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shorting mechine control using AT89C51

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Moderators: Ajay, Junied , abbas1707, Arun Kumar V, pdi33, Shailesh NAYAK, ۞ TPS ۞, shyam, sashijoseph

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caze

Fri Jun 27 2008, 08:37AM

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hi,
one more query..
do i need to connect a capacitor between groung and vcc of microcontroller.and between ground and power supply of PCB.

if it is so , what should be the values of the cap.

caze

pdi33

Fri Jun 27 2008, 11:42AM

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caze,
1.43A is too high a current for a small transistor like BC337. For that high a current u may have to use a power transistor like 2n3055 or mosfets like like he IRFXX series. just verify that the transistor /mosfet u use has a greater switching frequency than ur output PWM frequency.

As far as the capacitor is concerned, a 0.1uF cap across the uC is always recommended provided it is as close to the uC as possible. This is usually to eliminate the digital noise generated by the high frequency outputs of the uC. But in ur case that woud be hardly a problem as u are not using any analog ckt which may be affected so connecting the cap is only an option and not neccesarily required. But if the grounds of the +24V supply to the valve and the supply of the uC are same (i.e. if both are derived from the same power source, then the supply of the uC must contain a fast acting capacitor which can reduce the voltage spike the back emf of the valve coil generates when it is turned On/OFF as the load is highly inductive and the voltage surge can disrupt the flw of the uC (especially if it is of Atmel make as the AT89XX series does not have a efficient inbuilt EMI suppression like the phillips series of uCs ).

[ Edited Fri Jun 27 2008, 12:00PM ]

* inspired to develop,developing to inspire *

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caze

Arun Kumar V

Fri Jun 27 2008, 06:17PM

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hello caze,

No, you can't use a 800mA transi for a 1.5 amp job, it'll fry ! try a mosfet or a power transi like BDX series

Arun

caze

Sat Jun 28 2008, 08:09AM

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hi,

after a long search , i ve got IRFD014 power transistor , which has drain current of ID= 1.7 AMP.
can i use it .

caze

Arun Kumar V

Sat Jun 28 2008, 07:45PM

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Hi Caze,

as a general rule of thumb, you have to use a device which can handle 3-4 times your actual load. your load is 1.4 amp and your switching device is 1.7 amp, and since your load is a electromag device there is bound to be spikes and EMI stuff, so there should also be a protective diode across just like you have across a relay.

try using IRF540 or IRFZ44 ( price Rs.15-20)

Arun

caze

Sun Jun 29 2008, 04:51AM

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hi , arun,

but the current rating of these devices are too high.its 28 amp for irf540 and 50 amp for irfz44. will it not burn the valve...

and if u have little much time , i would like u to explain me the logic behind the "relationship between the seitching devices current and load current . And their working too."

i hope these are no silly questions ..

caze

caze

Sun Jun 29 2008, 01:03PM

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HI @ Arun,

the very first post which u have posted me regarding optocoupler , In that the load is connected to the emiter. what if we connect it to the collector. is there any difference ?

AND is the value of R2 depends transistor ,connected to coupler, if it is so , how to calculate it.

caze

Arun Kumar V

Sun Jun 29 2008, 02:33PM

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Hi Caze,

you can connect it that way, and you need R only in case of mosfet and not transistor

it can be of 100k-1meg value. for R2 you can use 10k.

but the current rating of these devices are too high.its 28 amp for irf540 and 50 amp for irfz44. will it not burn the valve...

thats really silly, there is no harm in using a higher rated device, since you are using PWM there will be rapid Switching On and Off the Load which results in EMI and high voltage spikes across the load.

by using a higher rating you have two advantages:

1) the mosfet or transi remains cool, no heat sink is required
2) Mosfet/transis operates at a safe limit

may be you haven't noticed the Thanks button, use it if the replies helped you

Arun

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caze

sashijoseph

Sun Jun 29 2008, 11:31PM

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Caze,it's typical of inductive loads to draw a pull-in current of ~10 to 20 times the hold current.So the mosfet suggested by Arun is just apt.With inductive loads it's always wise to have some headroom.

Let there be music........