Using a 3 Volt power supply to power LEDs with a 2N3904 will this work lower do

sam_sam_sam

Can you use the ULN2803A and a Basic Stamp this way

Powering the Basic Stamp on 5 volts and the ULN2803A on 3.3 volts


am I going to run in to any Problem·with this setup


I have use the ULN2803A many· time on more voltage than the Basic Stamp with no problem··but not with less voltage than the Basic Stamp voltage



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Sam

Post Edited (sam_sam_sam) : 11/23/2009 2:36:23 AM GMT

Mike Green

Have you looked at the datasheet for the ULN2803A? What do you think?

The ULN2803A isn't really powered by Vdd. It's a Darlington transistor array and there's an equivalent diagram of one stage in the datasheet. The only things that are connected to Vdd are the snubber diodes and those don't even conduct from Vdd.

sam_sam_sam

ABSOLUTE MAXIMUM RATINGS is 50 volts

It dose not·say any thing about a·working voltage or a min voltage that why I ask this question

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Sam

Mike Green

You have to look beyond the absolute maximum ratings warnings. For example, what's the minimum input voltage? It's abbreviated Vi. Like I said, it depends on the current being conducted. The collector to emitter saturation voltage (Vce-sat) tells you the voltage across the transistor (collector to emitter) when it's fully switched on. It needs to be greater than the applied voltage and affects how much voltage is "left" for the load to use.

sam_sam_sam

I do not understand what· it·is really telling me here in regard to what I am asking about

What would be max mill amps @ (3.3 volts power supply)·and what would be the· working voltage be·



VCE(sat) Collector-emitter saturation voltage············ ······· ··typ····· max
II = 250 μA,······ · IC = 100 mA,····························· ··0.9······ · 1.1
II = 350 μA,······ · IC = 200 mA,································1········· · 1.3·················· · V
II = 500 μA,······· ·IC = 350 mA,····························· · 1.3······· ·1.6

VI(on)·· On-state input voltage································ ·············· ·· ·· max
VCE = 2 V,·········IC··= 200 mA············································ · · 2.4
························ · ·IC = 250 mA··· ············································ · 2.7···················V
··························· IC = 300 mA···················································3

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Sam

Post Edited (sam_sam_sam) : 11/11/2009 4:05:44 AM GMT

Mike Green

Vi tells you the minimum input voltage to the ULN2803A from the Stamp I/O pin for the input to be treated as a HIGH and turn on the transistor.

Vce(sat) tells you the voltage measured across the transistor when the transistor is turned on (saturated). This varies with the amount of current being carried. If you're switching a 200mA current, the typical Vce(sat) is 1V. If you're using a relay as a load and the power supply is 3.3V, you'll have 3.3V-1V = 2.3V to activate the relay. Some 3V relays won't pull in at 2.3V. You may need a relay designed for a lower voltage (than 3V) or you may need to increase the supply voltage to compensate.

sam_sam_sam

Mike Green

Thank You For explaining how you read this Data Sheet I now I understand how to do this



I going to run LEDs banks·with this set up·...........>>>>>>>>>>>>>



I might·use a ADJ switch Regulator for· ULN2803A and ADJ the regulator so when I have a load on it ·I getting 3.3 volts out on the ULN2803A pins

.........>>>>>>>> So·do you think··this will ·work


Thanks for all of you help in this matter


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Sam

PJAllen

sam_sam_sam,

The ULN2803 does not output a voltage.· That is a·common misunderstanding.· The 2803 output, an open-collector output, is like a switch that opens/closes a/the current's·path to ground (V_ss, et al.)

[noparse][[/noparse]The·2803 does not run from a positive voltage connected to its pin 10 either.]

Mike Green

When you're driving LEDs, you will need a series resistor to limit the LED current. That resistor can be changed to provide the current you need at pretty much any supply voltage. The main reason for using a separate supply from the Stamp is to minimize the power wasted in these series resistors.

Other options might be to use an NPN switching transistor with the LED and a series resistor in the emitter lead, the collector directly connected to the + power source, and the base connected to the Stamp I/O pin. This is an emitter follower configuration and the negative feedback would keep the LED current fairly stable and the transistor would be doing most of the switching.

sam_sam_sam

·This is going to run on solar power and a battery set up· this going to be a· must

Mike Green Said

The main reason for using a separate supply from the Stamp is to minimize the power wasted in these series resistors.


Is there a chip like ULN2803··that can·use a separate supply··with this type of set up

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Sam

Javalin

Parallax to the rescue as always - have you read...

http://www.parallax.com/Portals/0/Downloads/docs/cols/nv/vol1/col/nv6.pdf

James

sam_sam_sam

Mike Green

Do I have this right what you had in your post above

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Sam