Need hardware advice: ULN2003A

[Christopher4187]

Hi everyone,

I have a device where I am driving two relays (Digikey PN#PB110-ND) with a ULN2003A (Digikey PN#296-1368-5-ND) from a 16F688. I have tested this layout quite extensively from January of this year without using flyback diodes to supress the voltage when the field collapses. I have not seen any failures yet and it does appear that the ULN2003A protects itself. I read the data sheet and it says, "Each consists of seven npn Darlington pairs that feature high-voltage outputs with common-cathode clamp diodes for switching inductive loads." Forgive me for not really understanding this situation as I don't have an EE degree but I would like to know if I really NEED to add flyback diodes. I know it's common practice when using a simple NPN transistor but again, do I really NEED them with a ULN2003A.

Thanks in advance for the help.

Chris

[sayzer]

Hi Chris,


Those internal diodes are not reliable for driving relatively large bobbins.

I always use 1N4001 in parallel.


If you are diriving a tiny relay, I do not think you will REALLY need an external diode.
Otherwise, I would not play a gamble. You never know when that flyback will occur.

[Melanie]

For driving inductive loads (eg Relays or Solenoids) up to say 50mA those internal protection diodes are just fine. I use the ULN2003, 2004 and 2803 extensively without problems.

[Christopher4187]

Thanks for the reply Melanie. I checked the data sheet and the coil current is 77mA. Have you ever tested the ULN at this coil current with no external diode or is this in the region of unknown?

Thanks,

Chris

[sayzer]

Chris,

Are you avoiding a diode from being used on the PCB board?


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[Christopher4187]

Well, it's a long story but I can't fit a 1N4001 diode on the board. However, if I need to, I can fit an SMT SOT-23 diode on the board for this purpose. I have some units in service now and my main reason for asking was to know if I should expect problems later on. Like I said, I can't add a thru-hole diode but I can add an SMT verision. It's 500ma SOT-23.

[Christopher4187]

Luciano,

Thanks for pointing out the pin 9. I setup one of my alarms to cycle the relays every second and I measured the voltage when the field collapses. Based upon what we discussed here, I should have expected this. The voltage when the field collapses is 64 volts but when pin 9 is connected, it is only 12.5 volts. Of the 90 units I have in the field, none of them have pin 9 connected. I guess it is not a matter of if but when they will fail.

I will let this test alarm cycle until the ULN2003A fails. At least I will have an idea when I should expect them be returned for warranty repair. Also, I will post when the ULN2003A fails if anyone is curious. If there is anything positive to come out of this, at least I caught the issue now with 90 alarms in service as opposed to 1000 alarms in service.

Thanks again for pointing that out. You probably saved me a lot of headaches wondering why my alarms are going to fail in the future!

[Luciano]

Hi,

Some interesting stuff.

http://www.circuitcellar.com/library...9/c99cdpdf.pdf
http://www.circuitcellar.com/library.../c109cdpdf.pdf
http://www.circuitcellar.com/library...99/c1299cd.pdf

(In the first link they talk about the ULN2003A).

Best regards,

Luciano

[Christopher4187]

Thanks for the info. I actually read this yesterday when doing a search on the ULN2003. My test alarm has been cycling on and off for over 21 hours and has simulated 11 years (26,000 cycles) of operation. The ULN2003 and the relays are still working great; it's not what I would have expected without connecting pin number 9.

[Christopher4187]

Just a follow up to all that are interested.

I tested the ULN2003 with the PB-110ND, with and without pin number nine connected. Both tests ran 30,000 cycles with no problems. Strange but everything works fine. I guess I answered my own question!

[sougata]

Hi,

1. The peak cuurent of a single array is 500ma and if your application needs for you can parallel the outputs. Keep in mind not to exceed the overall dispation of the device. Thus it is technically possible to parallel outputs for higher peak current at a lower duty cycle . For constant driving (100% duty cycle) it doesn't make much sense to parallel outputs as it will increase the overall dissipation of the device.

2. When you turn off a relay the voltage collapses much rapidly than the magnetic field in the coil and is manifested as a large reverse peak voltage [read BACK EMF]. This could kill the transistor array as it is likely to exceed the VCEO (collector emitter voltage) of them. So the clamping diodes come handy to eat this up. You can find in the datasheet that the clamping diodes can handle upto 2.5A (Iok). But to complete the circuit you need to connect them to the other pin of the relay coil which is most likely to be the relay supply. Thus connecting pin 9 to the supply completes the figure. The peak current through the clamping diodes would be high but for a short time and would not let reverse voltage buildup (limited to diode voltage).

So Chritopher it is always a good idea to connect the clamping diodes (pin 9). It does not mean essentially that your transistors will fry immmediately but may reduce operation life.

As for personal experience I use the ULN2803 in my designs. They are neat, saves PCB space and reliable. I parallel the ouputs in my LED Matrix design with no problem as the duty cycle is much less. In your application I suggest the use of pin 9 and you can rest assure that for a coil current of 77ma your product will not fail. You can parallel the outputs if you like. Even if you leave your unused input floating the transistors are unlikely to switch on as they have inbuilt base to emitter resistors. But as Luciano suggested it is a good paractice to gnd your unused inputs.