RE: Tachs and lights

Oct 05, 2003 12 Replies
D

I know that my Hobbico tach will read 3600 when pointed at a flourescent light. However, it will also read 3600 when pointed at an incandescent light!!!!



I was running an electric system indoors and wanted to read the speed while I was monitoring the watts with my Whattmeter. All I got was erratic readings. I had turned off the flourescent light after turning on the incandescent light, figuring the f-lite would mess with the readings. What a surprise to find that it would read the i-lite @



3600 as well.

So, beware if not using natural light to take tach readings!!!!!!!!



David



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F
60 cycles per second (HZ) light frequency, times 60 seconds in a minute = 3600 which is what the tach should read when pointing toward electric light. Remember it is revolutions per minute that the tach is calibrated for.
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D

I reckon you could use a torch just to take your readings - but...

David

PS: Of course if you really need to do that stuff on a regular basis, then get yourself one of those 12v ceiling or reading lamps, and run it off your charger power supply, or a car battery - with DC you will not get that flicker that causes the errors.

David AMA40795 / KC5UH wrote:

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A

Yes, but.... At 3600 rpm a two-bladed prop interrupts the light path 7200 times per minute =

120 Hz. Works out though, because the lamp turns on twice during each 60 Hz cycle. The count/sec is divided by 2 for display (or divided by 3 for a three blade prop).

Abel

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D

You guys are missing the point......... NORMALLY, it is flourescent lamps that flicker enough for the tach to read 3600. This is a normal incandescent bulb - they normally do not flicker like this.

David

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F

Yes regular bulbs with filaments flicker but your eye is not fast enough to see it.

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R

I wonder if they do. You are heating an element with AC, 120 pulses a second. Do you think the thermal mass is low enough to allow the filament to cool in such a short period of time?

Red S.

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S

Yes they do,...enough for a tach to pick up clearly. Also enough for a very few human eyes to pick up(react to). My eyes used to be sensitive enough to see this. It can cause quite a headache. Many more eyes at 50hz, that's why we use 60hz. I've lived with both and much prefer the latter. ;-)

Cheers,

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B

Forget theory, just try it. It works :)

Brian Hampt> I wonder if they do. You are heating an element with AC, 120 pulses a

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B

Incandescent lamps do indeed flicker. On 25 cycle power (still in use in some industrial settings) it's very noticible, and is still present at

60, though of much lower level. Flashlight (torch) bulbs which have a very low thermal mass filament can be modulated with audio to make a 'beam-o-light' communicator. The audio rolls of at higher frequencies obvioulsy, but still very effective for speech.

Bill (oc)

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P

Yes they do, you just don't notice it. Living in the US, 60hz just gets to be "normal" and your eyes Learn that this is steady. I took a trip to the UK, where they use 50hz (or used to) and I could "see" the flicker in the bulb. over time (a day or two), my eyes adjusted adn it wasn't so weird any more. an AC signal goes from negative to positive in cyclic form, the light does flicker, just not much. The tachs are sesnitive enough to ppick it up. Mark

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M

I recon so as well

"David" ???????/???????? ? ???????? ?????????: news: snipped-for-privacy@ozemail.com.au...

get yourself one of

supply, or a car battery -

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