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  1. #11
    Just a quick update, I've tried both the resistor and the diode, both get rather hot, although the diode seems to handle it better. Done some checks with a TC and the resistor sits at around 80degC whereas the diode goes up to around 60deg. The ideal solution would be to change the PSU to a 48VDC, but with the current solution costing less than a pound I don't think I'll bother:)

    T.

  2. Did you mount diode on a bit of board as I suggested? It's running a little hotter than I calculated. Junction temperature will be around 120 degC, still less than the maximum allowed of 150 degC, but 20 degC more than I'd like.
    Last edited by irving2008; 13-10-2014 at 05:50 AM.

  3. #13
    Hi you say your wanting to connect a LED to a 68VDC PSU. you can use a LED with a resistor in series If your machine is on permanently, I would suggest to drop the current to 15mA, LEDs work at 20mA but run constant shortens their life expectancy. If your LED is a 24 volt version check the spec on it the resistor already in situ may already drop the current to the LED to 12~15mA, just remember the power supply you are connecting to will either be PWM or linear my guess PWM either way the voltage (68VDC) will be constant

  4. #14
    This was only a quick test with a choc block, I'll need to cut up a piece of laminate and give it another go.

    T.

  5. #15
    Hi, you should not need a diode your power supply will be polarized +ve & -ve your problem is you are trying to drop 44 volts which creates a lot of heat, if your voltage was a bit lower you could have used a LM317 or similar the best you could input to these would be 32vdc its a weird voltage drop to are trying to use my suggestion is to try to get hold of a NUD4011 http://www.onsemi.com/pub_link/Collateral/NUD4011-D.PDF
    this should handle your voltage drop

  6. #16
    Quote Originally Posted by Binary View Post
    Hi, you should not need a diode your power supply will be polarized +ve & -ve your problem is you are trying to drop 44 volts which creates a lot of heat, if your voltage was a bit lower you could have used a LM317 or similar the best you could input to these would be 32vdc its a weird voltage drop to are trying to use my suggestion is to try to get hold of a NUD4011 http://www.onsemi.com/pub_link/Collateral/NUD4011-D.PDF
    this should handle your voltage drop
    Hi and welcome to the forum. Have you read this complete thread because I think you have missed the point and purpose about the diode also all the math has been done. I am not trying to put you down in any way but this guy has stated he wants a simple way and he is short of space ..Clive

  7. #17
    Thank you for the welcome. I understand he has a problem with space but if you are using a 5Watt resistor you need a lot of space the chip i suggested can be mounted on a 15mm x 8mm pcb depends how you proceed, that would include a smd resistor also i use its brother the nud4001 so i know how small it is, the chip is a current limiting chip that is why it can be used with such high voltages up to 200Volts input as for the price i pay around 30p for the nud4001 the price for the nud4011 is a little higher..its only a suggestion he does not have to use it but many brains make light work lol i'm not sure it may even fit on the pads of a piece of vero board,
    One other point " a LED is a Diode" a zener can be used to "clamp" the voltage" you just need to current limit it, I also have another chip in mind and the size of that is only 3mm looks like a smd diode 2 pins thank you clive

  8. #18
    I've made a small PCB 62mm x 40mm, it's got a larger section of copper on the banded area, roughly 38mm x 40mm. Just ran some tests and it levels at around 56degC, so better than before, but not massively.

    I do like the NUD chip, the issue is it's SMD (can't solder those) and for 68VDC and a single LED it's over it's power limit anyway. I guess I might have to get that 48W PSU anyway...

    Cheers,
    T.

    BTW. I'm not limited for space, I've got loads:)

  9. #19
    Quote Originally Posted by dsc View Post
    I've made a small PCB 62mm x 40mm, it's got a larger section of copper on the banded area, roughly 38mm x 40mm. Just ran some tests and it levels at around 56degC, so better than before, but not massively.

    I do like the NUD chip, the issue is it's SMD (can't solder those) and for 68VDC and a single LED it's over it's power limit anyway. I guess I might have to get that 48W PSU anyway...

    Cheers,
    T.

    BTW. I'm not limited for space, I've got loads:)
    This is what you quoted [Those 7seg displays are pretty neat considering the voltage range on them. I'll have to stick with a small LED indicator as I'm tight on space as it is.] This is why I mentioned that ..Clive

  10. #20
    Should've been more clear, I've got no space on the front panel, but loads in the guts, which basically means I need to stick with a single panel mounted LED and whatever internals it needs.

    T.

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