driving relay via ULN2803 !!

[loup-garou]

Hi there,

I tried to simulate (with ISIS) a simple circuite that drives a relay via the ULN2803 IC, but I faild.

the relay has the following caractarestics:

• nominal coil voltage:12V
• coil resistance: 200 ohm

the current (displayed by ampermetre) is 40mA insted of 60mA= 12/200.

what can be the reason ?

I attached circuit scheme + simulation file.


thanks in advance.

 

[Nigel Goodwin]

Measure the voltage drop across the IC, it's probably losing a fair amount.

 

[ericgibbs]

hi,
Nigel beat me too it.!

Does your simulator have a voltage probe, if YES, then measure the voltage at the ULN pin18 where the relay coil is connected to.

 

[loup-garou]

hi all,

thank you for your help,


when I push the button, I get 4V in pin 18.

 

[Boncuk]

The relay is OK. Just as already stated the transistor looses exactly 4V.

Here is the relay connected directly to 12V. The current is correct at 60mA and a coil restance of 200Ohm.

If your circuit is OK then the current should be 53mA (ULN2003 is equipped with darlington transistors)

The ULN2003A has a built in base resistor of 2K7. The ULN2001A has none. May be you should try that and use a lower resistance value at the base. On the other hand, why are you worried? 40mA instead of 60 for a G2R relay is good enough.

Also the switch is connected to VCC. If it has no explicit voltage label the voltage there is +5V which might be too low. Connect the switch to +12V and see what happens.

Boncuk

 

[loup-garou]

The relay is OK. Just as already stated the transistor looses exactly 4V.

Here is the relay connected directly to 12V. The current is correct at 60mA and a coil restance of 200Ohm.

If your circuit is OK then the current should be 53mA (ULN2003 is equipped with darlington transistors)

The ULN2003A has a built in base resistor of 2K7. The ULN2001A has none. May be you should try that and use a lower resistance value at the base. On the other hand, why are you worried? 40mA instead of 60 for a G2R relay is good enough.

hi Boncuk,

I've tried with ULN2001A, and it ran correctly.I also put 16V insted of 12V (in schematic with ULN2803) as nominal coil voltage to compensate the voltage lost (4V) due to the resistor and the simulation ran corretly also.(see attached)

Also the switch is connected to VCC. If it has no explicit voltage label the voltage there is +5V which might be too low. Connect the switch to +12V and see what happens.

Boncuk

I use 5V to simulate the output of a PIC which has low current level (15-20mA) cause I'll drive many relays with a PIC via the ULN2803.



but in pratical, what should I do when using this ULN2803 ? increase the coil voltage ? or use a resistor in // with ULN2803 output ?


thanks a lot Boncuk.

 

[ericgibbs]

hi,
The 2803 will work OK with a +5V input, the Vce saturation at the output for about 60mA should be approx +0.9V.

The high Vce sat, problem could be within the Simulator..

 

[loup-garou]

hi,
The 2803 will work OK with a +5V input, the Vce saturation at the output for about 60mA should be approx +0.9V.

The high Vce sat, problem could be within the Simulator..

hi ericgibbs,

I did not understand well how you did find the Vce sat value, but I used to think that the ULN is able the give the exact current needed by the relay's coil.

here, I think the problem also come from the resistance of the ULN.
(see attached)

 

[ericgibbs]

hi,
Look at this:

EDIT:
With a Vce sat of say 0.8v, leaves 11.2V/200R = 56mA

I would suggest thats OK to operate a 12V relay.