555 monostable, power transistor become warm

[abuhafss]

555 monostable, power transistor becoming warm

Hi

I am using the attached circuit to switch on a solenoid for 14sec. For Q1, I am using 2SD1897. The circuit is working fine, but the transistor is getting fairly warm (about 70-75 °C). The total current drawn by the circuit is 2.7A. Is it normal for the (5A) transistor to become warm in 14sec with about half of the rated current flowing thru it?

 

[BigBoss]

You should check the power dissipation instead of max. permissible current.That power dissipation heats the transistor, not current itself only..
During 14sec. 13.5x2.7A-3*2.7=29W.Is this transistor able to hold this power ??
Regarding to datasheet, max. Collector Power Dissipation is 30W@25C
So, it's working at its limit.

 

[Audioguru]

Like all other power transistors, this one needs a base current of 1/10th the collector current for it to saturate fairly well so it does not have a high saturation voltage loss multiplied by the current which makes lots of heat.
If your 13.5V does not sag when loaded then the 3 ohm load resistor has 2.7A x 3 ohms= 8.1V across it so the transistor has (13.5V - 8.1V)= 5.4V across it is and is not saturating and is heating with 5.4V x 2.7A= 14.6W which makes it very hot.

The output high voltage of a 555 with your 13.5V supply is about 12V and the base voltage of the transistor is about 0.9V so the base current is only 34mA when it should have 270mA for it to saturate with less than 1V loss.
But the maximum allowed output current from a 555 is only 200mA so this circuit will not work unless you cheat with the value of R2 a little (try 82 ohms).

 

[asdf44]

To add, in general you want a transistor that's rated for significantly more current than your load.

Without even opening the datasheets a 5A transistor driving a 2.7A load sounds like a problem. The front page current rating is typically specified at the ideal thermal scenario (in the case of a heatsinkable part it assumes a perfect heat-sink). In ambient air over typical operating conditions a transistor rated for 5A might only be useful for <1A.

Though I think Audioguru nailed your primary problem. That may help a lot given the limited 14S duration your looking for (note that a FET would have avoided this problem).

 

[crutschow]

Take asdf's suggestion and use an N-MOSFET rated for about 8-10A for Q1 and it will run perfectly cool.

 

[abuhafss]

I have tried IRF540, IRF630 and IRFZ44N. The first two are getting very hot and the last one is cool, but the 555 goes bad. Pin 7 and 8 gets short. Maybe I have selected wrong value for gate resistor (100 Ohms) . For IRFZ44N, I connected the gate directly to pin 5 of 555.

Shall be grateful, if anybody can suggest me the most suitable mosfet and the corresponding gate resistor.

 

[chemelec]

Any of those Mosfets Should work GOOD.
A 100 Ohm gate resistor is Good.

You Must have Something else Wrong?

Pin 7?
It should NOT be connected to anything.

 

[abuhafss]

Pin 7 is not connected to anything. I said that when 555 gets bad, pin 7 and 8 are found short internally.

 

[chemelec]

Not sure Why your 555 should go bad.
I Don't see any thing wrong with the circuit.
And None of those Mosfets should get Hot if your Relay Current is Just 2.7 Amps.

The Only thing I Might Recommend is:
Connect the Power and Ground of the 555 Direct to the Battery with one set of wires.

And Connect the Power of the Relay and the Source of the Mosfet, Also Direct to the Battery, But with SEPERATE Wires.

- - - Updated - - -

What is your POWER Source?
A Battery or a Power Supply?

 

[abuhafss]

Power source is 5A SLA battery.

It's working perfectly with IRFZ44N, FB59N10 and STP75N75. IRFZ24N and IRF630 are getting warm.

Anyway, thanks for your help.

 

[Audioguru]

Maybe you are buying fake Mosfets from ebay?
A real IRF540 from International Rectifier has a maximum drain to source resistance of only 0.044 ohms when its gate-source voltage is only 10V and the on-resistance is even lower with your 13.5V supply.
Then simple arithmetic shows that 2.7A squared x 0.044 ohms = only 0.32W which is barely warm. With the 12V from the 555 it will produce even less heat.

 

[abuhafss]

Maybe you are buying fake Mosfets from ebay?
A real IRF540 from International Rectifier has a maximum drain to source resistance of only 0.044 ohms when its gate-source voltage is only 10V and the on-resistance is even lower with your 13.5V supply.
Then simple arithmetic shows that 2.7A squared x 0.044 ohms = only 0.32W which is barely warm. With the 12V from the 555 it will produce even less heat.

I had bought that mosfet from a local store. It was lying in my mosfet collection box, maybe it is dude.

 

[crutschow]

I suggest double-checking all the wiring including verifying the pinout of the MOSFETs.

 

[abuhafss]

I suggest double-checking all the wiring including verifying the pinout of the MOSFETs.

The wiring is all okay, that's why it's working perfectly with IRFZ44N, FB59N10 and STP75N75.
But when I replace with IRF540, IRF630 or IRFZ24N, they get warm. All those mosfets are TO-220 package so the pin-out is the same for all.