Maker Pro
Maker Pro

Help with a PWM triggered switch.

Sidd1994

Aug 16, 2017
1
Joined
Aug 16, 2017
Messages
1
Hi,
I've been designing a pwm triggered switch with the following flow
Rc filter - >op-amp comparator->power mosfet as switch

I have posted the circuit below I wanted your recommendation before actually building the circuit. :) thanks.
PS would the circuit work (assuming my PWM design considerations are correct)
 

Attachments

  • IMG-20170804-WA0028.jpg
    IMG-20170804-WA0028.jpg
    27.7 KB · Views: 55

Harald Kapp

Moderator
Moderator
Nov 17, 2011
13,700
Joined
Nov 17, 2011
Messages
13,700
In principle the circuit looks fine. A few considerations:
  1. The LM358 output voltage is limited to V+ - 1.5 V. At 5 V supply this amounts to 3.5 V output voltage. The threshold voltage of the IRF540 is between 2 V ... 4 V.
    Therefore the output of the LM358 will not reliably turn on the MOSFET. You need either a comparator with rail-to-rail output or a higher supply voltage, e.g. 9 V, for the comparator.
  2. It is possible but not good practice to use an operational amplifier as a comparator. A TLV7211 for example is specifically designed as comparator and has a push-pull output. This one goes to Vcc -0.3 V at the output and therefore also remedies the above mentioned issue.
  3. When using only one component of a multi component packes (LM358 is a dual opamp ic) make sure the unused component's inputs are connected properly to defined potential to avoid unexpected behavior (e.g. oscillations due to open inputs, high power drain etc.)
 
Top