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Author Topic: need help with triac  (Read 3708 times)

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Offline Soeren

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Re: need help with triac
« Reply #30 on: April 11, 2010, 03:04:32 PM »
Hi,

Can you tell me more about R2? i got the rest but still R2 is unclear
Pin 7 (Discharge) on the 555 is an open collector NPN transistor (emitter to ground) made on the die.
You have quite a charge to get rid of, so bleeding it via a resistor takes the strain off the transistor, which will otherwise be shorting the (5*2/3)=3.3V charged 470µF. It wouldn't kill it off right away though and may not be that large a risk, but when you design for long term reliability, a resistor here and there is cheap insurance.

1k is just an arbitrary value, whatever reduces peak current flow, while still discharging the capacitor before the circuit should be reactivated is fine.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline aruna1Topic starter

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Re: need help with triac
« Reply #31 on: April 12, 2010, 04:08:10 AM »
so is that mean if i charge a cap i can use it to get more current?

say i charge 2200uf cap to 3V using solar cell and can i use that charged cap to run a motor?
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Offline Cristi_Neagu

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Re: need help with triac
« Reply #32 on: April 12, 2010, 09:53:38 AM »
Yes you could. But not for long.

Check this out: http://en.wikipedia.org/wiki/Farad#Definition

If you charge a 2200uF capacitor to 3V, and use it to run a DC motor that draws 100mA, the motor will draw all the current out of the capacitor in about 66ms (it's a bit more complex than that, because current draw is dependant of voltage, and voltage drops with time.... but it wouldn't make much of a difference).

You could go for one of those 5kF (yes... 5000 farads) at about a max of 3V ( http://en.wikipedia.org/wiki/Supercapacitor ). That would move a 100mA motor for 150.000 seconds. But you'd be better off with an ordinary battery. I'm quite sure those things cost a fortune.

Offline Soeren

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Re: need help with triac
« Reply #33 on: April 12, 2010, 11:05:26 AM »
Hi,

But you'd be better off with an ordinary battery. I'm quite sure those things cost a fortune.
Yes, they cost your soul (almost), but that's not the only reason to NOT use them for motor power.
The 5kF is only 2.7V max. and it's 16.5cm x 7.2cm x 6.0cm (712 cm^3) and weighs almost 1kg (930g to be exact).

Doing the numbers for a 100mA motor and accepting a useable range from 2.7V down to 1.5V:
For an average drain of 100mA, it is equivalent to 21 Ohm.
Discharge time is 17 hours, 8.5 minutes (that's the equivalent of ~1.7Ah).
During that time, around 6kC is removed at a start current of 128.6mA and an end current of 71.4mA

To compare, let's look at 2 NiMH cells of 1.7Ah
Each are 5cm long and 1.4cm in diameter (total 15.4cm^3, i.e. only 2.2% the size of the 5kF cap).
They'll drive the motors faster compared to the average speed on the 5kF, at ~2.4V steady through the entire period (i.e. motor speed reliable).

Or, take the same volume in NiMH and get close to 800 hours and you can get them in 2.7Ah capacity in the same size  ;D

Supercaps do have their purposes in low drain circuits, but they're not a cure all replacement for batteries and we fully agree that they should never be used for powering motors.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline aruna1Topic starter

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Re: need help with triac
« Reply #34 on: April 12, 2010, 11:14:24 AM »
he he
I'm just trying to work on a solar bot,not a electric car
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