Darlington Drivers - maximum output (sink) current?

I searched the forum but didn't find this. Can someone clear this up for me. I've always taken the maximum current of the darlington drivers as used in the project boards as 500mA maximum total current and that's what I've always understood, but if you read the datasheet it's a bit confusing.

From Picaxe website - 18m2 project board:

The board is supplied with all essential PCB components including IC and download sockets plus a ULN2803A Darlington driver buffer chip that enables you to connect higher power output devices (such as motors, solenoids, relays) directly to the board (current up to 500mA).

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From datasheet for darlington driver:

The ULN2801A-ULN2805A each contain eight darlington transistors with common emitters and integral suppression diodes for inductive loads. Each darlington features a peak load current rating of 600mA (500mA continuous) and can withstand at least 50V in the off state. Outputs may be paralleled for higher current capability.

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It's confusing as to what they mean by 'Darlington' as both the whole component and the individual transistors inside are being called 'Darlingtons'. If each individual darlington transistor of the 8 total can have a peak load of 500mA and two can be linked together for higher current draw, that would suggest a total maximum load of 4 Amps but I'm assuming that's not right as that's not what the graphs show.

But in that case what do they mean by "Outputs may be paralleled for higher current capability"?

I assume you can spread the 500mA over 1 output or 8 as long as you don't exceed the maximum.

The term Darlington Array merely refers to a popular transistor configuration. A darlington array can be built using individual transistors or several transistors integrated into a single chip.

DESCRIPTION
The ULN2801A-ULN2805A each contain eight darlington
transistors with common emitters and integral
suppression diodes for inductive loads. Each
darlington features a peak load current rating of
600mA (500mA continuous) and can withstand at
least 50V in the off state. Outputs may be paralleled
for higher current capability.

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Each array within the chip is capable of driving 500 mA but keep in mind the chip needs adequate heat sinking if you plan to run all eight arrays at 0.5 Amp.

Ron

jensmith25 said:

If each individual darlington transistor of the 8 total can have a peak load of 500mA and two can be linked together for higher current draw, that would suggest a total maximum load of 4 Amps but I'm assuming that's not right as that's not what the graphs show.

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That is my understanding; each output can sink 500mA.

The datasheet I have does not have any graph but reads to me that the maximum chip current is 2.5A so it would not be within spec to sink 4A, 8 x 500mA.

Careful! That's .5A Total for the entire chip!

The ULN2803A chips are useful for driving small relays because the suppression (spike) diodes are built in. Because the outputs are open-collector, they can be connected together (but if the inputs are not tied together, they should be enabled/disabled together). This would be useful for driving larger relay coils where the inrush current might exceed 600mA.
I use ULN2803A chips to drive +12v relays via a Picaxe or other uProc.

Look at the attached .pdf page 4 for example of ULN2803A parallel outputs. Have fun!

hippy said:

That is my understanding; each output can sink 500mA.

The datasheet I have does not have any graph but reads to me that the maximum chip current is 2.5A so it would not be within spec to sink 4A, 8 x 500mA.

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Hippy - this is the datasheet from the Picaxe website for the ULN2803A: http://www.picaxe.com/docs/ico050.pdf

mikeyBoo - so you are saying it's 500mA total?

Power Dissipation
(one Darlington pair) 1.0W
(total package 2.25W

I believe what it will come down to is what the total package can dissipate. Again, consider the heat sinking of the chip.

Ron

jensmith25 said:

Hippy - this is the datasheet from the Picaxe website for the ULN2803A: http://www.picaxe.com/docs/ico050.pdf

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Thanks.

It does seem from that, from Figure 11, that if all 8 outputs are continually conducting, each output should not exceed 150mA, 1200mA total.

And if only 2 channels are continually conducting, each output should not exceed 400mA, 800mA total.

I would guess why that 1200mA capability isn't available for just two channels is down to heat dissipation with some sort of squaring or similar in the calculation which makes it non-linear. Hopefully a proper electrical engineer will come by and explain it

Added: Just realised Figure 11 is for Tambient=70C and Figure 12 is for Tambient=50C which allows a bit more. Looking at Figure 10, one has more to play with at lower Tambient but I am not sure how one determines how much.

jensmith25 said:

mikeyBoo - so you are saying it's 500mA total?

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That has been my understanding (but I'm not infallible). The data sheet I have (Motorola) specs 500mA max Ic. Now, ALL of the drivers return through the same ground pin, so the individual Ic currents add up.
You are right, the data sheets can be a little ambiguous. However, can't get around objective reality, so has anyone got a few ULN2803A chips they wouldn't mind destroying? Would make a fun experiment to see when the smoke gets out (wear glasses & use a picofuse on the breadboard !!). Just need to monitor the current coming out of the ground (power supply Common) pin on the chip & measure the chip temp.
My rule of thumb is to put the side of my little finger on a component when it's running at maximum load. If I jump back & say a curse word, then it's overloaded or needs external cooling. Of course, if the smoke gets out, no need to burn your finger.
Hey, if it was easy, wouldn't be any fun!

mikeyBoo said:

jensmith25 said:

mikeyBoo - so you are saying it's 500mA total?

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That has been my understanding (but I'm not infallible). The data sheet I have (Motorola) specs 500mA max Ic.

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But does Ic not indicate Icollector which would only make sense per channel ?

Also 500mA per channel with "Outputs may be paralleled for higher current capability", suggests the chip must be able to support greater than 500mA total.

At least I don't feel like i'm the only one that's a bit confused!

hippy said:

Also 500mA per channel with "Outputs may be paralleled for higher current capability", suggests the chip must be able to support greater than 500mA total.

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That's what I thought and why I decided to ask the question.

hippy said:

But does Ic not indicate Icollector which would only make sense per channel ?

Also 500mA per channel with "Outputs may be paralleled for higher current capability", suggests the chip must be able to support greater than 500mA total.

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hi hippy (enjoyed your posts over the years),
Yep, I get what you’re saying, that’s why I suggested an empirical test. I checked my workshop & I’ve got plenty of ULN2803A chips, so I’ll set up a test this evening (or tomorrow morning). How ‘bout 800mA spread across 8 outputs at an ambient of 75degF? Then if no smoke, 1200mA?
I’ll take a few pics if anybody’s interested, might be fun!
How’s this for ambiguous:
from: http://www.jameco.com/Jameco/Products/ProdDS/34315.pdf
“Typical power loads totaling over 260 W (350 mA x 8, 95 V) can be controlled at an appropriate duty cycle depending on ambient temperature and number of drivers turned on simultaneously. “
Now quantitatively, what exact value(s) is “appropriate duty cycle” (well, it depends). Really all boils down to the thermal breakdown of the chip (which changes with ambient temp. & air flow).

I look forward to the results!

jensmith25 said:

Hippy - this is the datasheet from the Picaxe website for the ULN2803A: http://www.picaxe.com/docs/ico050.pdf

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Figure 11 shows that with 1 or 2 outputs used, 400 mA each at 100% duty cycle (continuous) rising to 500 mA as duty cycle reduces to around 75%.
The figure also shows that as more of the 8 channels/outputs are used, the current per channel reduces so that at the other extreme, with 8 outputs operating at 100 % duty cycle the max is 150 mA per output.

Thermal ratings are the constraint.

The UDN2981 (high side equivalent) which I have used has a similar graph.
I have operated with 2 or three channels at higher continuous current (~100 mA) and the rest at around 20 mA driving LEDs on numerous chips. They get warm but not excessively hot (I did do the power/heat dissipation Calc).

If you are going to run at higher total current levels and heat dissipation, you can buy various small heat sinks about the same area as the IC to fix or glue to the IC to increase the surface area and help keep the IC cooler.

EDIT:
Some heat sink info here:
http://electronics.stackexchange.com/questions/85483/how-to-cool-down-dip-devices

From SGS-Thomson datasheet: http://www.soemtron.org/downloads/disposals/uln28012805.pdf

Max dissipation for one Darlington is 1Watt ...roughly = saturation voltage x output current = 2V x 0.5A.
Max dissipation for package (total) is 2.25W. (Note: a heatsink won't extend dissipation beyond this limit).
...so 3 x parallel outputs might give you just over 1A, depending upon saturation voltage for each output.

The rising saturation voltage with rising output current should be noted.

You should be able to use several packages and cross parallel for better results (e.g. IC1a para with IC2a should give you 1A max) but this may need testing.

In datasheet see Absolute Max Ratings, Fig 8 and Fig 12.

Just to add to this I was looking at the datasheet for the Picaxe 28 project board and it states in the text that the ULN2803A can drive up to 800mA PER OUTPUT!

Each of the outputs is driven through a ULN2803A darlington driver, which will drive up to 800mA per output.

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http://www.picaxe.com/docs/AXE020.pdf

I believe the AXE020 "800mA" capability originated from a typo which was in the ULN2803A datasheet we were using. I will log that issue.

ULN2803A test

hi folks,
Well, I ran a little test to see how current effects ULN2803A chip temperature. (See .pdf file if you wanna' see test setup.)
Here are the results (used a Toshiba chip, test could vary by chip maker):

Total current is divided equally across the number of enabled outputs (each output sinks ~.25A when ON)

Ambient # of Outputs On Amps Chip Temp Ouch Factor (little finger test)

76degF 2 .51 129degF (53degC) no problem

78degF 4 .98 160degF (71degC) can’t leave finger on, but not instant burn

78degF 6 1.41 186degF (85degC) hot, get finger off quick

Decided to end test here, chip getting too hot!!

While it is possible to run any or all outputs at maximum collector current (0.5A), chip heating quickly becomes an issue. Since the outputs are open-collector, they may be combined for more current capability (of course inputs may also be combined).

@mikeyBoo,
Based on figure 11 in many of the various manufactures ULN2893 data sheets,
(See link originally given by Jen and also Steves link at post 16)
with 6 channels/outputs continuously on the current should be limited to around 190 mA for thermal/heat reasons.

For information as a "touch test" , around 60degC is the highest temp at which one can leave your hand continuously on the hot item.

Thanks mikeyboo for doing the test and westaust55 for the extra information. So it looks like you can push it higher than 500mA then but you need to do it with care and spread the load evenly or keep it to no more than 800mA for 2 outputs.

Hi Jen,

No, the absolute max current for any output channel is 500 mA.
With 2 channels operating continuously the max current will drop to ~400 mA for each of the two channels in use.

As hippy has indicated, the 800 mA was a typo from a "flawed" data sheet

Sorry, that's miscommunication on my part - I meant 400mA max each output if there's only 2 in use so 800mA spread between the two. Sorry for the confusion.

Since a darlington array is being used to SINK current rather that putting OUT current the title of this thread has a tendency to mislead... ;-0

premelec said:

Since a darlington array is being used to SINK current rather that putting OUT current the title of this thread has a tendency to mislead... ;-0

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Technically you are correct, however the data sheets from various manufacturers refer to the pins as "outputs".

Eg "Output current to 500 mA"

I have seen - over these many years in this forum - questions about the circuit by people expecting drive current from their boards and not getting any - so i thought a reminder of the common function could be useful... as is the darlington... ;-0

It looks like the ULN2XXX series devices are a bad choice for load currents above about 400mA; although the absolute max is 500mA, the recommended max is only 400mA (see SGS-Thomson datasheet).
Also, for a production circuit, you need to operate well below the 2.25W absolute package dissipation figure to ensure reliability over a range of conditions.


On the other hand, experimentation (sometimes crazy experimentation) is a cornerstone of DIY electronics!