Need hardware advice: ULN2003A

[Luciano]

@Luciano,

If your field is large, I guess it could. The problem is that the clamp diodes within the ULN2003A are most likely not matched and you are forcing that large field into one diode for a very short time. Maybe one way to mitigate that problem would be to put discrite resistors on each output to ensure the current does not go above an acceptable level. I don't have formal education to confirm this but it sounds like it may work.

The diode is not a problem because when the outputs are paralleled you will
have from two up to seven protection diodes in the circuit.

The datasheet says that: (Sorry I did not see that).

The collector-current rating of a single Darlington pair is 500 mA.
The Darlington pairs can be paralleled for higher current capability.

The datasheet says that, go head and connect the buffers in parallel
following the rules of figure 14 and figure 15 of the TI datasheet.

To put resistors on each output to ensure the current does not go above
an acceptable level is a good idea. (For the buffers and for the diodes
if they are not matched).

Best regards,

Luciano

[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.

[rwskinner]

I've been using ULN2803's on about 200 systems for 3 years now driving 24 volt relays which mine require 80 ma, using only the ULN2803's onboard freewheeling diodes and had absolutley no problems.

But I most likely have been lucky according to what you folks are saying.

The only problem I had was when they drive filliment type lamps, the element burns out, shorts, and causes 24 volts to directly sink via the 2803 and fry it.

We're going to fix that by limiting the current going to the 2803 for protection.

[mvs_sarma]

I've been using ULN2803's on about 200 systems for 3 years now driving 24 volt relays which mine require 80 ma, using only the ULN2803's onboard freewheeling diodes and had absolutley no problems.

But I most likely have been lucky according to what you folks are saying.

The only problem I had was when they drive filliment type lamps, the element burns out, shorts, and causes 24 volts to directly sink via the 2803 and fry it.

We're going to fix that by limiting the current going to the 2803 for protection.

Hi
what is the wattage of the filament lamps you are trying to drive with ULN2003--
the issue is with cold resistance of filament lamp and the surge currewnt that flows before the filament becomes HOT-- the driver device should haVE DOUBLE THE CAPACITY OF THE INRUSH CURRENT II believe.

[rwskinner]

Hi
what is the wattage of the filament lamps you are trying to drive with ULN2003--
the issue is with cold resistance of filament lamp and the surge currewnt that flows before the filament becomes HOT-- the driver device should haVE DOUBLE THE CAPACITY OF THE INRUSH CURRENT II believe.

It's not an overload issue, well, it is, but not caused by normal operation. It's when a lamp burns out and we get unlucky enough that the element rolls around or puddles and causes a direct short. We have done two things to stop this.

1) Changing all the Lamps with LED replacements
2) Active current limiting circuit

When I was researching 2803's "after" the fact, I found that lots of arcade games used them to control light arrays, and they too had many problems with lamps shorting and burning up the 2803. There were some pretty neat ideas on the net on how to avoid it.

[mvs_sarma]

Hi Rwskinner

If the failure rate is not high and things are running-- i feel no need to change ULN2003 unless it gives you an advantage-- like cost reduction-- etc,

perhaps you need not change for technology upgradation - as far as failure is occasional and tolerable without letting reputation down-- please continue

even the diode matrix inside need be used only when load is inductive like a relay coil.