ULN2803 Driving 5Amp Output

Discussion in 'The Projects Forum' started by Tasp, Sep 28, 2010.

  1. Tasp

    Thread Starter Active Member

    Aug 10, 2008
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    Hey guys, I'm new to electronics so go easy please!

    The project, is to control solenoids from a PC, the PC part I've done using PIC's to fire ouputs to a ULN2803 darlington driver. I've tried using just the ULN2803 but it burn't out as the solenoid drew to much current.

    I've created a diagram of what I think the circuit should work like and would like everyone's opinion whether changes need to be made. However after destroying quite a bit of kit with shards of plastic flying about I'd rather have a second opinion.

    The ULN2803 should drive the TIP3055 transistor which uses the secondary 16vdc higher power output supply to drive the solenoid. This also has a 2200uf capacitor to help give a surge to the solenoid.
    Hopefully this will fire 8 solenoids so will mimic the same circuit for all 8 each having it's own transistor.
    It has to use a separate PSU.
    I do not want to use relays unless absolutely necessary as I've seen this done without also the cost of using high output relays.

    These solenoids usually need a capacitor discharge unit (CDU), http://www.brian-lambert.co.uk/CDU.gif the original page of this image is http://www.brian-lambert.co.uk/Electrical-2.htm there is a lot of info on this page most not relevant your best off using the gif link.

    The datasheet for the TIP3055 is http://www.rapidonline.com/netalogue/specs/81-0812.pdf

    The datasheet for the ULN2803 is http://www.datasheetcatalog.org/datasheets/90/366828_DS.pdf

    There is no datasheet for the solenoid, however this is what it is, http://www.gaugemaster.com/item_details.asp?code=PEPL-10

    I hope that I've given all the info for you to help.
     
  2. Markd77

    Senior Member

    Sep 7, 2009
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    The ULN2803 doesn't source any current or voltage. It only allows current to flow or blocks it so you need something to turn the transistors on.
    A resistor from the base of the TIP transistor to the 16V line would work (I think) sized to supply 1/10th of the current you want through the solenoid.
    The ULN2803 could then turn off the transistor by pulling the base low (the current would flow through the resistor into the ULN2803 instead of the transistors base.
    Things are going to get pretty warm.

    I'm pretty sure there are much better solutions, hopefully someone will have one.
     
  3. tom66

    Senior Member

    May 9, 2009
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    Since you have 16V a better solution is a power MOSFET.
     
  4. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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    To give a bit of isolation and keep things a bit cooler you could switch some small relays instead of the transistors.
     
  5. Markd77

    Senior Member

    Sep 7, 2009
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    I've had a bit of a think and a look at the link to the CDU.
    It seems the solenoids only need the high current for a brief time to move them and then a lower current to hold them in position, which is why the CDU is used. If you run high current through them continuously I'm not sure if they will overheat.
    I think this needs more consideration.
    MOSFETs would almost certainly be a better choice.
    What will be controlling these solenoids - microcontroller / switches / logic?
     
  6. Tasp

    Thread Starter Active Member

    Aug 10, 2008
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    Prefer not to use relays for this job, thought about it but not really viable, need to have about 30 of these, and the solenoid works in both directions so will need 60 relays.

    @Markd77
    Your right the solenoids don't latch, they only need a breif surge of power for around 50-250ms.
    They were originally going to be controlled by a PIC via ULN2803 darlington driver, but after looking at the MOSFET they will be controlled directly via the PIC outputs which I beleive are logic (0v going 5v).
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Don't forget that you must use reverse-EMF protection diodes across each solenoid for your points motors. If you fail to do so, you will exceed the voltage rating of just about any semiconductor when you try to turn off the current flow, and will let the smoke out of them.

    Connect the common motor terminal to +V, and use N-ch logic level power MOSFETs to sink the current from the "ends" of the solenoids.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Farnell has 45 of these N-ch logic-level MOSFETs in stock:
    http://uk.farnell.com/international-rectifier/irlu8721pbf/mosfet-n-i-pak-30v/dp/1551913

    You can also use these:
    http://uk.farnell.com/international-rectifier/irlu7821pbf/mosfet-n-30v-i-pak/dp/1436998

    Consider them roughly equivalent, but the latter is obsolete. You can either wait until they get more of the former in, or look elsewhere - perhaps RS components has them, or maybe Maplin (although I'm not very optimistic about them having such things).

    You won't need heat sinks, and the package is a handy size; in-between the large TO-220 and the small TO-92 plastic package.

    MOSFETs are static sensitive, but once installed in a circuit, they're pretty robust.
     
    Last edited: Sep 29, 2010
  9. Tasp

    Thread Starter Active Member

    Aug 10, 2008
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    Thanks for the responses,

    I found this one which I think is suitable, seems the same specs as the farnell one Logic MOSFET, http://www.rapidonline.com/netalogue/specs/47-0552.pdf

    I was aware that the solenoids needed back emf protection and no heatsinks would be required as the short burst of power shouldn't heat them up, but good to point out anyway just in case thanks. Also thanks for confirming it was N type I required, I was 95% sure but needed clarifying.

    I've put together another diagram using the MOSFETs and the CDU, could you have a look and tell me if there's anything wrong (or right!).

    I'm a bit concerned that

    1, There maybe back emf into the PIC on 0v.
    2, Is the CDU circuit ok as this was orginally meant for AC, and can it be used like this?
    3, C3 & D4 even required?
    4, Is a 10k pull up resistor required between PIC O/P and MOSFET?

    I think thats all!! ;)
     
  10. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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    If I had actually read you original post correctly I would have known what you were talking about. I have actually built somthing similar and I am trying to find the circuit on my PC. The only difference it mine isn't controlled by a PIC but a PC parallel port but the resultant circuit is the same.

    I am assuming you are only applying a pulse to the FET to switch it on and then off again. Points solenoids only need an instantaneous switching current hence the CDUs. They just apply a sudden high current pulse. If you leave the FET on continuously you will burnout the coils.

    I will try and dig up my circuit. It involved a CDU that supplied multiple points. It works very well.

    I am not sure you would even need the CDU in your circuit. Try it with just the FET and the coils between 16V and ground.

    Have a look at this page. http://home.cogeco.ca/~rpaisley4/CircuitIndex.html

    And this one in particular. http://home.cogeco.ca/~rpaisley4/CDPSU.html

    It has a heap of great ideas for trains.
     
    Last edited: Sep 29, 2010
  11. Tasp

    Thread Starter Active Member

    Aug 10, 2008
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    Yes it will pulse for 50-200ms
     
  12. windoze killa

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    Feb 23, 2006
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    I just updated my previous post. Check out the 2 links.
     
  13. Markd77

    Senior Member

    Sep 7, 2009
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    I'd scrap c3 and D4. They would stop it working properly. Most designs I've seen have a small resistor 47 or 100 ohms directly between the PIC output and the gate.
    It might be worth having a 10K from gate to ground to keep the MOSFET turned off while the PIC gets powered up.
     
  14. fanie

    Active Member

    Jan 20, 2007
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    Just use a logic level fet like the IRLZ44 to drive the solenoids. The PC and solenoid commons will have to be connected so the gate signal has a reference.

    If you need to isolate the PC then look at using a TLP590

    BTW, 4 circuits like this makes a very nice H-bridge !
     
  15. Tasp

    Thread Starter Active Member

    Aug 10, 2008
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    Ok c3 and D4 are going, cheers.

    really feels like I'm getting there.

    @windoze killa did you find those diagrams?
     
  16. SgtWookie

    Expert

    Jul 17, 2007
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    The IRLZ44 is outdated, has an unnecessarily high Vdss rating, a lower current rating, and a HUGE total gate charge requirement compared to the MOSFETs that I recommended and that Tasp selected.

    Tasp, the MOSFETs you selected should work OK. They don't have the current rating of those I pointed out earlier and the gate charge is higher, but they would still work OK.
     
  17. fanie

    Active Member

    Jan 20, 2007
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    Hi SgtWookie,

    Give us a part number there, would gladly upgrade my old stuff.
     
  18. Tasp

    Thread Starter Active Member

    Aug 10, 2008
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    @fanie, as is the earlier post in this thread.
    And the one I chose was http://www.rapidonline.com/netalogue/specs/47-0552.pdf

    Cheers for confirming the MOSFET I chose is ok Sarge, Before I go ordering stuff I just see in windoze killa can find those circuits.
     
    Last edited: Sep 30, 2010
  19. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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    AAAAAAHHHHHHHHH it seems as I have deleted them or backed them up to some unknown CD somewhere. I will try and recreate it from the built board unfortunately I am going away for work for the next week.

    It was based on one of those circuits on the links I posted.
     
  20. Tasp

    Thread Starter Active Member

    Aug 10, 2008
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    Now I know this is an old thread, however I started this originally and am now concerned the diode part as a issue.

    I originally was fitting the diodes across the point motor (solenoid) I have since found that with vibration they come off the PCB as the PCB has a cooper pad rather than a hole.

    My question really is will the diodes still stop the current discharging across the coil if they are approx 4 inches away attached to a fly lead?

    Or do I have to have them fitted right across the coil?
     
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