Darlington issues

[Pat13]

Hello all, I am having some issues with a darlington circuit. Here is the setup:
08M2 driving the darlington (ULN 2803) which is switching on/off 12VDC vent motors under minimal load.
The 08M2 outputs are going high/low in accordance with the code. The darlington is switching in accordance with the code. The problem is when the pins go low, the darlington is still showing 6.33V at the outputs which means the motors are still running, albeit slowly. I have tried several different darlington chips with the same result, which leads me to believe it is a wiring issue. Should I put resistors between the picaxe outputs and darlington inputs? In manual 3 it shows the circuit without resistors.
thanks in advance for your help.

 

[oracacle]

circuit diagram and program will be extremely helpful

 

[Technical]

Something is not right, so we need a circuit diagram, and also the current rating of the motors?

 

[Pat13]

Something is not right, so we need a circuit diagram, and also the current rating of the motors?

Sorry, do not have a diagram. However, the circuit is exactly like the one on page 7 of manual 3. I have a uncoupling cap across the +V and 0V of the 08M2, but do not have one across the + and 0 of the darlington. 12VDC to power the motors. As well no resistors between picaxe and darlington, or between darlington and motors (as stated, exactly like manual 3). No diodes on motors, as the darlington already has them. Unloaded current draw of the motors is 20 ma. Here is the code

'#08M2 random arm movement
symbol SHOULDER=C.1
symbol ELBOW=C.2
symbol MAGICHAND=C.4

symbol SHLDR_MOVE=w3
symbol ELBO_MOVE=w4
symbol MAGICHAND_MOVE=w5

Main:

			
	
		random SHLDR_MOVE
		random ELBO_MOVE
		random MAGICHAND_MOVE
		let w3=w3//7000+2000
		let w4=w4//7000+2000
		let w5=w5//7000+2000
		High	SHOULDER
		pause w3
		High ELBOW
		pause w4
		Low SHOULDER
		PAUSE W3
		High MAGICHAND
		pause w3
		Low ELBOW
		pause w4
		Low MAGICHAND
		pause w5

	goto main

 

[eclectic]

High-resolution photographs of the board?

e

 

[hippy]

Have you tried a simple HIGH / PAUSE 1000 / LOW / PAUSE 1000 loop ?

A program which quickly toggles an output pin could well give the illusion of 6V appearing on a Darlington output when it is actually repeatedly alternating between 0V and 12V at high speed.

 

[SAborn]

Do you have pin 10 of the ULN2803 connected to 12v+ as it should be, it sounds like you might have pin 10 connected to 5v+.

 

[Pat13]

Do you have pin 10 of the ULN2803 connected to 12v+ as it should be, it sounds like you might have pin 10 connected to 5v+.

Thanks SAborn that's exactly it. I knew it would be simple and obvious. Thanks again.

 

[westaust55]

Should I put resistors between the picaxe outputs and darlington inputs? In manual 3 it shows the circuit without resistors.thanks in advance for your help.

If you look in the ULN2803 datasheet (see:http://pdf.datasheetcatalog.com/datasheets/105/366825_DS.pdf)
you will see on page 2 with the link I gave that the ULN2803 has a 2k7 Ohm resistor in series with the input base for each Darlington channel.

While you have resolved the problem now, You can speed up the process of fault finding by providing a couple of clear good resolution photos showing both sides of any strip/vero/specific and similar project printed circuit boards.

 

[g6ejd]

The problem is the PICAXE output does not have a low enough output impendence to pull the Darlington input to the off-state, try adding a resistor, say 47K from the PICAXE output pin to ground.

 

[westaust55]

The ULN2803 uses BJT technogy, not FET's
With the Darlington needing in excess of 1 volts at the base to switch on and a PICAXE output in low state down to <0.3 volts I have never had a problem with switching off a ULN2803.

The ULN2803 already has a string of 3 resistors from the input to ground that total 12.9 kOhm

 

[SAborn]

I think the problem PAT13 had was he had 5v to pin 10 (com) and not 12v, 5v plus the diode loss will give about 6v back feeding to the 5v rail and hence why he thought the darlington did not switch off, the darlington was switching off and the voltage on the output was from the differential of the 2 voltages on the output side.

Its a problem i have seen often on different forums and a common mistake many make.

 

[srnet]

This also highlights the benefits of drawing circuit diagrams, even if its only on low tech pencil and paper.

With a 'hard copy' circuit diagram in front of you, its very easy to check that the circuit (especially when copied from a book) is actually wired as it should be.

Get out the multimeter, and tick each connection as its tested, I usually use a coloured marker to mark the connections as checked.

 

[SAborn]

Yeah, we always ask for a circuit diagram, but often it can be solved with just text to the problem, its really a matter of if someone has crossed the problem before hand and knows the solution.
If not then perhaps pen to paper might be needed, but often a text question can provide the answer, so lets not get too hung up on requiring a schematic first up unless all else fails.
I do agree a picture speaks a thousand words, but often we just need to air the problem to the wide view to see what one has missed.

I have asked many questions to the forum and always got an answer or help, And not yet supplied a schematic, even though a few members have always requested a schematic, and really it was not required for the question i asked.

I often hear on other forums how on this forum members wants to see everything in a schematic to solve a problem, and often its a bigger task for someone to draw the circuit than it took to build it, and yes i agree a schematic can solve a problem faster, but it should not be the first question always asked by a member in reply to the post.

 

[ValueAdd]

It can also depend upon how well the poster of a question/problem can explain in a clear manner (hopefully avoiding slang) what is wrong and what tests they have done.
Also providing a web link to less frequently used parts.

A schematic can help however, in its own right does not guarantee that the project was wired correctly and a couple of good clear photos as also often requested can help by showing where the problem lay when it is wiring/connection orientated.
Often posters will surmise that the problem is either hardware or software and base their question accordingly when in fact the problem is the converse of their surmise.