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Power Output vs. Power Dissipation of ULN2803A

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
 
P

Paul Hovnanian P.E.

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

The dissipation of the device* is the current controlled by each
Darlington pair times the voltage drop through the Darlington, which may
be a few tenths of a volt. The load power is this same current times the
voltage drop across the load which, for your application is 24V but
could be as high as the maximum voltage this device is capable of
controlling (I don't have the data sheet, so I don't know hat that is).

* plus a little bit of power that it consumes internally.
 
J

John Popelish

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.

http://us.st.com/stonline/books/pdf/docs/1536.pdf

You are seeing a bit of specmanship, here. Each of 8 outputs is
individually rated for 500 mA, continuous or 600 mA peak. With a full
50 volt supply, and all 8 outputs passing 600 mA momentarily, the peak
power being controlled is 8*0.6*50=240 watts. The power dumped into
the chip is the load current times the on state voltage.

The on state voltage is not specified any higher than 350 mA, but even
that low, that saturation drop can be as high as 1.6 volts. So at
that current times 8 outputs, the chip power would be 8*0.35*1.6=4.61
watts. So it is pretty obvious that no where near 24 0watts can be
continuously powered through this chip.

It looks like your steady state load current is about 5.4W/24V=0.23A.
It is safe to assume that since this is lower than the 0.35 amps that
the 1.6 volt saturation voltage is specified, you can safely assume
that the chip dissipation is less than 0.23A*1.6V=.37 watts. This is
well below the 1 watt power rating for a single output in a 25 C
ambient. In fact, you might get away with using 6 of the 8 outputs
this way, before you exceed the total power rating of 2.25 watts.

Of course, if you need only one driver, why not use a single
darlington or mosfet, instead.
 
L

legg

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.

The switches are rated to 95V.

Obviously they won't control the same power in a 24V system.

RL
 
B

Ban

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?

I have looked at the TI data sheet, which states 500mA(single output) and
50V. Nowhere anything about 260W. There are specs for combined current, and
combined dissipation, which you do not seem to have understood.

Why is the chips power dissipation rating 2W

Because the package can not dissipate more, and even then only with extra
cooling measures.
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

You seem to argue like a digital designer, who has never build anything. You
should learn how to read the data sheet. You will find everything inside.
But even without reading it, common sense will tell you that it is not
possible to simply multiply and add the maximal values for a single channel.
 
S

Spehro Pefhany

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

If your solenoids are DC, then that's 225mA/coil. (If they are AC (!)
you'll likely need something different to drive them-- check this
point because wattage ratings are more typical with AC coils).

I'm not going to look at the data sheet for you, but I'd guess that
you won't be able to drive more than one or two of these safely,
allowing for reasonable ambient rise, chip package and so on. That
chip is a darlington array so it has a *lot* of voltage drop, maybe
over a volt typically. A logic-level MOSFET and 1N400x catch diode per
channel whould allow you to drive this level of current very safely
and they can be designed to run cold, because of the low voltage drop.



Best regards,
Spehro Pefhany
 
Thanks to everyone for all the help!

I guess I should have been more specific about some things:

The 260W power was at 95V, and was not for the 2803A, but rather
another version. The 260W power figure was given by an Allegro data
sheet for a now-discontinued version of the device.

I need to control 80 valves, and will be using a 96-channel PCI digital
I/O card, so I will need several devices, not just one channel.

To my perceptive friend in Italy: You are correct, I do argue like a
digital designer who has never built anything. I am a mechanical
engineer by trade, and although I've used lots of off-the-shelf
industrial control equipment, I've never tried to build any.

Thanks to all for the help.
 
J

James Meyer

Thanks to everyone for all the help!

I guess I should have been more specific about some things:

The 260W power was at 95V, and was not for the 2803A, but rather
another version. The 260W power figure was given by an Allegro data
sheet for a now-discontinued version of the device.

I need to control 80 valves, and will be using a 96-channel PCI digital
I/O card, so I will need several devices, not just one channel.

Also take into account the fact that chip dissipation will increase
momentarily during the time that the valves are being turned on or off. In
other words, If many rapid on/off cycles are anticipated, the dissipation could
be much higher than the DC or steady-state predictions.

I suggest getting one chip and connecting eight solenoids to it.
Exercise all the outputs at a speed somewhat greater than you expect in the
final product. Adjust the supply voltage to a bit higher than you expect the
normal supply voltage to be. Let the test chip run for ten minutes and if you
can then hold a finger on top of it for longer than a few seconds, you'll be
reasonably assured that the completed systen will be OK.

Jim "empirical designs-r-us" Meyer
 
S

Spehro Pefhany

Also take into account the fact that chip dissipation will increase
momentarily during the time that the valves are being turned on or off. In
other words, If many rapid on/off cycles are anticipated, the dissipation could
be much higher than the DC or steady-state predictions.

Really? I have not done the calculations, but I find it hard to
imagine that an inefficient Darlington switch that can switch in the
microsecond time range would get that much more inefficient switching
every 50 or 200 msec or whatever speed a solenoid valve could keep up
with.
I suggest getting one chip and connecting eight solenoids to it.
Exercise all the outputs at a speed somewhat greater than you expect in the
final product. Adjust the supply voltage to a bit higher than you expect the
normal supply voltage to be. Let the test chip run for ten minutes and if you
can then hold a finger on top of it for longer than a few seconds, you'll be
reasonably assured that the completed systen will be OK.

Jim "empirical designs-r-us" Meyer

I think the DC power dissipation is going to dominate. Place 8 chips
side-by-side with tinfoil on top. Apply 64 loads for a few minutes or
until the oil begins to sizzle. Add the beaten egg with a bit of
butter, chopped chives and cilantro. Turn lightly and serve.


Best regards,
Spehro Pefhany
 
C

Chris

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
 
B

Ban

Spehro said:
I think the DC power dissipation is going to dominate. Place 8 chips
side-by-side with tinfoil on top. Apply 64 loads for a few minutes or
until the oil begins to sizzle. Add the beaten egg with a bit of
butter, chopped chives and cilantro. Turn lightly and serve.


Best regards,
Spehro Pefhany

But with 225mA*1.6V=360mW dissipation for each channel there is no way
driving 8 solenoids at the same time from 1 package.
Here is a better advice: You can parallel two drivers and have guaranteed
some headroom left. It will be too difficult for a beginner to design
drivers from scratch, so the choice of an array is a good one. There are
also all protection circuits incorporated, nevertheless I would still solder
some 1A schottky-diodes directly across the coil connections with the
cathode(where the stripe is) to the 24V line. Fairchild sells the SB150,
which would be appropriate for this application.
Unfortunatly this means double the driver ICs, but now you do not need to
think about cooling, if you leave enough space between the devices.
I hope you have some helper with a bit of knowledge to look over your work
before you order circuit boards. In case you need some support, drop me a
mail.
 
J

James Meyer

By the way, questions of this type usually are asked at
sci.electronics.basics or sci.engr.control.

Quite correct. This news group, sci.electronics.design, is more
properly reserved for posts concerning the death of fishes, lame jokes, and
celebrity trials.

Jim
 
But with 225mA*1.6V=360mW dissipation for each channel there is no way
driving 8 solenoids at the same time from 1 package.


I see that. Actually, I've decided to use the ULN2803A to drive some
small relays to control the solenoids. Thanks to all for the help.
 
Thanks for all the straightforward help, Chris. I've done exactly as
you suggested; I've looked at a lot of data sheets and schematics, and
found the best solution (after getting all the great advice here) is to
use the 2803 to operate some small relays. Thanks for the tip on using
the diode across the load.

Thanks again to all who've helped with this question.

gm
 
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