PWM with 555 am I ingenious or barmy ?

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Thunderchild

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ok just check this schematic out for me I simulated it with elctronics workbench 5.12 and it works exept that for max and min pot after a second the simulation jams but I would say that that is a software limitation on the program. it seems that I can get a 0 % straight thru to 100 % duty cycle. the idea is to speed control a motor.
is there another way of doing it with out the transistor or have I hit the nail on the head for max duty cycle range ?
 

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Barmy

ok I just played around with diodes in the circuit and got the exact same result
 

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I looked at your circuit and it seems to me that you're achieving duty cycle adjustments by having a variable ON time and a fixed OFF time. This results is very low frequency when the duty is close to 100% and the PWM frequency goes up to maximum as the duty cycle drops to 0%.

There are applications when this is undesirable. The frequency can go up to where switching losses could be intolearable. Or the frequency will drop to audible levels and there will be an annoying hum from the motor.
 
Hmm, i've seen something similar here:

**broken link removed**

It does work quite well, but the frequency changes by around 15% across its range.
 
there is no drastic frequency change. the total off+on time is constant becase one part of the pot is used to charge and the other to discharge. the transistor is keeping the supply cut off during discharge as if you look closely the pins that sense the voltages on the cap while it is discharging will be subject to a voltage devider that upsets the circuit and recs havoc. the result is the same as the secon circuit where diodes are used to seperate charge and discharge sections.
so the diode solution is probably best. just one question though: a diode can only work up to like 200 Hz or is this not a problem because it is all DC so no reversing and rereversing problem ?

EDIT:

ok I took a look at that other diagram you linked to. hm well yea i get how it works but why the hell reverse the dis and out pins. the result is exactly the same. especially as the output is push pull anyhow so if you need a clamp down to GND thats ok as well.
 
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What makes you thing diodes are limited to only 200Hz?, diodes are available that equal (or exceed) transistors.

Normal slow recovery rectifiers are only suitable for low frequencies, but that's what they are designed for - a diode (such as a 1N4148) will massively out perform a 555.

BTW, the two diode trick for adjustable mark/space is a VERY old technique, and is probably shown on some of the datasheets?
 
yea i suppose it is i remembered seeing it in my school electronics book that just reproduces component datasheets. i couldn't remember hoe it was done and came up with the transistor solution first but having understood what was going on I worked out the diode solution for myself
 
This is one of the best ways to do PWM with a 555, or better still a 7555.
 

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The reason for using the discharge pin in that design is so the boost start system will work, as the discharge will be the inverse of the output pin in this arrangement, i'm pretty sure thats it.

Anyhow, I built that design again today, and used one of my new IRF540 MOSFETS (I wanted to test them). It works really well, and I managed to run two 12v lamps and a motor off of it with no problems, taking over 2A total. The MOSFET didn't even get warm and the scope showed good fast on and off times. I have a video if anyones interested. My power supply didn't take it (LM317 job) so I used the drill battery for power.
 
yea hero999 thats definitely a better way for one reason: as the output is charging/discharging the condenser and has a higher amp capacity there is no need for that minimum R of 1K that is to prevent the discharge TR from burning out. having looked at that Diagram I begin to wounder why the hell the discharge pin was inserted
 
I am being told that the file format is not acceptable what codec did you use ?

EDIT it was a download error. please email to "thunderchild64 (at) yahoo.com"
 
thanks mate

say i want to control 10-20 amps what mos do you advise. bvy the way i have never used a mos so tell me how to cenect it to the 555
 
10-20A is a lot, but I would think 7 IRF540 in parallel would work, each will need just a small heatsink. I was testing another design earlier using one IRF540, it was a voltage controlled PWM I intend to post under projects soon, and it handled around 3A comfortably with its small heatsink. I was using nichrome wire as a resistance, it gave a good control over it, from cold to red hot or fused depending on its length. The IRF540 datasheet is readily avialable for download, it shows the pinout. As its an N-channel, its connected so the + rail is like the load's ground, and the output drops from +v to -V. Check out when I post my design.
 
so what is the max amps of the irf540 ?. I wish to control a car fan and either it is a good mechanical design or that is an awsome motor as it is so powerful it might be too much lol. so i want control over it without loosing tons of power on heat in a transistor (BJT) and causing a cooling problem. i suppose i will put the power mos in front of the fan intake so it is cooled ok. the more amps going thru the more air so it will stay balanced with a smaller heatsink
 
Its rated at 22A, but its rds on is 0.06ohm typically, so it would need a very big heatsink if used singly, I would reccomend at least 3 used. There may be more suitable MOSFETs, but the IRF540 is at least easy to obtain.
 
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