How much current does a Grimes beacon really draw???

Lopez · Post by **Lopez** » Wed May 22, 2013 4:39 pm

I'm doing an electrical load analysis for a field approval, and I can't find a spec on this anywhere? The case on the beacon has a part number (D7080-1-12 if it matters) and 13V but no current. I know I could measure it, but a written spec would be better.

Anyone have a reference???

Post by **Bruce Fenstermacher** » Wed May 22, 2013 5:30 pm

Did it give a wattage? Divide it by volts and you will have the amperage. Ohms law.

I once measured just a Grimes bulb for a beacon at 5 amps.

Lopez · Post by **Lopez** » Wed May 22, 2013 6:03 pm

No wattage specified. 5A sounds like a bunch for a bulb, are you telling me that a Grimes beacon uses 2 60W bulbs??? That sounds a wee bit high.

bigrenna · Post by **bigrenna** » Wed May 22, 2013 6:03 pm

delete

Post by **GAHorn** » Thu May 23, 2013 2:12 am

The typical 14V Grimes motor requires 1.4 amps, and the lamps (Grimes 7079B-12) draw 40 watts each (3 amps). If you have the dual-lamp model rot. beacon ...and if it's motor is not dragging due to age and/or lack of lubrication to it's gearset, then it draws 4.5-5.0 amps (single lamp with reflector) or 9-10 amps (dual rotating lamps).

Yep...it's a lot. And the motor uses brushes that can add electrical noise to your audio system. (but the good side is that if you forget to turn the master-switch off....the grinding-noises echoing thru the fuselage will remind you.)

Lopez · Post by **Lopez** » Thu May 23, 2013 3:33 am

Well I hooked up an ammeter today and it was drawing 4.7A, with dual lamps. I had just finished completely disassembling, cleaning, lubing, and reassembling the gear train. Turns faster now too...

Post by **GAHorn** » Thu May 23, 2013 2:52 pm

Lopez wrote:Well I hooked up an ammeter today and it was drawing 4.7A, with dual lamps. I had just finished completely disassembling, cleaning, lubing, and reassembling the gear train. Turns faster now too...

If it's a dual-lamp Grimes, then someone has likely installed lesser-wattage lamps than the 7079B's. (It probably does not meet the FARs under which it was installed. Ordinary Nav lamps typically draw 24-28 watts, while beacon lamps draw 40 watts, according to the Grimes charts.)

Part No. A7079B12 General Characteristics
Primary Application Aircraft - Rotating Beacon
Design Volts 14
Design Watts 40
Base Type S. C. Indexed Bayonet
Average Life 150 hrs
Equivalent & Interchangeable Part Numbers:
•Whelen Engineering Part Number: WA7079B12
•Whelen Engineering Part Number: 34-0070371-02
•Trimcraft Aviation Part Number:T7079-12

Part No. T7512-12 General Characteristics
Primary Application Aircraft - Navigation
Design Volts 12
Design Watts 26
Base Type S. C. Bayonet
Average Life 300 hrs
Equivalent & Interchangeable Part Numbers:
•Whelen Engineering Part Number: W1290-12
•Whelen Engineering Part Number: 34-0414020-65
•Trimcraft Aviation Part Number:T7512-12

(As a further matter of possible interest....the expected life ratings of lamps are dependant upon their rated operating voltages. In the two examples above, notice that one is rated at 12v/300hrs while the other is rated at 14v/150hrs. A healthy C170 charging system will produce about 13.5 volts, which will tend to decrease the nav lamp life and increase the rot. bcn expectancy.)

Lopez · Post by **Lopez** » Fri May 24, 2013 6:11 am

Well I checked it with 2 different meters, and both read 4.7-4.9A. When I had it apart, I checked the bulbs and they were both stamped 7079B, 40W. My electrical math doesn't add up, but I measured it twice, I'm not sure what else I can do.

Post by **GAHorn** » Fri May 24, 2013 6:29 am

Volts X Amps = Watts
Therefore 40 watts divided by 14 volts equals 2.86 Amps, times two lamps, equals 5.7 Amps, plus the presumed motor draw of 1.4 Amps, equals 7.1 Amps.

Your meters may not be accurate, or there may be other complicating factors..... different motor, repaired motor, damaged motor field w/reduced capacity (If you've measured the lamps at lower voltage, such as 12 volts, the Amps INCREASE to 3.33 for each lamp....and conversely, if you've measured them at 15 volts...it would decrease.)

There,....I've told you more than I know... so my goal for the evening has been accomplished.

Post by **Bruce Fenstermacher** » Fri May 24, 2013 1:37 pm

Lopez wrote:Well I checked it with 2 different meters, and both read 4.7-4.9A. When I had it apart, I checked the bulbs and they were both stamped 7079B, 40W. My electrical math doesn't add up, but I measured it twice, I'm not sure what else I can do.

I wouldn't do anything. You measured the actual draw and you pretty much confirmed it with a different meter. I'd use 5 amps as the draw of the unit.

The reason the math might not add up is we don't know the actual condition of the bulb. We don't know the voltage of the bulbs to get 40W with the resistance of the filament. For example if the bulb was rated at 18 volts then using Ohms law 40/18=2.2 amps. In other words the bulb can only achieve 40 watts when supplied with the rated voltage and in the case of this installation it does not.

Lopez · Post by **Lopez** » Fri May 24, 2013 1:39 pm

I agree Bruce, and that's my plan. I have a hard time believing that 2 meters are identically screwed up. Particularly when I checked them against several other components of known current draw and found them to be very close to published spec there.

Post by **GAHorn** » Wed May 29, 2013 11:06 am

Actually Bruce, we DO know the design voltage for the bulbs. It's just unlikely he measured them at that exact voltage. (Regardess however, the wattage ...the total current consumed...within reason should have been the same...it's just that electrical load analysis tables do not require wattage listings.)

The meter-measurement likely also does not account for line-losses. Someone here has to know something more about empiricle math than me! I'll have to consult Jamie's Elect. Engineering brother, who is actually into the window-replacement business.

Lopez, I suspect you'll have to list the make/model of your beacon if your airworthiness representative is worth his salt.

(Otherwise, how will you ascertain a replacement beacon will not obviate your analysis? And, if you knew that information, you'd also know it's electrical requirements.... a vicious circle, heh?)

BEEZERBOY · Post by **BEEZERBOY** » Tue Jul 09, 2013 2:03 am

nothing wrong with your meters. the bulb elements heat up when under power.... that increases resistance. the motor develops counter emf when running, that increases it's effective resistance as well. there is a chart in AC34.13 for switch derating factor that might help explain... some loads draw high for a few milliseconds, but the steady state draw is much lower. its called inrush current & when these types of load are encounered the switch has to be of a higher current rating so the contacts will last.

you have an accurate reading on the run current, use that

The International Cessna^® 170 Association