UDN & ULN Protection

Go To Last Post
7 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Hi all ,
I want to protect ULN2803 (NPN Array Transistors , Active Low) and UDN2981(PNP Array Transistors , Active High) , How could I do that ?
The board I'm designing is a tester so I don't know what is going to happen ,
Maybe just for a simple mistake in assembling the parts , ULN & UDN see unexpected voltages at their outputs . I can't use a series resistor cause it limits the current .

For ULN , I decided to use a relay to connect emitters to 2 resistors to verify the voltage between these 2 resistors and finally decide if the collector is ok or it's directly connected to +12v and then if the condition is ok I switch the emitter to GND .

For UDN , I decided to use a relay to connect emitter to 2 resistors to verify the voltage between these 2 resistors and finally decide if the collector is ok or it's directly connected to GND and then if the condition is ok I switch the emitter to +12v .

Is it correct ?
In other way :
Are Va & Vb different enough for ADC ?
Are Vc & Vd different enough for ADC ?

Thanks in advance

Attachment(s): 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

If you want protected outputs, then choose another device. The uln2803/UDN2981 like to go pop pretty easy if you short their outputs. Used within their specs they are perfectly reliable, but do some thing wrong, you'll smell burnt silicon. In the real world, doing something wrong to make them pop is particularly easy. Protecting them is a lot more difficult. A discrete solution might be adviasable or going for some Freescale automotive parts with inbuilt protection might be the go.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Kartman wrote:
some Freescale automotive parts./quote]Do you know ULN & UDN equivalent from Freescale ? Unfortunately I don't have enough time and for now must protect ULN & UDN under any circumstances , I must find a way , even with the price of using a relay .

What about Va , Vb , Vc and Vd ?

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

"...I can't use a series resistor cause it limits the current..."

Just how much current do You need?

And You can not have anything significant in the emitter circuit.

All switches are joined internally and are made available at only ONE common emitter pin.
So no emitter resistors can be left in circuit as the switches will interact via the emitter feedback (voltage).

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

ignoramus wrote:
"...I can't use a series resistor cause it limits the current..."

Just how much current do You need?

And You can not have anything significant in the emitter circuit.

All switches are joined internally and are made available at only ONE common emitter pin.
So no emitter resistors can be left in circuit as the switches will interact via the emitter feedback (voltage).

The amount of current I need is different from one output to another , on the other hand I need to sense the currents from different outputs so the idea of using a resistor doesn't seem to be correct .
About the common emitters I'm going to turn off all switched except one which is supposed to be connected to the emitter resistors mentioned . So I think other switched won't have any effect on the emitter resistors . Isn't this true ?
If it's true how about the voltages I'm connecting to ADC ?

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Some transistor fundamentals

The transistor is a current controlled device ( collector current = beta * base current)

If the transistor sees a resistor between the emitter and ground then the resistor will have a current of beta * base current plus original base current flowing through it.

Base current will stop flowing if emiter voltage to ground is greater ( for the purposes of this argument) than the base to ground voltage.
And in fact the base voltage in a properly biased conducting transistor is one diode voltage drop above the emitter voltage in case of NPN transistor.

Since You are not going to have more than one transistor switched on in the package You can use an emitter resistor dimensioned for maximum circuit current to sense transistor state.
Just keep in mind that the collector load will now be limited in voltage swing( supply - emitter voltage- transistor on state voltage)

As for monitoring the load current ( that is emitter resistor voltage) yes quite OK provided the input to the ADC is correctly scaled.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Quote:
As for monitoring the load current ( that is emitter resistor voltage) yes quite OK provided the input to the ADC is correctly scaled
Thanks alot , So the circuit is ok