Amount of current the prop power LED draws?

turbosupra · 2010-12-20 10:41

I'm having trouble with depleted car batteries in the morning, and I'm wondering if the prop red LED and a green LED with the same config as the props red power indication LED?

Could these 2 tiny LED's deplete a car battery to not being able to crank the engine over, over a 12 hour period?

Mike Green · 2010-12-20 10:56

Typically, an LED will draw on the order of 20mA, maybe less. Two will draw 40mA. If they're being driven off a regulator, that will draw a few additional mA. Overnight, that's maybe 12 hours, so you've got maybe 12 x 50mA = 600mAh. A typical starter battery's capacity is maybe 100 times that, so that's not your problem. Leaving LEDs like that on for days on end will be a problem, but not overnight.

You didn't say what else is running. A Propeller can draw more than the LEDs if it's running at high speed. You'll have to look at the various charts in the Propeller Datasheet for exact values for your circumstances. Usually, you can get all the cogs idling for most of the time, tremendously reducing the power consumption of the Propeller itself.

Kevin Cook · 2010-12-20 10:58

Hi turbosupra,

On the high end, the LED will dissipate 120 mW. In a 12 hour time frame even if it runs constant, your power loss is roughly 1.5 watts (that would be max). Hardly enough to drain your battery to a point of non-operation.

-Kevin

turbosupra · 2010-12-20 11:36

Thanks all, I thought that same thing but wanted a second opinion or two.

My prop is running at 80mhz, is there something I can buy and put inline or around a wire (inductance) that I can see how much current is going through each wire?

I guess I could code it to have most cogs sleep if the key was off? Or should I have it run at a slower speed if the key is off?

Mike Green · 2010-12-20 11:49

Usually, you put a small resistor in the power lead and measure the voltage across it to determine the current draw. Something in the range of 10 to 33 Ohms should work for currents on the order of 50 to 100mA. That'll give you a voltage drop from 0.5V to 3.3V. Use a 1/2W resistor. You should have at least 4V of "head room" for your voltage regulator since you're running off the car battery, so the extra voltage drop shouldn't be an issue.

Putting the Prop into low power mode when the key is off is a good idea. Switching to the slow internal clock is easy and effective as long as you don't need the speed or clock accuracy. Having the cogs waiting for either a signal or a time in the future most of the time will also markedly reduce the power draw.

turbosupra · 2010-12-20 19:18

Thanks Mike, I'll try and see how this works ... and start searching for code examples about reducing the clk

I think an LED was keeping a transistor on and that was my issue, I'll know tomorrow. If that's the issue, I'll have to figure out how it is causing such a drain.

Mike Green wrote: »

Usually, you put a small resistor in the power lead and measure the voltage across it to determine the current draw. Something in the range of 10 to 33 Ohms should work for currents on the order of 50 to 100mA. That'll give you a voltage drop from 0.5V to 3.3V. Use a 1/2W resistor. You should have at least 4V of "head room" for your voltage regulator since you're running off the car battery, so the extra voltage drop shouldn't be an issue.

Putting the Prop into low power mode when the key is off is a good idea. Switching to the slow internal clock is easy and effective as long as you don't need the speed or clock accuracy. Having the cogs waiting for either a signal or a time in the future most of the time will also markedly reduce the power draw.

LoopyByteloose · 2010-12-20 23:12

Nobody seems to be mentioning that you might just have a bad automotive battery or a problem with charging. The average car battery will run a Propeller board for many, many days - if not months - from a properly charged battery.

Additionally, batteries tend to substantially reduce their output in severe cold. In the Arctic, battery powered devices often are buried 6 feet below the surface just to moderate this effect.

You battery may simply be near the end of its useful life. The 'average' installed automotive 12 V battery is somewhere between 150AH and 300AH, depending on the climate you live in.

turbosupra · 2010-12-21 07:38

Hi, that is a good point.

I too should have mentioned that I had already ruled that out. I have a battery load tester that I put on the battery and it passed, as well as I also unplugged by prop rig and did not have any issues when it was all unplugged.

I believe I found the issue, I'm just not sure why. My green LED is turning on a FET transistor, just not sure why it is doing that so I will figure that out and get back to this thread.

Thanks for everyones input!

Loopy Byteloose wrote: »

Nobody seems to be mentioning that you might just have a bad automotive battery or a problem with charging. The average car battery will run a Propeller board for many, many days - if not months - from a properly charged battery.

Additionally, batteries tend to substantially reduce their output in severe cold. In the Arctic, battery powered devices often are buried 6 feet below the surface just to moderate this effect.

You battery may simply be near the end of its useful life. The 'average' installed automotive 12 V battery is somewhere between 150AH and 300AH, depending on the climate you live in.

turbosupra · 2010-12-27 10:55

Update

It was definitely the FET/LED combo, using an NPN transistor I have remedied this. Thanks to everyone who replied.

Cluso99 · 2010-12-27 11:52

turbosupra: FWIW I always use a superbright LED and 3K3 to 10K. Most superbright leds will give sufficient brightness when drawing =<1mA these days.
Easiest way to measure the current is place a multimeter in the power path. I presume you know this so there must be a reason you cannot do this.

Amount of current the prop power LED draws?

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