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hardware question on cFP-DO-403 module

We are trying to use a pulse-width modulated DC drive (28V at 20KHz switching frequency) to drive some aircraft lights.  We have this variable lighting voltage going to one side of the bulb and sinking the other side with the cFP-DO-403 Vout pin.  The problem that we are having is that when the DO is off, we are getting a glow on the lights that is caused by a voltage drop accross the DO-403 module.  This happens at the lower end of the pulse-width values (1/4 of the duty cycle on), where we mesure ~7 volts on the high side of the bulb and ~4.5 volts on the low side (Vout) of the DO module.  When we turn the channel of the module on the voltage goes low (~.1 volt) and the bulb turns on fine, it is just in the off state where we are getting a drop across the module.  We are unclear about if we need to wire up the V or the Vsup connections since we have our variable voltage going to the bulbs directly.  Any ideas about what we are doing wrong?
Message 1 of 13
How have you connected the COM/C terminals for the 403?
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Message 2 of 13
We have them tied to a DC ground.
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Message 3 of 13
I'll assume that this is the GND from the switching 28V supply?  Operating instructions for the DO-403 show that depending on your current draw, the Vsup and and maybe V terminals need to be connected.  Since these are optically isolated outputs, this voltage applied to the V terminal is probably necessary to help keep the outputs from conducting any current in the off state.  Suggest you connect the supply terminals as called out in the user manual.
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Message 4 of 13


   I'm not sure about your application, but remember that in DO-403 modules, the "common" terminal is the positive one, and each load has to be connected between Vsup (+) and Vout (-) , and you don't have to connect all the (-) of the loads toghether, because they're the effective control terminal of the switching.  Have you a scheme of your loads connection?



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Message 5 of 13
I made a quick word doc with the block diagram showing our wiring as it is connected (attached).  We will try changing the wiring to bring in the switching power to the V pin and then taking the Vsup to the bulb, but as for now we have not been able to make this change.
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Message 6 of 13

What you show won't work at all.


Look at page 4 on

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Message 7 of 13
Sorry to question a bit but I am not sure why?  Does the module get its power for the control logic or the isolation circuit from V or Vsup?  If not, isn't V just passed through to Vsup that would be tied to our bulb?  Thanks for your help.
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Message 8 of 13

Ravens Fan wrote:

What you show won't work at all.

Hi All!

   I'm wondering why... Actually, NI RECOMMENDS to wire Positive voltage to Vsup, but also the scheme with Vsup unwired should work, looking at the internal diagram.  The Vout terminal should only switch, from closed to open, on COM terminal. Since COM is wired, it shouldn't matter if Vsup is or not! I'm trying to understand...


   On the other hand, I don't know if it'll be good practice to wire a modulatedVoltage to Vsup terminal, form diagram you see a pure DC alim connected to Vsup...



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Message 9 of 13

Here's the scheme...



Message Edited by Graziano on 10-06-2008 03:42 AM
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Message 10 of 13