# Help designing a PWM w/ 19.5 Vin and 12 Vout

Sep 24, 2015
4
I'm new here. First post. Please advise if this thread is in the wrong place or if there is any problem w.r.t. forum guidelines and rules.

I'm building a variable speed motor controller.
I have a brushless DC motor (12V @ 180 mA). It's a 110 mm computer fan, re-purposed.
I have a DC 19.5V power source.
Desired operational speed is ~30 rpm and up to ~500 rpm. The high end is not so important as the low end.
Additional design parameter to include a switch and an LED power indicator.

I have found many circuit diagrams for designing a PWM using a 555 timer. However, they all stipulate 5V for Vcc.

I imagine that I can use an NPN transistor on the output of the PWM to control the 12V signal to the motor. However, I don't know how to split the voltage to give 5 V to the timer and 12 V to the transistor, allowing for the operation of the LED to be steady on whenever the switch is on.

Thank you in advance for taking the time to help.

Sep 24, 2015
4

#### davenn

Moderator
Sep 5, 2009
14,179

Sep 24, 2015
4
OK, I've drawn up a sketch. I can wire the resistor and LED in parallel and then put the LM7812 in series with the PWM loop.

Will I need to worry about heat in the LM7812? It will be constantly ... shunting(?)... the excess voltage to ground, right?

Also, I've found another circuit diagram with a quite similar circuit, but with subtle differences.
http://www.555-timer-circuits.com/pwm-controller.html

I have enough curiosity and ambition to want to know the practical differences.

The following questions are about the circuit in the link in this post:

I think I understand the use of both halves of the pot to adjust the pulse width while maintaining a constant period. What is the purpose of the additional 1k resistor in series with the pot?

I think I understand that the diode in parallel with the motor is to protect the MOSFET. Is the 47 Ohm resistor between the 555 and the MOSFET also to protect it?

How is the capacitance that connects the V_in to GND determined?

What is the function of the additional 100 nF cap between pin 5 (control voltage) and GND?

Finally, and this is probably the dumb question, why do some of the capacitors use a rectangle and a bar in the circuit diagrams, while others use 2 bars?

#### Colin Mitchell

Aug 31, 2014
1,416
http://www.555-timer-circuits.com/pwm-controller.html

That's my circuit you have suggested. It is designed for very high current. You just need 1 amp buffer.
The chip will get very hot if you drive it directly at 180mA.
The cap with a rectangle is an electrolytic - generally a capacitor over 1u.

#### davenn

Moderator
Sep 5, 2009
14,179
Will I need to worry about heat in the LM7812? It will be constantly ... shunting(?)... the excess voltage to ground, right?

it wont be drawing much current to drive the 555 prob. less than 50mA in this case and another 180 mA for the motor. The 555 can source or sink up to 200mA.
There would be no harm bolting the 7812 to a small heatsink

I think I understand the use of both halves of the pot to adjust the pulse width while maintaining a constant period. What is the purpose of the additional 1k resistor in series with the pot?

look closely, its not just purely in series with the diodes and pot ... it initially goes to pin 7 of the 555

This is an Astable Operation use of the 555
If the circuit is connected as shown (pins 2 and 6 connected) it will trigger itself and free run as a
multivibrator. The external capacitor charges through the top resistor + the variable and discharges through variable resistor. Thus the duty cycle may be precisely set by the ratio of these two resistors.

In this mode of operation, the 100nF capacitor between pins 6 and 2 to gnd charges and discharges between 1/3 VCC and 2/3 VCC. As in the triggered mode, the charge and discharge times, and therefore the frequency are independent of the supply voltage.

I think I understand that the diode in parallel with the motor is to protect the MOSFET. Is the 47 Ohm resistor between the 555 and the MOSFET also to protect it?

No, it limits the current output from the 555 to protect the 555 against excessive current draw

How is the capacitance that connects the V_in to GND determined?

its just for smoothing ... there prob is a formula, but the value isn't critical and would be partly be determined by rule of thumb of 1000uF per amp that the load is drawing

What is the function of the additional 100 nF cap between pin 5 (control voltage) and GND?

its mainly a noise bypass capacitor ... sometimes it isn't used, rather a voltage is applied to that pin

Finally, and this is probably the dumb question, why do some of the capacitors use a rectangle and a bar in the circuit diagrams, while others use 2 bars?

rectangle and a bar is a polarised capacitor it may be an electrolytic cap or a tantalum cap, also commonly shown as a solid rectangle, - leg and an open rectangle as the + leg
2 thin bars is a non polarised cap eg a mylar, ceramic, polyester etc

Dave

Last edited:

Sep 24, 2015
4
Thank you so much for your help. This is my first project that I'm doing solo. I'm amused at how much I felt like I was learning in my undergrad, and yet how confused I am now that nothing is handed to me.
(My degree is in physics, not electrical engineering.)

Back on topic:

I understand that the 2nd circuit diagram I posted (Colin Mitchell's design) is more robust than my project requires.
Do I have this correct?
If yes: Is there a compelling argument to use Colin's design, anyway? (The difference in cost and complexity is basically 0, but I strongly doubt I will ever re-purpose this circuit.)

I was going to ask about using the MOSFET vs. an NPN, which I had initially suggested, but I found a thread on this site which has me thinking I should simply use whatever is in the circuit diagram I am copying, and to not worry about it unless there's a performance issue.
Do you agree with my assessment?

Finally: Are there any questions which I didn't ask, and which you feel are worth addressing?
I.e. Do you have any other advice for me before I go and purchase the components for this project?

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