555 PWM wont work with split voltage

Thread Starter

Hamlet

Joined Jun 10, 2015
376
555 pwm for split voltage.pngSorry for the poor schematic...
What I am trying to do is use a 555 pwm circuit to drive a mosfet to operate a motor or bulb at higher voltage.
I added a L7812 to isolate my 555, so I don't burn it up.
The problem is that no matter what I try, when I try to supply my motor with more than 16.5v, the PWM function
stops working, and I get full on. I want to operate a motor at 20v, pwm. All my grounds are common, but I've tried
to isolate them, use diodes, separate power supplies, omit the pull-up resistor, omit the driver, whatever I can think of...

This is day 3, so I'm at my wits end. Any help would be appreciated.
 

MrChips

Joined Oct 2, 2009
22,468
NE555P Vcc max is 18V.

It is generally easier to use an N-type MOSFET on the ground side and the motor on the high side (high side load).
Then you can drive the gate of the MOSFET directly via a series resistor.
 

Thread Starter

Hamlet

Joined Jun 10, 2015
376
Yes, but I need an P-Mosfet, as this is a module for another applicaton.
I understand NE555 is only good for 18v, (Someone said 16v) but I need
20v, maybe 24v... That's why I am attempting to isolate it from the mosfet
side... I thought it should be easy, since mosfet gates are high impedance.

At any rate, it all goes phlewy when the voltage goes over 16.5v, and I measured
the 555 side when this happens, and it's still around 12v... (L7812).
 

wayneh

Joined Sep 9, 2010
16,481
Your GATE voltage must be near the motor voltage, the DRAIN pin. In your drawing it can never be above 12V. So cranking up the Drain beyond a certain threshold, apparently about 1.5V above the Gate, turns on the MOSFET.
 

MrChips

Joined Oct 2, 2009
22,468
If you must use the P-type MOSFET then the driver and the 5kΩ resistor must be on the source side of the MOSFET, i.e. +20V and not the +12V.
 

MrChips

Joined Oct 2, 2009
22,468
You are not driving the 20V MOSFET from the 555-timer 12V output directly.

That is why you need the MOSFET driver to deliver the necessary voltages that the gate requires.
 

Thread Starter

Hamlet

Joined Jun 10, 2015
376
You are not driving the 20V MOSFET from the 555-timer 12V output directly.

That is why you need the MOSFET driver to deliver the necessary voltages that the gate requires.


See my updated schema... I just tried this, and I blew the smoke out of the BD139...

555 pwm for split voltage2.png
 

Thread Starter

Hamlet

Joined Jun 10, 2015
376
Okay, I tried something new. Upgraded my driver BJT's, and plugged in a 100ohm resistor, & omitted the pullup.
Works much better, however, at full output at 19.5v, the 100ohm 1w resistor gets ouchy hot, eventually. The TIP42 also gets just a little warmer. The mosfet & TIP41, are stone cold. I tried a 200ohm resistor before the driver, but I couldn't get as nice a range, uh, something from middle to high, but no lower control.

1609743080772.png
 

Kjeldgaard

Joined Apr 7, 2016
435
How about regulating the supply to the NE555 on the negative side?

An L7912 with Input from Motor GND, L7912 Output to NE555 pin 1 and various decoupling capacitors and L7912 GND for Plus Motor Supply.
NE555 Pin 3 either directly to the Power MOS Gate or a few ohms in series.

It also provides regulation of the Power MOS Gate-Source voltage.

There is also a lack of a Fly Back Diode across the Motor.
 

Thread Starter

Hamlet

Joined Jun 10, 2015
376
How about regulating the supply to the NE555 on the negative side?

An L7912 with Input from Motor GND, L7912 Output to NE555 pin 1 and various decoupling capacitors and L7912 GND for Plus Motor Supply.
NE555 Pin 3 either directly to the Power MOS Gate or a few ohms in series.

It also provides regulation of the Power MOS Gate-Source voltage.

There is also a lack of a Fly Back Diode across the Motor.
That's quite a mouthful. I'm game, especially for the regulation of gate voltage... which I am concerned with.
My mind hasn't had much experience with neg. regulators,,, I'm even new to the idea of pull-up-down resistors.
 

crutschow

Joined Mar 14, 2008
25,980
I'm game, especially for the regulation of gate voltage... which I am concerned with.
You can reduce the gate voltage in my circuit by adding a resistor in series between the MOSFET gate and the BJT collector.
For example, adding a resistor equal in value to R2 will reduce the ON peak gate-source voltage to 1/2 of the MOSFET source voltage.
 

Thread Starter

Hamlet

Joined Jun 10, 2015
376
I soldered up a test, works great, thank you.
I'll drop in my redrawn schema/notes for future reference.
 
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