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Using ULN2803 between board and device as a voltage converter

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Feb 27, 2008
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From My SBC Board supply of +3v is coming and External Device need 12votl supply can I use ULN2803 between board and device as a Voltage convertor??

uln2803 schematic

dinesh.4126 said:
hI,From My SBC Board supply of +3v is coming and External Device need 12votl supply can I use ULN2803 between board and device as a Voltage convertor??
Are you asking how to convert 3 volts to 12 volts or you need a level converter?

uln 2803 ic

My point is I am having ULN2803 IC can I used this IC to convert it 3v into 12v.Because 12v I/P need for External Device.
uln2803 led schematic

The ULN2803 is a NPN type darlington arrangement and you'll get Ground from the outputs when inputs are fed with 3 volts. You may use an Optocoupler like MCT2E where the 3 volts can be fed to the LED through a resistor and the collector connected to the +12v and take the output from the emitter. This arrange ment will also isolate the 2 circuits. Hope it helps you. Good luck.

uln board

Please allow me to inject some correct information here. In order to use the MCT2E opto device shown above, you do NOT feed the LED side 3 volts.

The LED side needs to have a controlled electrical current flowing through the LED in a forward biased fashion. Saying feed 3 volts through a resistor to the LED inside the MCT2E you are effectively accomplishing this but to think of the LED as a voltage operated device is bad thinking. Better to say pass a current not to exceed 60 millamps through the LED side of the MCT2E.

By the way that is a odd looking diode in the above picture. Pin 1 is the anode and pin 2 is the cathode, but the cathode in the picture has a line missing.


If the original poster must control a electromechanical relay who's coil voltage rating is 12 volts DC from a logic circuit who's supply voltage is only 3 volts, then a "level shifter is in order". A "current" from the logic that is 3 volts can be used to turn on a bipolar transistor (passing current though the base emitter junction) to switch current through the collector emitter. A 2N2222 NPN transistor (or equiv.) can be used, no need for a MCT2E opto isolator (unless you need electrical isolation as well).

My point in getting involved in this thread is to point out to all readers that LEDs are current controlled, and so are bipolar transistors. If you think of them as devices that turn on and off with voltages you are never gonna understand these solid state devices, and you'll destroy them in a microsecond. You must control the "current" flowing through these parts. Talking about them as if they are like light bulbs does no one a service.

The ULN2803 is basically 8 bipolar NPN transistors in a 18 pin package. (They are darlington transistors but please don't dwell on that).

Using one of the 8 transistors in the ULN2803 to "level shift" from 5 volt logic to 12 volt logic is great. ULN2803 internal transistors have the gain to take tiny current from the 5 volt logic, and switch heavy currents (upto 500mA) on the 12 volt board and are set up internally to deal with 5 volt logic. HOWEVER, the original poster said they had 3 volt logic, not 5, the transistor array you mentioned is NOT the best array choice. Look at the ULN2801...the difference is that the ULN2803 you mentioned has 2.7K resistors on the transistor bases, all ready for 5 volt logic. The ULN2801 I mentioned has no base resistors in the package so you put one in your circuit and you calculate the proper resistor for your 3 volt logic to get the proper behavior from the darlington inside the ULN2801. If you don't need 8 transistors in the array look at the ULN2001 chip. It costs much less and has only 7 transistors. If you only need to switch one 12 volt relay from 3 volts, use a single transistor like 2N2222 or equiv. biased properly with the appropriate base current drive. The ULN2803, 2801, 2001 are arrays for when you need to switch 7 or 8 different things on and off. If you use the 2N2222 device, don't neglect to put a snubbing diode across the relay coil ! If you use the ULN2xxx array be sure to tie the clamping diode pin to +12v to snub back EMF.

Per the MCT2E.... that can be used as a level shifter (for logic) but it is not going to have enough current gain capability to drive the relay coil directly on the transistor side of that part. A transistor is going to be required anyway, so unless (electrical) isolation is required (and the original poster did not say it was) then the MCT2E is basically useless.

If the original poster needs to produce 12 volts from 3 volts, then he needs charge pumps or a DC to DC switching converter / up switching power supply, but I don't think that is what he meant.

uln2803 required external protection diode?

I think the original poster might be talking about boosting the 3v power supply to 12v so he can power the external device that runs on 12v.

the answer to that is "depends".

you certain cannot level shift as you don't already have a 12v power source.

you can configure one of the npns into a switch and create a boost converter to raise the 3v to 12v, using additional parts and with some limitations.

uln2803a led drive circuit drawing

Using the original posters choice of words, in particular the words "convert it 3v into 12v" I'm still unsure, but I guess if I was forced to choose, he might be saying he wants to make 12 volts from 3 volts, in that case, he'll need to create or purchase a DC to DC step-up converter.

The OP's addition of the word "it" between the words "convert" and "3v" adds to the confusion.

I have created them (DC to DC step-up converters). Not really a big deal..... you don't need a ULN2803 to do so. In fact the darlingtons are not the best choice if you are endeavoring to create a roll your own DC to DC switching power supply (step-up). These days MOSFETS are the way to go, or just get a step up switching regulator IC such as Unitrode UC2577 and use it per the app note in the data sheet.


It gets tough on the internet with language. I'm sure I would be bad trying to communicate in his native language, but for me to help him he has to do better at using my language.

how to connect 2 pin single hooter

My Query I have to drive Hooter at 12V using ULN2803 with I/p of 5V coming from Board.So is it possible using ULN2803 because ULN2803 is high voltage high current darlingtion drive.

supply voltage of uln2803ag

No. It is not possible/recomended. The hooter may take some what more than 500 ma which is the maximum capacity of the ULN. Feeding the input of the ULN with 5volts will pull the related output pin to ground where you may connect a relay which will then drive the hooter. Good luck

uln2803 8 pins

Hooter is 12v at 1A.So Final its possible.But you have also
Feeding the input of the ULN with 5volts will pull the related output pin to ground where you may connect a relay which will then drive the hooter??

circuit schematic of hooter

If it is mandatory to use ULN2803, you may us as given in the schematic below. As you are using only one channel if ULN just of single relay to drrive the hooter, you may also choose to driver the relay with single transistor as given in the second schematic. Hope it helps you. Good luck

uln2803 protection

Thanks for prompt reply.
In case of ULN2803 will I supposed to connect Vcc of 12V or I need to connect common to External Supply of 12v.

uln2803 pin configuration

ULN2803 as it is darlington arrnagement inside, doesnt require Vcc to worlk. The Vcc pin (10) is the anode of the protection diode built inside the chip. It will prevent relay chatters in case any EMI problems across the relay coil. You may see the internal circuit on the datasheet of the same. Good luck

uln2803 5v 12v

Just clear me one more thing Is Relay connected to +12v external supply??

uln2803 current

This thread is getting really complicated!

1. ULN2803 is a transistor array with internal clamp diodes. Nothing more.
2. It cannot CREATE a 12V supply from 3V, 5V or any other voltage.
3. It is capable of switching higher voltages but the voltage has to come from somewhere else.

So your answer is, Yes, you have to provide another source of power.


uln2803 schematic

Now Just tell me simple way for DC-DC voltage Boost so that I can get 12v o/p.

uln2803 12v

You will have to use a step-up converter (also known as a boost converter). The easiest to use is probably the UC2577 as mentioned earlier but you must be aware of the amount of power you can draw.

Your hooter needs 12V and your supply is only 3V. So, you want to increase the voltage by 4 times. Even if the boost circuit is 100% efficient (which it isn't) you would require 4 times more current from your 3V supply than you draw from your 12V line.

You say the hooter needs 1A so your would need to draw more then 4A from your board, I would allow at least 5A to take into account that the converter is less than 100% efficient. Is this more than your board can supply?

It might be simpler, and would certainly cost less, to build another 12V supply and use that instead. You can still control the hooter from your SBC of course.


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