intermittent connector at a condenser fan. That thermal fuse did not let go just for grins.
Thanks for sharing your experience and I agree that there could be more to the issue. We've followed all of our processes at the end of repair including keeping the vehicle vehicle overnight for next day Q/A and road testing. We normally follow up with customers a few days after the service visit and with your info, I'll be sure to include this knowledge in the next touch-base.
It is a heat detecting fuse and at 14v /3.2 ohms the watts is <> 60 With a 1 amp 250 fuse at 14 volts it can handle <> 20amps At a draw of 20 amps the watts = 280 At 280 the fuse blows but would have to draw 20 amps So the fuse can handle Why it needs the thermal is beyond me unless the clutch is used elsewhere (enclosed area)and the flash point of the clutch material is…
Very common to have a thermal fuse in those clutch coils. I think it is a bit of a "Hail Mary" designed in.
Thanks for the info George. Glen Stutzman pointed me to the Valeo website where I found that their technical specification for the clutch coil power consumption is rated at 45 watts.
Some info on thermal fuse found here patents.google.com/patent/JP20062…
From the patent - "When the drive shaft is locked due to a failure such as burning of the compressor, excessive torque is applied to a transmission member such as a belt for transmitting power from the power source to the driven device, and the transmission member such as this belt or the like. As a result, the power source such as the engine may be damaged. In order to eliminate such…
Sounds like that if the compressor locks up but the coil will still energize and to create this heat the clutch starts slipping creating this heat
Is this your compressor? valeocompressors.com/catalog/en/veh… Take this info with a grain of salt since I do not work on the posted vehicle...In case you don't already know the posted vehicle has a build date cut off on the comp and or clutch according to my info
Hi Glen, Your links didn’t take me to where you wanted. Please let me know what you were pointing to and I’ll have a look.
Click on the Valeo icon go to catalog, I used a like a model Nissan. Build data info comes from the from repair link
Thanks Glen, The most interesting data I discovered there was that the power rating for the clutch coil is 45 watts. Was there something else you were trying to point out?
From the Emerson Thermal fuse spec sheet; The fuse is temperature operated only. The current rating is the maximum current that it is rated to interrupt at it’s rated voltage. The 250V rating is ac. The higher voltage rating is needed because DC is harder to interrupt than AC. What I find odd is that the heat damage of the coil is isolated to the fuse and it’s leads. It will open on overcurrent…
My assumptions over the years as to the causes of this phenomenon from postmortem's, range's from multiple conditions that may cause rapid compressor clutch cycling and or worn compressor clutches with increased clearances between the clutch plate and the pulley surface from my understanding would require more energy to keep the clutch engaged, all to me would create conditions of excessive…
Thanks goes out to all that took the time to dive into this question for me. I learned quite a bit about how a thermal fuse is rated vs other fuses and their general use. Thanks to Glen Stutzman for pointing me to the Valeo compressor website where I found data sheets on this compressor that included the rated wattage of the compressor clutch coil (45 watts at 12v).
I see a lot of starters with a thermostat fuse ...They always check good when rebuilding the starter ...Some are replaced due to broken plug socket.The fuse is attached to a brush holder and the external leads are integrated into the ignition. which can be bypassed. As far as the 45 watts in the data sheet In my calculations I used 14 volts which would give you around 60 watts