Ballast resistor/oil & AC fan low speed resistor - "redesigned"
12-17-2008, 09:29 PM
#31
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Test results for max. running temperature ....
Ran an operating test tonight to measure temperature rise rate and max. temperature experienced at these ballast resistors: before jumping to the results, note that each circuit is fused in the CE panel at 30 amps ( #'s 2 & 39). Also note that the two fans are different, although each specify the same ballast resistor. The AC condensor, 964 264 035 01, has a coil resistance of ~0.2 ohms [edit - entered 2 in error initially - pesky decimal point left out]. The oil cooler, 964 264 035 00, has a coil value of ~0.5 ohms.
To each of the above values, add 0.5 ohms ballast for low speed operation - the basis of the test , and one would expect different temperature curves in operation measured at the ballast. I did not attempt to measure amperage draws; however, if one is inclined to play with Ohms Law, proportional power curves can be calculated. I cheated a bit by running these at battery voltage (12.5v), with a charger attached to assure consistency: were the engine running, fan speeds and efficiency would be proportionally better, as everything runs at an alternator regulated voltage of ~14v.
I used a laser pointer type hand held IR gun, and selected the 'hot spot' of each individual resistor to determine the time/temp curve: the background temp. of the aluminium heat sink/mounting bracket was also taken.
It is snowing & blowing like mad here, so this kid was not about to run the engine with a closed door
: that is to rationalize the above mutterings .... but more importantly, to note that ambient temperature in the garage was a nice 0 deg C - not the nominal 25C test zone .... oh well , can't wait for spring .....
Both resistor packs reached max temperature at 7 min. of running: these values were sustained without further change for 15 min. of operation - so steady state was established. Readings were taken each 30 sec for the first 4 min., then by minute intervals .... no surprises.
The AC pack hit a max of 43C, and the oil cooler followed at 28C: the heat sinks never rose above 13C and 7C respectively, ambient being 0C.
It would appear that the finned resistors are doing a reasonable job of energy dissipation, as the aluminium mounting brackets/ heat sinks, stayed very cool.
In actual service, what is the true duty cycle of eack of these fans? I didn't check, but the AC may cycle continuously between the two speeds, wheras the oil cooler is on demand only - low or high speed determined by which thermistor is triggered.
Anyway, there are some numbers that may be of interest.
To each of the above values, add 0.5 ohms ballast for low speed operation - the basis of the test , and one would expect different temperature curves in operation measured at the ballast. I did not attempt to measure amperage draws; however, if one is inclined to play with Ohms Law, proportional power curves can be calculated. I cheated a bit by running these at battery voltage (12.5v), with a charger attached to assure consistency: were the engine running, fan speeds and efficiency would be proportionally better, as everything runs at an alternator regulated voltage of ~14v.
I used a laser pointer type hand held IR gun, and selected the 'hot spot' of each individual resistor to determine the time/temp curve: the background temp. of the aluminium heat sink/mounting bracket was also taken.
It is snowing & blowing like mad here, so this kid was not about to run the engine with a closed door
: that is to rationalize the above mutterings .... but more importantly, to note that ambient temperature in the garage was a nice 0 deg C - not the nominal 25C test zone .... oh well , can't wait for spring ..... Both resistor packs reached max temperature at 7 min. of running: these values were sustained without further change for 15 min. of operation - so steady state was established. Readings were taken each 30 sec for the first 4 min., then by minute intervals .... no surprises.
The AC pack hit a max of 43C, and the oil cooler followed at 28C: the heat sinks never rose above 13C and 7C respectively, ambient being 0C.
It would appear that the finned resistors are doing a reasonable job of energy dissipation, as the aluminium mounting brackets/ heat sinks, stayed very cool.
In actual service, what is the true duty cycle of eack of these fans? I didn't check, but the AC may cycle continuously between the two speeds, wheras the oil cooler is on demand only - low or high speed determined by which thermistor is triggered.
Anyway, there are some numbers that may be of interest.
Last edited by Garth S; 12-18-2008 at 02:32 PM. Reason: corrected to a 0.2 from a 2 initially entered above
12-18-2008, 01:12 AM
#32
Nice work Garth!
All this at 0 deg C for science and Porsche. I am impressed. I think you might have mixed up the two results. I think the power dissipation and temperature rise will be highest for the case with the lowest resistance (highest current) the one with a total resistance of 1 ohm. There with ~12.5 V one gets 12.5A *6.25 V or about 78W. In the other case (total resistance of 2.5 ohms) one gets a current of 5 amps*2.5V or about 12.5W. (if I did not make a mistake) . The difference in power dissipation between the 2 cases is rather large vs. the temperature rise so I am guessing the total resistance is not 1 and 2.5 ohms. These results are somewhat consistent with the expected temperature rise in the data sheet. What specific resistors are you using?
In any way, a temperature rise of 43C is probably OK, but not negligible. Keep in ind that the temperature rise is not linear with ambient temperature. If we assume the ambient temperature is about 75C (i do not know how realistic this is in hot weather, air conditioning running etc.) then the resistor temperature is going to be above 120C! What is the maximum temperature of the insulation , black plastic etc before it starts to burn? I do not know, however it would be interesting to find out at what temperature these things ignite.
It is good to see that the heat sink is very effective- at least at 0deg C!
All this at 0 deg C for science and Porsche. I am impressed. I think you might have mixed up the two results. I think the power dissipation and temperature rise will be highest for the case with the lowest resistance (highest current) the one with a total resistance of 1 ohm. There with ~12.5 V one gets 12.5A *6.25 V or about 78W. In the other case (total resistance of 2.5 ohms) one gets a current of 5 amps*2.5V or about 12.5W. (if I did not make a mistake) . The difference in power dissipation between the 2 cases is rather large vs. the temperature rise so I am guessing the total resistance is not 1 and 2.5 ohms. These results are somewhat consistent with the expected temperature rise in the data sheet. What specific resistors are you using?
In any way, a temperature rise of 43C is probably OK, but not negligible. Keep in ind that the temperature rise is not linear with ambient temperature. If we assume the ambient temperature is about 75C (i do not know how realistic this is in hot weather, air conditioning running etc.) then the resistor temperature is going to be above 120C! What is the maximum temperature of the insulation , black plastic etc before it starts to burn? I do not know, however it would be interesting to find out at what temperature these things ignite.
It is good to see that the heat sink is very effective- at least at 0deg C!
12-18-2008, 10:35 AM
#33
Three Wheelin'
"To each of the above values, add 0.5 ohms ballast for low speed operation " 
Look at the cct diagram and tell me that the ballast resistor is not in parallel with the fan. ie when the relay closes current is drawn through the resistor that drops the VOLTAGE at the fan.
Look at the cct diagram and tell me that the ballast resistor is not in parallel with the fan. ie when the relay closes current is drawn through the resistor that drops the VOLTAGE at the fan.
12-18-2008, 12:13 PM
#34
Rennlist Member
Very nice. I like this.
Isn't it simpler than that? Porsches unit dissipated 50w and yours would need to do the same. Seems like your heat sink has more surface area and thus should be a lower average temp than the Porsche unit.
I really like the idea of putting a thermal link in so that if the fan shots and you are putting 300w into the resistor, you don't start a fire. Was there any into on the value of the Porsche thermal link?
Next time my resistor goes, I'll have to do this.
Isn't it simpler than that? Porsches unit dissipated 50w and yours would need to do the same. Seems like your heat sink has more surface area and thus should be a lower average temp than the Porsche unit.
I really like the idea of putting a thermal link in so that if the fan shots and you are putting 300w into the resistor, you don't start a fire. Was there any into on the value of the Porsche thermal link?
Next time my resistor goes, I'll have to do this.
12-18-2008, 02:34 PM
#35
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"Look at the cct diagram and tell me that the ballast resistor is not in parallel "
Having looked at the wiring diagram, & to my untutored eye, it appears that the resistor is in series with the circuit for low speed operation.
Stage 1 of the relay operation shunts current through the resistor to the fan & stage 2 shunts current direct.
Having looked at the wiring diagram, & to my untutored eye, it appears that the resistor is in series with the circuit for low speed operation.
Stage 1 of the relay operation shunts current through the resistor to the fan & stage 2 shunts current direct.
12-18-2008, 02:37 PM
#36
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So this is why you earn the big bucks
- you, Sir, are likely correct: Block L/21 of the wiring diag shows stage I fan operation with the ballast in -----? Truly, I am not certain, for I cannot trace back the trigger functions that control the dual solenoid. I need something better than my crappy pdf file copy of the wiring diags.EDIT PS: 'on reflection', is there any other way to effect speed control on a DC motor other than dropping voltage? ( ie., max "run away" speed is set by armature back EMF, drag of the impeller , etc). To run at less than max rpm, voltage must be decreased .... which is why a ballast resistor is in series. If it were in parallel, the motor would still see full (max rpm) voltage - but the amperage carried by the wiring would increase, making the resistor an effective heater, but poor speed controller.
Curious ( as ever
), I plugged the low speed jumper in the oil cooler fan to get low speed .... then opened the ballast connector: fan died! ie - resistor in series .... or am I missing something?
Last edited by Garth S; 12-18-2008 at 03:30 PM.
12-18-2008, 02:38 PM
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Another question: is the ballast resistor really necessary? What if it were omitted & the fan ran fast in stage 1 & stage2 operation from the relay?
The oil in the cooler would just cool down more rapidly: what's wrong with that?
The oil in the cooler would just cool down more rapidly: what's wrong with that?
12-18-2008, 02:48 PM
#38
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Hi perlfather,
Glad to see your input - and wise caution regards potential overheating: I will repeat these runs when weather improves.
BTW, 75C ambient is 167F
I thought MA was a little more temperate, but if I come by your way, hope you've got lots of cold ones on ice 
12-18-2008, 03:26 PM
#39
BTW, 75C ambient is 167F I thought MA was a little more temperate, but if I come by your way, hope you've got lots of cold ones on ice [/QUOTE]
Garth, I just thought that things are pretty hot in the engine compartment on hot day after some spirited runs and heat rising from the exhaust. There must be a reason why automotive electronics are rated to +125C (from my experience with ADXL accels.)
The interesting experiment that needs to be done is to see at what temperature does oil coated insulation or plastic (typical of the 911 engine compartment) ignite. I bet it is not that high. I think we can do it with a healthy soldering iron.
Please let me know when you are in MA next time, would love to invite you for a cold one. Our spring and fall are quite spectacular and last several weeks!
I thought MA was a little more temperate, but if I come by your way, hope you've got lots of cold ones on ice [/QUOTE]Garth, I just thought that things are pretty hot in the engine compartment on hot day after some spirited runs and heat rising from the exhaust. There must be a reason why automotive electronics are rated to +125C (from my experience with ADXL accels.)
The interesting experiment that needs to be done is to see at what temperature does oil coated insulation or plastic (typical of the 911 engine compartment) ignite. I bet it is not that high. I think we can do it with a healthy soldering iron.
Please let me know when you are in MA next time, would love to invite you for a cold one. Our spring and fall are quite spectacular and last several weeks!
08-25-2013, 12:59 PM
#40
Rennlist Member
I PMed Garth regarding the above 100W dissipation capability a couple of weeks ago.
The parallel combination is 0.5 ohms but the 50W power handling of each resistor is in fact lower than the OEM resistor. The voltage drop across the parallel resistors is still the same (only the current is halved to each resistor).
I measured the voltage drop across my new OEM resistor at ~5V (leaving 9V across the fan motor), so the power is V^2/R or 25/0.45 = 55.5W based on an engine running battery voltage of 14.1V. Ideally, I'd want to measure the current and voltage drop to calculate the actual resistance during operation.
Garth's 50W power resistor is running at ~25W (5^2/1 = 25), which is fine provided the ambient temperature is 25C or less and the correct sized heat sink is used. Vishay specs.
Note that a resistor's specified power rating is de-rated based on ambient temperature and reduced heat sink area.
Anyway, back to looking for an easy way to get to the oil cooler fan series resistor.
05-04-2014, 03:22 PM
#41
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Bumping this thread back up.
On a NB, accessing the oil cooler resistor is not too bad. On my WB with the turbo S lip, this would require removal of the entire front bumper. I came across this thread and thought I may reposition the new oem series resistor in the same area as Garth did on this thread. Anyone know what's behind the double walled area a few cm above where Garth mounted his home-brewed resistors? I'm thinking drilling and tapping there to mount. Thoughts?
On a NB, accessing the oil cooler resistor is not too bad. On my WB with the turbo S lip, this would require removal of the entire front bumper. I came across this thread and thought I may reposition the new oem series resistor in the same area as Garth did on this thread. Anyone know what's behind the double walled area a few cm above where Garth mounted his home-brewed resistors? I'm thinking drilling and tapping there to mount. Thoughts?
05-04-2014, 05:15 PM
#42
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P-daddy: I left the failed original resisters in situ, and mounted new OEM resistors accessible from the headlight opening. This was almost ten years ago, and the resistors still look new and test within spec. No need to remove the bumper or anything other than headlights to do this. Here is how:
1. Remove the headlight.
2. Locate the ballast resistor wiring harness--it is fastened to the outer wall of the luggage bay with a black plastic loop.
3. Pull gently on this harness to unplug it from the ballast resistor.
4. Install the new ballast resistor in a suitable location. I use one of the fender bolts. Remove it from inside the boot, and drill the hole out slightly to accept a 25X6mm bolt. Secure the bolt with a nut and lockwasher. Insert a locknut into the recess in the resistor, and spin it hand-tight onto the bolt. Plug the resistor into the wiring harness. Reinstall the headlight, and you are done.
I did the same resistor relocation on a friend's 993 a couple of months later. His are still working too.
1. Remove the headlight.
2. Locate the ballast resistor wiring harness--it is fastened to the outer wall of the luggage bay with a black plastic loop.
3. Pull gently on this harness to unplug it from the ballast resistor.
4. Install the new ballast resistor in a suitable location. I use one of the fender bolts. Remove it from inside the boot, and drill the hole out slightly to accept a 25X6mm bolt. Secure the bolt with a nut and lockwasher. Insert a locknut into the recess in the resistor, and spin it hand-tight onto the bolt. Plug the resistor into the wiring harness. Reinstall the headlight, and you are done.
I did the same resistor relocation on a friend's 993 a couple of months later. His are still working too.
05-04-2014, 06:09 PM
#43
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P-daddy: I left the failed original resisters in situ, and mounted new OEM resistors accessible from the headlight opening. This was almost ten years ago, and the resistors still look new and test within spec. No need to remove the bumper or anything other than headlights to do this. Here is how:
1. Remove the headlight.
2. Locate the ballast resistor wiring harness--it is fastened to the outer wall of the luggage bay with a black plastic loop.
3. Pull gently on this harness to unplug it from the ballast resistor.
4. Install the new ballast resistor in a suitable location. I use one of the fender bolts. Remove it from inside the boot, and drill the hole out slightly to accept a 25X6mm bolt. Secure the bolt with a nut and lockwasher. Insert a locknut into the recess in the resistor, and spin it hand-tight onto the bolt. Plug the resistor into the wiring harness. Reinstall the headlight, and you are done.
I did the same resistor relocation on a friend's 993 a couple of months later. His are still working too.
1. Remove the headlight.
2. Locate the ballast resistor wiring harness--it is fastened to the outer wall of the luggage bay with a black plastic loop.
3. Pull gently on this harness to unplug it from the ballast resistor.
4. Install the new ballast resistor in a suitable location. I use one of the fender bolts. Remove it from inside the boot, and drill the hole out slightly to accept a 25X6mm bolt. Secure the bolt with a nut and lockwasher. Insert a locknut into the recess in the resistor, and spin it hand-tight onto the bolt. Plug the resistor into the wiring harness. Reinstall the headlight, and you are done.
I did the same resistor relocation on a friend's 993 a couple of months later. His are still working too.
05-04-2014, 07:01 PM
#45
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P-daddy:
Here are a couple of shots. The only advantage over the location you showed is easy access via the headlight opening if future replacement is required. Also, the long bolt I specified stands the resistor well clear of the body sheet metal, so that heat can be dissipated through the heat sink provided with new resistors.
The pictures are of the oil cooler fan resistor, so right headlight opening. The first picture shows the wiring harness, while the second picture is with the wiring harness tucked behind the plastic headlight bracket.
Here are a couple of shots. The only advantage over the location you showed is easy access via the headlight opening if future replacement is required. Also, the long bolt I specified stands the resistor well clear of the body sheet metal, so that heat can be dissipated through the heat sink provided with new resistors.
The pictures are of the oil cooler fan resistor, so right headlight opening. The first picture shows the wiring harness, while the second picture is with the wiring harness tucked behind the plastic headlight bracket.