Hello,
I have a project that requires the control of some small (5.4W) 24V
solenoids from a TTL logic source. I'm thinking of using a ULN2803A to
drive the small coils . I found a data sheet that lists the IC's power
dissipation as 2.25W for the whole package. However, the same data
sheet states that the chip can handle a total combined load of 260W.
What gives here? Why is the chips power dissipation rating 2W when
each darlington pair has a load current rating of 500 mA continuous?
Am I missing something? Just what is power dissipation rating, and how
does it relate to the load power for a device like this? Can this chip
supply the 45 watts of power I need? Thanks.
gm
Good morning. Keeping it simple, the power dissipation in any given
transistor, regular or darlington, is approximately the voltage across
the transistor (Vce) * the current going through the transistor (Ic).
(This ignores base current.) Device power dissipation doesn't have
anything to do with load power dissipation -- two different things.
The darlington transistors in the ULN2803 will typically have a
saturation voltage of about 1.0V to 1.2V. Each transistor that is on
will then typically dissipate about 0.225 to 0.27 watts when on, with
your 0.225 amp load. If you multiply that by the eight darlingtons in
the package, that will give you 1.8 watts to 2.16 watts of power
dissipation in the IC (which is different than load power). Given that
the data sheets specs a maximum Vce(sat) of 1.6V, that _will_ exceed
total maximum power dissipation.
As a practical matter, that's way too close to the absolute maximum
power dissipation. In addition, if you consider variances in coil
resistance, power supply (current through the coil will go up 10% if
your power supply goes up 10%), and variations in Vce(sat) (which will
probably be a little higher than 1.0V), it looks like a mistake to use
this IC.
However, you may have an application that only requires one or a couple
of these solenoids to be on at one time. In that case, the ULN2803
might be an ideal solution. Personally, I would worry about going much
over 1.0 watt (4 of your loads) on these ICs. (N.B.: By the way, make
sure to connect the diode pin of the ULN2803 to your +24V supply, or
the diode protection won't work and your chips will smoke).
So, you have several potential solutions here:
* If you can guarantee that you're not going to be driving all of the
solenoids at once, you might be OK with your setup. In fact, if you
can guarantee you're not going to be lighting up more than four at one
time, it's a great solution.
* If you have to do all eight at once, you might want to consider
doubling up on your ULN2803s, and only using half the outputs on each
IC. That will guarantee you will not go much above 1 watt power
dissipation for each IC.
* You might want to take a tip from the programmable logic controller
people, and use your ULN2803s to drive eight small relays capable of
handling the loads. If you do this, be sure to place 1N4002 diodes
across the load to avoid arcing at the relay contacts when they open.
* Instead of a "one IC solution", you might want to consider using
discrete darlington transistors to do the job. Assuming you have a TTL
signal, you might want to try something like this (view in fixed font
or M$ Notepad):
' VCC VCC
' + +
' | |
' 1N4002| C|
' - C|
' ^ C|
' | | Solenoid
' | |
' | |
' | |
' '-----o
' |
' |
' ___ |/
'TTL o-|___|- -|TIP120 or equiv.
' 10K | |> (NPN Darl.)
' .-. |
' 10K| | |
' | | ===
' '-' GND
' |
' |
' ===
' GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta
www.tech-chat.de
This is something like what you'll see with a programmable controller
with a "high current" DC solid State output module. If you pop one
open, you'll see the discrete darlington transistors all in a row, just
like this.
Unfortunately, there aren't any slick answers here. Your load is right
at the point where you have to start thinking about alternative, higher
power control solutions than the trusty, ever-popular ULN2803.
A lot of the digital control of "real world" stuff like solenoids,
heaters, and other stuff isn't that complicated. Look around at the
stuff that's out there, learn from it. Take the time to look at
websites, schematics, and learn from what you see. And take the time
to carefully read all data sheets. They're made to sell the product as
well as accurately characterize it. Most IC manufacturers make a
special effort to make their data sheets as clear as possible to a wide
range of potential customers.
By the way, questions of this type usually are asked at
sci.electronics.basics or sci.engr.control.
Good luck with your project.
Chris