mestora70
Posts: 11
Joined: Wed Aug 15, 2012 10:46 pm

Is 3V3 GPIO HIGH voltage enough to fully turn on ULN2803a?

Wed Aug 15, 2012 10:56 pm

Looking at the datasheet for the uln2803 (Rev E) I see the following for Vi(on) on state input voltage

Ic=200mA 2.4v
Ic=250mA 2.7v
Ic=300mA 3.0v

Extrapolating, even if pulled up, it looks like the Gertboard can only be relied to switch about 350mA on the open collector driver, not the device rating of 500mA. Am I missing something?

Mike Stora

mjtessmer
Posts: 27
Joined: Sun Nov 20, 2011 12:50 am

Re: Is 3V3 GPIO HIGH voltage enough to fully turn on ULN2803

Thu Aug 16, 2012 12:31 am

You are not missing anything. Transistors are current amplifiers.
Collector current equals hfe(current gain) * base current.
Base current is determined by voltage input divided by base
resistance (which is approx 10k ohms).

User avatar
mahjongg
Forum Moderator
Forum Moderator
Posts: 12672
Joined: Sun Mar 11, 2012 12:19 am
Location: South Holland, The Netherlands

Re: Is 3V3 GPIO HIGH voltage enough to fully turn on ULN2803

Thu Aug 16, 2012 12:35 am

I don't think extrapolating works here.
If you look at the VCE(sat) (saturation voltage, that is the voltage with the darlington fully open) then you can see that at an input current of 500uA (micro Amp) the darlington is fully open, they do not go further than 350mA as 500mA isn't a normal operating current, but the absolute maximum current (beyond which you would damage the IC)

The input as seen by the GPIO consist of a 2K7 resistor and two diode drops. (see page 2).
That means that over the diode 3.3V minus two diode drops will fall, A (base diode) diode drop is typically 0.6V, so 1.2 Volt falls over the diode drops. So over the 2K7 resistor falls 3.3 - 1.2 = 2 Volt.

That means that through the resistor runs a current of I = U/R = 2/2.7K in mA = 0,74 mA or 740 uA, or 50% more than what is needed to drive the darlington into saturation.

That means the transistor will be driven fully open!

Also see figure 8, where you can see that the test is done with an input voltage of just 3.0 Volt.

So yeah, with the 3V3 drive of the PI the darlington will be driven fully into saturation.

You are advised to choose the darlingtons load resistance so that the 500mA absolute maximum limit is not exceeded.

Neil
Posts: 98
Joined: Thu Sep 29, 2011 7:10 am
Contact: Website

Re: Is 3V3 GPIO HIGH voltage enough to fully turn on ULN2803

Thu Aug 16, 2012 12:18 pm

Also take note of the maximum substrate-terminal current of 2.5A. So that's the total current for all outputs combined as it all flows out of the common GND pin. At 500mA you can only drive 5 outputs.

User avatar
Gert van Loo
Posts: 2487
Joined: Tue Aug 02, 2011 7:27 am
Contact: Website

Re: Is 3V3 GPIO HIGH voltage enough to fully turn on ULN2803

Thu Aug 16, 2012 5:46 pm

The data sheet I have from an ULN2803a says 500mA TOTAL maximum.

User avatar
Grumpy Mike
Posts: 932
Joined: Sat Sep 10, 2011 7:49 pm
Location: Manchester (England England)
Contact: Website

Re: Is 3V3 GPIO HIGH voltage enough to fully turn on ULN2803

Sat Aug 18, 2012 6:19 pm

Gert van Loo wrote:The data sheet I have from an ULN2803a says 500mA TOTAL maximum.
My data sheet says:-
Each darlington features a peak load current rating of 600mA (500mA continuous) and can withstand at least 50V in the off state.
So that is each one of the 8. However, thermal considerations restrain the total for the chip to about 650mA.
Here is the reasoning:-
http://www.thebox.myzen.co.uk/Tutorial/ ... mples.html

mestora70
Posts: 11
Joined: Wed Aug 15, 2012 10:46 pm

Re: Is 3V3 GPIO HIGH voltage enough to fully turn on ULN2803

Wed Aug 22, 2012 12:11 am

Thanks Grumpy Mike. It looks like driving 8 160mA automotive relays (using the max saturation VCE--typical is better), gives 1.56W dissapation and a package temp 122C above ambient--just about the limit for this chip.

Unless . . .
Image

shortcircuit
Posts: 14
Joined: Sat Aug 04, 2012 5:14 pm

Re: Is 3V3 GPIO HIGH voltage enough to fully turn on ULN2803

Fri Sep 28, 2012 6:55 pm

mahjongg wrote: You are advised to choose the darlingtons load resistance so that the 500mA absolute maximum limit is not exceeded.
A late reply but hopefully not too late: You should never ever go near the absolute maximum ratings for any component !
Doing it is begging for trouble :)
Read the fine print slightly below in the data sheet:
Stresses beyond those listed under “absolute maximum ratings” may cause permanent damage to the device. These are stress ratings only, and
functional operation of the device at these or any other conditions beyond those indicated under “recommended operating conditions” is not
implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
So in this case do not exceed 350 mA per channel.
The worst part of censorship is ###### ########

Return to “HATs and other add-ons”