3-12V Voltage regulator

Discussion in 'General Electronics Chat' started by MWG, Apr 15, 2009.

  1. MWG

    Thread Starter Member

    Apr 15, 2009
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    Hi Guys

    I'm new here and kinda new to electronics, have dabbled with them but never really learnt abut circuits etc. I've had a search and found a couple of threads that have given me a lot of help but have a couple of questions before i start buying parts

    I'm trying to make a voltage regulator to control a DC motor, range is from 3-12v but it needs to be able to deliver 10 amps. I've found plenty of diagrams and how to's for voltage regulators up to 3A but nothing very detailed for anything above that

    I've found a circuit diagram that was linked to in another thread which i've attatched

    I can understand that i need 2 IC's, i've found these but have no idea if they are suitable or not?

    http://www.maplin.co.uk/Module.aspx?ModuleNo=46054

    other than that i'm pretty stumped, i know i need capacitors and diodes and i think a variable resistor of some kind? as to what ratings they need to be i don't know. If someone could give me the specs of each of the parts i would be greatful

    also why are there so many grounds??

    thanks in advance for any help
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Those 78L12 are not suitable. The L means they are in TO-92, and able to handle only about 100 ma current. The circuit is a means of paralleling two of them for more current - but still under 200 ma. All those grounds are one and the same - it's easier to draw the circuit like that than with the additional lines it would take to connect each ground point.

    An LM317 is a variable output regulator that can do your voltage range, but not the current. Examples on the data sheet show ways to boost current up to 5 amps.

    You might look into pulse width modulation (PWM) for your speed control. The nice feature is that full torque is maintained over the speed range. We have many threads on the subject in these forums.
     
  3. MWG

    Thread Starter Member

    Apr 15, 2009
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    that's just a circuit diagram i want to base the controller on, the 78L12's will be replaced with TS1084's which are supposedly rated at 5 amps and capable of the voltage range i want.

    its all the other components i'm unsure of, i know i'm gonna need some pretty large capacitors and diodes etc

    i did wonder of all the grounds were common but wanted to make sure before i did anything

    how much more complicated is pwm? i wanted to keep things fairly simple, pwm might be something i'd use in the future but as i said i'm kinda new to this
     
  4. MWG

    Thread Starter Member

    Apr 15, 2009
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    Since my last post a lot of my rather limited electronics knowledge has come back to me with a little help from one of my colleagues. I've also been doing a lot more digging/ research into PWM

    Turns out the TS1084's i wanted to use are only good for 3.3v :( so i either parallel up a load of 1.5A voltage regulators as shown in my revised circuit diagram. All Diodes are rated at 3A, the resistor was 470Ω and the variable was 0-4.7kΩ I simulated it in circuit maker 2000 but couldn't regulate the voltage properly, i could get 10+ V out but couldn't regulate it down unless i used a ridiculously high R value? Is it even possible to make this work?

    On another note i've found a circuit diagram for making an 'H' bridge controller which the designer says it is capable of 1-5 amps continuous which is not quite enough to drive the motor i want to use, ideally i want to be able to double it up to 10A so i don't fry the circuit of the motor stalls which is possible. How would i go about interfacing it with a 555 timer and making it capable of 10A ??

    I'm looking at the TIP107 and TIP102's as something that would need to change, don't really know much about transistors though? Diodes would also be beefed up, again not sure what too though, would they need to be large enough to cope with the motors draw?

    Thanks
     
  5. tkng211

    Well-Known Member

    Jan 4, 2008
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  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    The schematic on the page tkng211 linked to has errors and omissions.

    This revised version will work much better:
    [eta: see corrected 2nd revision below]

    A 220uF cap should be added from U1's Vcc pin to the GND pin.
     
    Last edited: Apr 23, 2009
  7. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Hi Sgt,

    You must be not yourself today.:(

    I have spotted three mistakes in your revision:

    1. The POT value is now 100Ω, which is too low. Should be 100K.

    2. The 555 output is connected directly to +12V, this will kill it instantly.

    3. The MOSFET IRFZ46N gate drive is connected to +12V so it will conduct forever with power ON.
     
  8. MWG

    Thread Starter Member

    Apr 15, 2009
    28
    0
    Thanks for the replies guys, Sgt thanks for making those revisions.

    elbc - i would never have seen the problems you've mentioned, not even sure if i understand the problems with the 555 and MOSFET at the moment

    from what i've read DC motors tend to generate high pitch noise at some PWM frequencies, i don't particularly want to have to put up with this. Is it a case of just guessing or is there a way to work it out or a ball park figure i should be aiming for to try and avoid this??

    thanks again guys, i'm gonna try to model that circuit tonigt and see if i have any problems with it
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    elbc1388/L Chung,
    I agree, I must have indeed been having an "off day" - actually, I was interrupted multiple times while I was re-drawing it, and failed to do a "virtual bench-check" on the changes. Thanks for pointing them out.

    The corrected revised version is attached; apologies if our OP suffered "smoke damage" due to the mistakes.

    [​IMG]
     
  10. MWG

    Thread Starter Member

    Apr 15, 2009
    28
    0
    Hey Sgt, thanks for revising that. I'll have another go with the new version and see if i can get it to 'work'

    couple of questions though

    where does pin 7 on the 555 go to ? i can only think of putting it to earth??

    the +ve side of the motor is connected straight to the source right? and then the timer and related circuit is used to pulse the -ve side? Just want to try and understand how the circuit works a little better

    and again about the PWM frequency, using the equation f = 1.44/(R1*C1)

    Changing R1 in steps of 2.5k i've got a range from 0.144 - 5.76kHz, so when i power up the circuit i should have the POT at 100K then bring it down to increase speed? how low down should i go? do you get to the point where reducing R1 no longer has an effect?

    Thanks
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Leave pin 7 disconnected. It's an open-collector connection that's frequently used for discharging the timing capacitor. In the original schematic, they were using it to discharge the gate of the MOSFET, but pin 7 has a limit of around 15mA current. Pin 3 can source/sink 10 times that much. Also, in the original circuit, a 10k resistor was being used to charge the gate; this would result in the MOSFET staying in the linear region for a long time, causing it to rapidly heat up.

    I would use a fuse or circuit breaker in series with the connection to the power supply.

    Yes.

    OK, this is the part that's a bit tricky to understand. ;)

    R1 always has a total resistance of 100k in the path. However, the diodes control the current flow through R1. All of the charge or discharge current goes through the wiper (middle terminal) of R1. When timing capacitor C1 is charging from pin 3 through R4 and R1, (conventional) current flows through the wiper of R1 and D2. When discharging, current flows through D1, out the wiper of R1 through R4 to pin 3.

    The output frequency doesn't change much; just the ratio of ON to OFF time. R4 is necessary to keep the C1 charge/discharge current under control when R1 is at one end or the other of it's travel; otherwise the MOSFET gate charge/discharge times would suffer.

    R3 (10k) now serves as a safety; in case the 555 timer output becomes "open" or the timer is removed from the circuit, the MOSFET's gate will be pulled low, turning it off.
     
  12. MWG

    Thread Starter Member

    Apr 15, 2009
    28
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    I was going to put a fuse in the circuit just before the motor, don't really want to fry it and its going to be under a fair amount of load. I've also added a couple of LED's so i know when the circuit has power and one after the fuse so i know if it has blown

    I've modelled the circuit up in multisim, had to replace D1 and D2 with 1N5820's as the 1N5818's aren't in the library and i've put 3 1N4001's in parralell to act as D3

    I've got a load of probes around the circuit to see the voltages etc and a scope across the motor to view the pulses.

    When i have R1 at 50% i have a ~4ms pulse width, now this is where things started to confuse me slightly.

    Basically i want to know if the movement of R1 is symetrical so to speak? and that either side of 50% is meant to give you the same result? from what you said previously i get the impression thats what i'm meant to be seeing? Either thats right or i've done something wrong lol

    Thanks again
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    That's fine.

    You can use 1N4148 or 1N914 diodes for those; they are cheap and plentiful. The 1N5158's are Shottky diodes which have a low Vf, but for this circuit it's really not very critical.
    That's not so good. 1N4001's are only rated for 1A average current. 1N5400 series diodes are rated for 3A average. What other diodes are in your library?

    Try setting R1 to 25%, and run the simulation.
    Then try setting R1 to 75%, and re-run the simulation.
    With one setting, you'll see the 555's output to be high roughly 25% of the time, and low 75% of the time. With the other, you'll see the output high roughly 25% of the time, and high roughly 75% of the time.

    It's not going to be exact.
     
  14. MWG

    Thread Starter Member

    Apr 15, 2009
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    D1 and D2 are now 1N4148's and i'm using 1N5401's to act as D3. In terms of what's in the library there's 810 listed so way to many for me to even know what to do with :confused:

    Now i've changed the diodes its working properly, seems you can't just guess when it comes to diodes lol

    I'm fairly confident this will be my last question. Are any of the components going to need heat sinking? The only thing that i'm thinking that will is the transistor?

    Thanks for all of your help Sgt
     
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