Hi All,
I've built this pwm motor controller using a 555 (NE555P to be exact). The output feeds an IRF540 to control a motor. The circuit works great. The only problem is as soon as I up the speed to max or thereabouts, the 555 starts getting hot very quickly. Almost too hot to touch. Any ideas?

 

There is no component to change the frequency. What are the component values when it gets hot? And what is the supply voltage?

 

There is no component to change the frequency. What are the component values when it gets hot? And what is the supply voltage?

R1 on the far left is a trim pot; is that not the speed adjustment? Also, speed in the original post could be taken to mean 555 speed (frequency) or motor speed (most likely a function of duty cycle?)

I agree that knowing the supply voltage is key.

 

Also, speed in the original post could be taken to mean 555 speed (frequency) or motor speed (most likely a function of duty cycle?)

Yes, I assumed '555 frequency, but I think you're right, TS means motor speed.
The circuit looks fine given a sensible supply voltage so we will wait for that information.

 

Thanks for the replies guys. The supply voltage is 12V from a 1.2A wall power brick. The motor is a 6-15V motor. Mosfet stays cool during operation. The speed of the motor is adjusted by R1 on the left hand side of the schematic.

 

So there's nothing in your schematic to indicate what would cause the 555 to heat.

Double check all connection and component values, especially R3 and R4.

 

So there's nothing in your schematic to indicate what would cause the 555 to heat.

Double check all connection and component values, especially R3 and R4.

Checked and rechecked. All is good. I've even etched a pcb for it and soldered everything. Its weird because the circuit works great except for the heating. Maybe I should try another 555?

 

Give us good pictures of both sides of the PCB.

 

You've checked and rechecked component values - have you checked resistance in the whole path? Any chance there's an etching issue, solder bridge, or some other such thing creating a low resistance path from output to ground? Have you measured resistance from output to ground?

 

Hello

Your title and initial post indicate you're using PWM to control the motor speed but unless I am mistaken you have no way to vary the duty cycle, only the frequency. If you increase the frequency of the 555 to near max I would expect it to get hot. Would you not be better off using a constant frequency and variable duty cycle?

Like some of these.

Designing 555 Astables

 

Your title and initial post indicate you're using PWM to control the motor speed but unless I am mistaken you have no way to vary the duty cycle, only the frequency.

He stated frequency but in the circuit he posted, R1 varies the duty-cycle, not the frequency.

 

He stated frequency but in the circuit he posted, R1 varies the duty-cycle, not the frequency.

Ahh good catch, I missed that wiper connection there So would the frequency just be depended on R2C2 in this case?

 

Fig. 4.4.7 Duty Cycle Control Using Ctrl (pin 5) from Marcus2012's link shows a circuit that looks almost identical, but connects the variable resistor/diode network to output, not discharge. Are these both acceptable variations of the circuit, or is the discharge connection in this thread a mistake?

 

So would the frequency just be depended on R2C2 in this case?

Actually a function of R1, R2 and C2.

 

555 PWM Oscillator

The article explains why the 555 is getting hot

 

Thanks Wendy. Does that extra 10 ohm resistor also apply to the other variation of the circuit where you use pin 7 instead of the output pin 3?

 

I'd like to update this. I made the changes to my pcb so that it looked more like Wendy's (i.e. Disconnected pin 7 from everything and took pin 3 to my Pot wiper through a 10 ohm resistor. I still had the heating problem, just slightly better. So I tried another 555 and now the problem is gone. No heat from the 555 at all. I'll mark this as solved. Must have been a dodgy 555, which will be the first one I've ever come across.

 

If something caused the lower transistor in the 555 totem-pole output to short with a high resistance (perhaps an accidental output short to V+) then it could still work but dissipate more power, and the power would be proportional to the duty-cycle, as you noted.