5amp switching

[Napst3r]

hi everybody

im looking for help with regards to the switching of a 2n3055 transistor,
the scenario we sitting with is that we have a PWM (pulse width modulator) that varies the pulse width at the output (pin 3) of the 555 timer, now we need to use that modulated signal to switch a light bulb which has a 60W rating at 12Volts, this means we need 5A into the bulb...

can anybody help with the circuit diagram from the 555timer to the bulb

much appreciated

 

[Reloadron]

While a 2N3055 transistor can be used I would think about using a logic level MOSFET to take care of switching the load on and off. You don't mention the pulse amplitude out of your 555 circuit? Anyway, I would likely go the MOSFET route.

Ron

 

[Napst3r]

okay i have attatched the circuit diagram of the PWM that i want to use...

and if i am not mistaken the logic output level at pin3 should be 5volts

 

[Reloadron]

If you look at this 555 PWM circuit you will notice it is pretty much as your circuit. Pretty common 555 type PWM design. Note the MOSFET used to drive the load. Note the output is taken off pin 7 but that is just a design variation. This link provides examples of logic level mosfets. The 2987 would suite your needs or just use the circuit linked to. The IRFZ46N would handle your load. Your circuit runs on 12 volts so I would just use the IRFZ46N. For what you want to do it isn't all that critical.

Ron

 

[crutschow]

Just to clarify the reasons for not using a 2N3055 bipolar transistor for this application:

One is that it is rather slow to turn on and off, so the efficiency in a PWM circuit may suffer.

Two, it requires a lot of base current to fully turn on (at least 1/10th of the maximum collector current or in this case 500mA) which would require an additional buffer driver and further reduces efficiency.

 

[MikeMl]

Use an NFET or a NPN Darlington.

 

[Mr RB]

A NPN darlington at DC 5A will dissipate 1v (5W) which is a bit nasty. Definitely go for the FET.

Also your 555 schematic needs a resistor between pin 3 and pot wiper, to reduce the peak current in/out of pin3 at extreme duty cycles. A 470 ohm resistor will work fine.

 

[Napst3r]

okay so i think the MOSFET will be the best option, i will go out today and buy one and see if it works..

thanks everybody for the help and suggestions, if anybody comes up with bright ideas just let us know

thanks

 

[audioguru]

You cannot select a Mosfet since you don't know how much gate voltage it will get from the 555 since your schematic has no supply voltage shown.

An incandescent light bulb is almost a dead short when it is cold. Make sure your Mosfet can supply enough current to heat it when its resistance increases to its operating power.

 

[Reloadron]

You cannot select a Mosfet since you don't know how much gate voltage it will get from the 555 since your schematic has no supply voltage shown.

An incandescent light bulb is almost a dead short when it is cold. Make sure your Mosfet can supply enough current to heat it when its resistance increases to its operating power.

I saw it as BT1 = 12 volts? The text is a little obscure with a diode in there. That was VCC or did I miss something here?

Ron

 

[Napst3r]

yes that is correct, the Vcc is 12V

 

[audioguru]

A 60W old fashioned incandescent light bulb draws more than 600W when it is turned on until its filament reaches 2000 degrees C when it draws 60W. Guess why an incandescent light bulb always burns out when it is turned on.

 

[Reloadron]

A 60W old fashioned incandescent light bulb draws more than 600W when it is turned on until its filament reaches 2000 degrees C when it draws 60W. Guess why an incandescent light bulb always burns out when it is turned on.

Uh....Uh....Uh....

Incandescent light bulbs have a filament made of tungsten. The filament is heated to a white-hot temperature by the electrical current passing through it. The filament is so hot that the tungsten slowly evaporates off the filament and plates the inside of the glass envelope (bulb). The evaporation of metal causes the filament to get thinner over time. Repeated heating and cooling from turning the lamp on and off causes the filament to become brittle. A large surge current flows through a cold filament when the bulb is first turned on (the resistance of tungsten increases with temperature reducing the current as the bulb heats.) The surge passing through a brittle, thin filament can create a hot spot that completely melts the filament causing the light to burn out. This is why light bulbs tend to burn out when being turned on.

Do I get a cookie?

I ripped it off from Wiki.

A 100 watt 120 volt bulb draws something like 12 to 13 amps of inrush current but within about 13 mSec. it is hot and down to about 0.8 amp. Not to say 13 mSec is not enough time to destroy a component but for the planned application I don't see the inrush current as a major player.

Just My Take
Ron

 

[crutschow]

The transistor may come out of saturation and limit the current during the bulb turn on. But as Ron noted, this is for a short period of time and likely will not harm the transistor.

 

[audioguru]

There is an incandescent light bulb in Alaska that has been lighted for 100(?) years or more. Because it has been turned on only one time then it doesn't burn out. If you flash an incandescent light bulb on and off many times then it burns out soon. But if you apply a small current to pre-heat it when it is turned off then it lasts a long time.

 

[Mr RB]

If you add a cap on the base of the transistor (or FET gate) and the drive voltage has some resistance you will get a soft turn on and turn off feature.