daft question on LED indicators and high VDC

[irving2008]

Well a series resistor could be used.. 68-24 = 44v @ 20mA = 44/.02 = 2200ohm and 44 * .02 = 0.88W so need a 2200ohm, 2W resistor.
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Alternatively could use a 43v zener diode like this which might actually be cheaper since can be bought 1 off whereas resistors generally need to be bought 10 off. When using the zener the end with the band goes to the + side of the supply. Get it the wrong way round and both zener and LED will fry faster than you can say magic smoke!
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For heat dissipation purposes the zener should ideally be mounted on a bit of single sided circuit board approx. 60mm x 30mm. Make a single 2mm cut across the copper 40mm from one end to create two electrically separate areas. Form the leaas of the zener so it sits flat and solder the lead from the banded end to the larger area, keeping the lead full length. the body of the zener should just straddle the cut. Solder the other lead to the smaller area. If possible mount the board vertically to aid convection.

[EddyCurrent]

Alternatively could use a 43v zener diode like this which might actually be cheaper since can be bought 1 off whereas resistors generally need to be bought 10 off. When using the zener the end with the band goes to the + side of the supply. Get it the wrong way round and both zener and LED will fry faster than you can say magic smoke!

Irving, good idea using zener but should the connection go like this ?

supply +ve to lamp +ve
banded end of zener to lamp -ve
plain end of zener to supply -ve

[dsc]

I thought there's two ways:



I might be wrong though:)

T.

[irving2008]

I thought there's two ways:

I might be wrong though:)

T.

Umm.. the first way is correct - the resistor and zener are interchangeable in this diagram, just different ways to drop the volts.

The second is unnecessary as uses two components when one will do. LED doesn't need regulated supply so the zener is unnecessary.

Irving, good idea using zener but should the connection go like this ?

supply +ve to lamp +ve
banded end of zener to lamp -ve
plain end of zener to supply -ve

You could, but convention is that indicators always have one side grounded as often, in times past, the panel was the ground.

Or you could use one of these:- http://www.amazon.co.uk/99-9V-Digita.../dp/B008KXMNTU ..Clive

You could, but a pain to mount, unless you're CNC'ing the panel and overkill :lol:

[Clive S]

You could, but a pain to mount, unless you're CNC'ing the panel and overkill :lol:

Mount like this ..Clive

[dsc]

Umm.. the first way is correct - the resistor and zener are interchangeable in this diagram, just different ways to drop the volts.

The second is unnecessary as uses two components when one will do. LED doesn't need regulated supply so the zener is unnecessary.

The second provides a solid voltage on the zener regardless of the load, which is different to a standard resistor divider. I'll go with the first one, it's 36p for the Zener vs. 9p for a resistor but as you said before minimum order quantity is 10 (doesn't really matter as I need to go over £5 for the order anyway).

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.

T.

[dsc]

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.

[Binary]

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