PWM circuit from 20vdc

Discussion in 'General Electronics Chat' started by nitrochicken, Mar 7, 2008.

  1. nitrochicken

    Thread Starter Member

    Mar 7, 2008
    11
    0
    Hi,
    This is my first post, I have searched the forum for answers but can't find quite what I need. I would appreciate if anyone could give me some advice.

    I'm attempting to make a speed controller for a fan(20vdc, brushed)
    I was hoping to modify a circuit on this website:http://ourworld.compuserve.com/homepages/Bill_Bowden/page6.htm#dimmer.gif

    The power is coming from a 20v transformer that I have rectified to dc.
    The problem I have is that the 555 can only handle max 15v so I need to lower the voltage to the control side of the circuit somehow whilst still supplying the motor with 20v(pulsed)
    Can I just use a resistor in series? if so, how would I calculate the size required?

    Also, what is the best frequency to use for controlling the motor speed, this circuit states 200hz. I have read on somewhere that 20khz+ is best for motor speed control? I'm assuming I can change this by using a smaller capacitor?

    Thanks
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    You can use a 12V linear regulator to supply your 555.
     
  3. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,682
    900
    There is quite a debate on that question. I believe the answer is that each has advantages and disadvantages. Whenever a question is un-resolved, that always seems to be the answer.

    For a simple fan motor, I would go to the higher frequency for peace and quiet. The coils on motors act as speakers at lower frequencies. In fact, some controllers communicate with the user by using the motor coils in that way.
    John
     
  4. nitrochicken

    Thread Starter Member

    Mar 7, 2008
    11
    0
    Hi,

    Thanks for your help, I've made the circuit up this afternoon using a 7.2v battery for the 555 circuit and it seems to work well. I have changed the capacitor for a 10nf ceramic one which I have worked out gives me approx 15-30kHz, and there doesn't seem to be any noise!

    The mosfet seems to be getting hotter than I expected(too hot to touch), it has a small heat sync on and the motor only draws 2A. It is rated at 55A 60V. Is this normal?

    As far as dropping the voltage for my 555 from 20v, is there a way I can do it easily without buying a voltage regulator(don't want to wait!) I have a few transistors, various resistors and caps etc.

    Thanks
     
  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,682
    900
    Please post your circuit. You may just need something simple like a capacitor or an active turn off for the mosfet gate. John
     
  6. nitrochicken

    Thread Starter Member

    Mar 7, 2008
    11
    0
  7. rwmoekoe

    Active Member

    Mar 1, 2007
    172
    0
    yes you can get a lower voltage unregulated for the 555. if you have a zener, it'll be better. otherwise, just go with resistors to form a divider. couple with a cap, followed by an npn, to supply the 555.

    btw, the heat on the mosfet should be taken care of. maybe the working freq is too high, the transition between hi and lo become dominant. you could lower the freq, or find a better way to drive the gate like jpanhalt said and/or find a mosfet that can work better at that high freq.
     
  8. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,682
    900
    Nitrochicken,

    I would try the following, in order. First, reduce the gate resistor from 100 ohms to something like 10 ohm. Some devices don't need them at all, but 100 ohms will slow down both turn on and turn off. The gate capacitance of your mosfet is about 1.5 nF.
    Second, try a large capacitor (100 to 2000 uF at 35V) from the +20 V to ground. Keep leads short. Your motor current drain is 2A and I doubt you will need a heatsink on the mosfet for that. The capacitor will be absorbing a lot of power during the off cycle so its lead may get warm, but not hot. John
     
  9. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Your MOSFET isn't working properly. There is no way that 2A through a ~0.018 ohm rdson MOSFET is causing a ~30+ degree temperature rise. Ignoring switching losses, you should be dissipating 0.072W from the MOSFET. Take the typical 50-60 degrees/W spec. for a to-220 package, you should only be getting a 4.32 degree rise.

    I am willing to bet that you're mainly operating in the linear region of your FET. Apparently, the 555 can only sink/source 200mA, which will take a very long time to switch your FET gate charge; thus keeping it in the linear region for high frequencies. You can either slow your switching speed down significantly, or use a FET driver. Better yet, find a lower gate charge FET that will do what you need. You don't need 55A and such a low Rdson. Check out the following link, you can either buy or build a good FET driver.

    http://www.zetex.com/3.0/appnotes/apps/an18.pdf

    Steve

    Oh, I just noticed the post above, getting rid of the 100 ohm resistor is a great idea. These are typically 10-20ohms and they prevent overshoot.
     
  10. rwmoekoe

    Active Member

    Mar 1, 2007
    172
    0
    think the gate voltage is not high enough. should be at least 10 volt at on state. why don't you use 12 volt supply for the 555. doesn't need to be too regulated.
     
  11. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    With a 4V maximum gate threshold voltage, then 7.2V is plenty enough. It's the current source/sink that isn't adequate.

    Steve
     
  12. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Hi nitrochicken.

    It looks like you have accidentally used a bipolar transistor symbol in your schematic instead of an n-channel mosfet symbol.

    I think the output of the 555 timer is sufficient to drive the stp55nf06 to saturation. The spec sheet I examined indicated that a couple of volts should be enough.

    I would suggest you put a flyback diode in parallel with you motor to protect your mosfet possible damage from back-emf that will be generated by the motor.

    hgmjr
     
  13. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I'd get rid of the gate resistor entirely; just connect 555 pin 3 directly to the gate - and use a minimum of 10V for the 555 supply. You need that gate voltage rise/fall times as short as possible, from 0V to 10v and back down again.

    Using a large cap across the 555's supply pins won't do much good unless it's a low ESR cap. a 0.1uF to 1.0uF tantalum or ceramic should be fine. Don't use electrolytics; too much ESR (internal resistance).
     
  14. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,682
    900
    My suggestion for the capacitor was not based on the need to de-couple the 555. I assumed that was already or would be done. It was based on 4QD-TEC's recommendation (http://www.4qdtec.com/pwm-01.html). It solved a lot of overheating problems for a controller I built. That controller was a lot larger, and the cap was probably not necessary for lower power, which is why I listed it as a second step, if getting rid of the gate resistor didn't solve the problem. John
     
  15. nitrochicken

    Thread Starter Member

    Mar 7, 2008
    11
    0
    Thanks for the help, I have now tried the following and I'm still getting a lot of heat.
    - removed 100ohm resistor, tried 10ohm and also without.
    - changed cap for 0.1uf to lower frequency
    - used 14.4v input for 555(didn't have 12v handy)

    I have not tried using a larger capacitor accross the supply yet, closest one I have is 2200uf 25v electrolytic, would this be ok and where would I connect it?

    Thanks

    Dale
     
  16. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,682
    900
    Just to be clear on the capacitor discussion, SgtWookie and I were referring to different capacitors. He was referring to the decoupling capacitor across the pins and as close to the 555 as practical. I was referring to the motor capacitor as described by 4QD. It goes across the supply. The 4QD link gives a very nice discussion of what it does and how to connect it. The 25V electrolytic you have should be OK for that. One cannot rule out the need for that cap until you try it; however, at such low power, I don't think that is the real problem.

    The 555 turns on and off so quickly that it can cause dips and spikes in the supply line. The decoupling capacitor reduces those effects, thus "decoupling" the 555 from other components, including itself, that depend on the same supply. The decoupling cap needs to be low esr, such as 0.1 uF ceramic. The decoupling cap is often omitted from schematics, because "everyone knows" it should be there.

    I looked at your schematic again and it doesn't show a capacitor from pin 5 to ground. That pin (control) usually has a 0.01 uF cap to ground. It may be needed to prevent unwanted oscillations in the 555, which would lead to heating as well. I typically use a mylar/polyester cap there. Sometimes the pin 5 cap is also omitted from schematics, and a note is added that one might be needed. It won't hurt to add it. In fact, I would recommend you try this change next.

    Finally, confirm that the mosfet is on the "low" side. Your schematic shows it like that , but it is easy to get mixed up. The battery positive goes to the motor, the negative from the motor goes to the Drain of the mosfet, and the Source of the mosfet goes to ground. The mosfet leads are GDS from left to right with the tab at the top and the label facing you. If you reverse the D and S, the gate will still turn the mosfet on, but it will get very hot because the intrinsic diode will be shorting across the supply. Once turned on, mosfets conduct in both directions, so in low-power situations (i.e., a high resistance supply), it might appear to be working but getting very hot because of the diode.

    Some or most of this comment may be obvious to you and I don't mean to insult you by repeating it. But, since your circuit is not working, it is kind of a checklist to review. Here is another link to the datasheet for your series of mosfets. The one you posted is not working this morning.

    http://pdf1.alldatasheet.com/datasheet-pdf/view/24443/STMICROELECTRONICS/STP55NF03L.html

    Please keep us updated on your progress and success. John

    Edit: Some after thoughts: Are there caps or a diode across the motor leads. That is, right at the motor there are oftens caps from each lead to the case or just between the two leads. The caps stop rf noise, but they may also help in your situation. They can be small, like 0.1 to 0.01 ceramic. Finally, you should consider a diode across the motor as well. The dark-banded end should be attached to the positive side. It's purpose is to conduct the reverse spikes ("inductive kick") from the motor back to the supply. A 1n4001 through 1n4007 should work. While it is true the mosfet already has such a reverse biased diode (the intrinsic diode), an additonal, faster diode can really help. Again, see the 4QD discussion. All About Circuits ebook probably discusses it too.

    As a sequence to try, put the decoupling cap and the cap at pin 5 first, confirm all component orientations, try the diode across the motor, then try the large motor cap.
    John
     
  17. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Adding to what jpanhalt wrote,
    I suggest that you need both a reverse-EMF diode and a small capacitor (470pF to 2200pF) across the motor. The purpose of the small cap is to absorb the beginning of the large EMF pulse that occurs when the MOSFET switches off, and before the diode can begin conducting. Rectifier diodes like the 1N4000 and 1N5400 series are quite slow to turn on and off; the cap "buys them time" to turn on.

    There are fast Shottkey diodes out now that have very fast switching times and lower reverse leakage than the older incarnations, but they're still quite expensive.
     
  18. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Hi,

    How low of a frequency are we talking about here? As I had said earlier, I believe that the problem is the 555's inability to source/sink the necessary current to switch the MOSFET fast enough to keep it out of its linear region. By lowering this significantly, you should be able to run free of heat. Again, you may need a proper driver to do this.

    People get confused about the notion of a voltage controlled device requiring high current to get it to function correctly. This is because there is a gate charge that needs to be overcome in order to extend the depletion region through the channel. Many of the drivers I had used in the past were able to source/sink 1.5 to 3A and they were not driving much larger MOSFETs than what you present.

    Steve
     
  19. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    As scubasteve_911 has pointed out, if the switching frequency of the 555 is too high this could result in the 555's output driver spending too much time in its linear region due to the mosfet's gate capacitance. This could result in overheating of the 555 and most likely the mosfet as well.

    hgmjr
     
  20. nitrochicken

    Thread Starter Member

    Mar 7, 2008
    11
    0
    Hello again and thanks to everyone for the help.

    I decided to start the circuit again from scratch with the modifications that everyone has suggested.
    I now seem to be having the same problem with heat plus another strange one. When I turn the pot, top end is fine for full speed but when I turn it down the motor speed drops by perhaps 20% but not down to zero as before.
    I thought I had made a mistake somewhere but when I checked it all looks ok.

    I also noticed that my diodes in the bridge rectifier were getting very hot, they are rated at 3A so I wasn't expecting this. I'm not sure if this was happening before as I never checked.

    I don't think that the frequency is too high as I'm now using a 0.1uf capacitor which should give me 288hz max

    I have attached a new picture of my circuit for you to see.

    Thanks

    Dale
     
Loading...