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  1. #61
    Ok I'll see what the smallest 100v caps are. The only bit I'm stuck with is the high power resistors. The Original dosnt show a rating and the similar Burger design shows the bootstraps as 2 watt and the sense res as 5 watt! dose this seem right?

    The other problem is that although the original board used this configuration the motors are only 2 amp per phase so I will need to lower these values to set a temporary 2 amp max current. This shouldnt affect the pcb layout tho. maybe just use a large res mounted vertically?

  2. Quote Originally Posted by Ross77 View Post
    Ok I'll see what the smallest 100v caps are. The only bit I'm stuck with is the high power resistors. The Original dosnt show a rating and the similar Burger design shows the bootstraps as 2 watt and the sense res as 5 watt! dose this seem right?

    The other problem is that although the original board used this configuration the motors are only 2 amp per phase so I will need to lower these values to set a temporary 2 amp max current. This shouldnt affect the pcb layout tho. maybe just use a large res mounted vertically?
    Vref = 0.5v, phase current = 4A therefore sense resistor is 0.5/4 = 0.125ohm and dissipates 0.5 * 4 W = 2W so need to be 5W rated.

    Why would you need to reduce them for 2A, just turn down Vref to 0.25V. I would recommend a 10 turn pot for the Vref trimmer, makes it so much easier to set.

    What have you planned for heatsinking the 6203s?

    From the datasheet, at 4A you are going to dissipate between 10 and 15W. Max junction temp is 150, max case temp 90 so for an ambient of 25degC you'll need better than (90-25)/15 = 4degC/W heatsink per 6203 - I'd go for ~3degC/W. thats going to need a finned heatsink around 60mm x 100mm each chip...

  3. #63
    Vref = 0.5v, phase current = 4A therefore sense resistor is 0.5/4 = 0.125ohm and dissipates 0.5 * 4 W = 2W so need to be 5W rated.
    If it only = 2W then why do the res need to be 5W? I thought vref was up to 1v as well? if so then 1/4 = 0.25 hence the two 0.5r, but this would mean max current is 4W? probably got it wrong again :whistling:

    Theres not much available in 2 or 5W resistors in that range as they cant be wirewound.......Why is it possible to get a 2W smd res 30p? dosnt make sense.

    For the heat sinks I was going to reuse the originals, their about 50mm square and 140mm long with fan at each end. The 6203's mount on both sides with the fins top and bottom.

    Does the shunt regulator have to be on each board or could I just put one on the PSU distribution board?

  4. Quote Originally Posted by Ross77 View Post
    If it only = 2W then why do the res need to be 5W? I thought vref was up to 1v as well? if so then 1/4 = 0.25 hence the two 0.5r, but this would mean max current is 4W? probably got it wrong again :whistling:

    Theres not much available in 2 or 5W resistors in that range as they cant be wirewound.......Why is it possible to get a 2W smd res 30p? dosnt make sense.

    For the heat sinks I was going to reuse the originals, their about 50mm square and 140mm long with fan at each end. The 6203's mount on both sides with the fins top and bottom.

    Does the shunt regulator have to be on each board or could I just put one on the PSU distribution board?
    You are right, Vref for the L297 is 1v - I was confusing it with the TBA6560 chip on the other thread... so the sense resistor should be 1v/4A = 0.25ohm non-wirewound, rated at 1v x 4A = 4W you would derate by 25% - 50% so really you need something rated at 5W - 8W

    Look at the datasheet for this 0.5ohm SMD resistor from Farnell (you'd need 2 in parallel to give 0.25ohm/4W). At its rated dissipation it needs, and I quote "2 Watts with total solder pad and trace size of 300 mm2" - I doubt you'll be close to that amount of copper, and even if you were the temp rise is 90degC above ambient... thats 105degC+ - ouch!

    A better product is this nickel chrome metal resistor from Vishay, also from Farnell. This is rated at 5W and seemingly doesnt need a heatsink, but will hit 120degC at rated load so some airflow would help.



  5. #65
    And this is why I like low-current digital electronics

  6. #66
    You are right, Vref for the L297 is 1v - I was confusing it with the TBA6560 chip on the other thread... so the sense resistor should be 1v/4A = 0.25ohm non-wirewound, rated at 1v x 4A = 4W you would derate by 25% - 50% so really you need something rated at 5W - 8W
    Phew I thought I was going mad. thanks for the smd links but I think I'll stay away from them at the mo. i just couldnt understand how such a small part was so powerful and cheap.....

    Looks like digi-key are the best for this project. in fact they are cheaper on a lot of things, I just need to spend 50 for free delivery. Any body need parts? I can get 2watt res for 13p each, if I use 3No. 750 ohm in parralel = 250 ohm at 6 watt correct?

  7. Quote Originally Posted by Ross77 View Post
    Phew I thought I was going mad. thanks for the smd links but I think I'll stay away from them at the mo. i just couldnt understand how such a small part was so powerful and cheap.....

    Looks like digi-key are the best for this project. in fact they are cheaper on a lot of things, I just need to spend 50 for free delivery. Any body need parts? I can get 2watt res for 13p each, if I use 3No. 750 ohm in parralel = 250 ohm at 6 watt correct?
    That is correct...

  8. #68
    Quote Originally Posted by Ross77 View Post
    Any body need parts?
    Let me know how short you are off your 50 - I may have some bits that I can order from Digikey.

  9. Ditto, i need to order a few small bits, not urgent but if you need a couple of quid extra let me know... stuff i need can be posted to me with a 2nd class stamp...

  10. #70
    Let me know how short you are off your 50 - I may have some bits that I can order from Digikey.
    Ditto, i need to order a few small bits, not urgent but if you need a couple of quid extra let me know... stuff i need can be posted to me with a 2nd class stamp...
    Sure pm me with what you want. it would be good to help a little, after all the help you guys have given me. Dosnt matter if I go over, just said 50 min.

    just about there with with the design. I've had a crack at laying out the pcb but I dont think it will fit on the 100x80 board in eagle, I'm sure you will prove me wrong.....

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