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Electronics => Beginners => Topic started by: 2DrTahoe on December 26, 2016, 09:34:11 pm

Title: PWM Soft Start w/555?
Post by: 2DrTahoe on December 26, 2016, 09:34:11 pm
Hello everyone,

I am new to electronics and I have been searching for a 20khz pwm circuit with soft start. This will be used for a 12v, 30 amp automotive radiator fan. The soft start is needed for the 80am draw on initial start up. There are 12v relays involved to handle the input from various conditions such as Low, High, AC etc. All that has been worked out.

I have seen both direct dc soft start and 555 astable pwm circuits but never a soft start 555 astable pwm (see attachments- the DC soft-start says 24v but I only need it for 12v so I assume some values will have to be changed). I am going to use N-Channel mosfets in parallel and have also been looking at gate drivers to handle them.

I also have been souring the web for a simple chip that doesn't require programing that can accomplish everything, other than the gate driving. I think I have found some ic's but they become very complicated and seem to be geared for brushless motors.

I need to have two speeds, which to my understanding will require two 555 chips. I have found 555 calculators on the web which when using the correct resistors and capacitors will give me duty cycles around 53% for the slow and 98% for the high speed at 25khz.

I have run a simple 555pwm in Mouser's Milti-Sim Blue software and it seems to work ok except for what looks like an occasional hick-up on the oscilloscope.

I have taken this as far as I can go and really need some professional insight. If anyone can make some suggestions I would appreciate it.

Title: Re: PWM Soft Start w/555?
Post by: Seekonk on December 27, 2016, 04:25:39 am
I was around when the 555 was invented, so I won't use them for anything.  Get over it and use a $2.50 328 NANO board. They are ideal for a solution like this.  I did a fan controller much like this for my wife. Fan runs for two hours till she is asleep and then ramps down for the next half hour till the last useful low speed.  Took 15 minutes to build and program. I guess everyone has to go through their 555 stage.  Don't take too long to join the future.
Title: Re: PWM Soft Start w/555?
Post by: Ian.M on December 27, 2016, 05:50:30 am
+1.  A small MCU is the way to go to generate the PWM. You'll still need the MOSFET driver and a beefy MOSFET.

However if you are determined to use 555 chips, the best approach would be to use two of them - the first to generate a very narrow low going pulse at the desired PWM frequency, and the second as a monostable using the same timing component values to control the PWM duty cycle.  Vary the voltage at the second 555's CV pin to control the duty cycle.  To get soft start, you need to arrange for CV to ramp up smoothly when the duty cycle increases. 

Nobody does it that way commercially any more as you'd need a whole mess of 555s, OPAMPs and passives to do the job of one MCU, and its far far harder to customise its behavour.
Title: Re: PWM Soft Start w/555?
Post by: Seekonk on December 27, 2016, 01:40:12 pm
You could consider using a single switch regulator like a LM494, Basically a 555 that isn't crap and is stable....
Title: Re: PWM Soft Start w/555?
Post by: 2DrTahoe on December 28, 2016, 01:56:08 am
Thanks for the feedback. I'm not stuck on the idea of using a 555. Its just that there is as much info and schematics on the web using that chip as there is porn! I think the idea of using an MCU is a bit too much considering the environment the unit will be in.

I do however like the idea of the TL494. I'll do some research on available schematics, modify them and then run them past the forum.