What value pullup, 5V logic to 3.3V actual?

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rds2112

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I'm using a DAQ's digital output to sink a fet, to control some solenoids. There are 10 of these. I've noticed that when I only have 1 solenoid populated, it will not work, I have to have all 10 populated. The digital out signal is tied high to 3.3V through 22k. However, my DAQ is a 5V logic device. Could it be that the pullup is too weak and the DAQ thinks it's floating?
 
3.3V (or even 5V) is likely too low a voltage to adequately turn on the FET to drive the solenoids. What FET are you using and what is its gate threshold voltage?
 
Thanks for your reply Carl,

it's actually sinking a fet, not driving it. The circuit is attached.
The load HAS to be there, all 10 solenoids, just to get one solenoid to work. In other words, if you want to test one solnoid, you can't just install one, you install 10 to test the one. I'm getting around this by using a load resistor/LED accross the load in the unused spaces. This isn't a big deal, but I wonder why the DAQ is having issues.

(note: in some places the Solenoid volateg is 12V, this diagram shows a 5V one).
 

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The DAQ's job is to sink the FET, not the solenoid. That is also why the problem is strange. The DAQ shouldn't care about the solenoid being there or not, since it's on the other sode of the FET.
The DAQ is a NI 6259
 
But the current through the Solenoid, FET & DAQ *according to your schematic* will all have the same current through them. Your DAQ cannot sink that kind of current. Rethink your design.
 
Bill,
Sorry I mis-spoke. Indeed the DAQ supplies the current, but due to the gate being supplied the PWM, the current is very low, on the order of 20mA.
 
Pin 4 on the FET will want GND. You circuit won't work as designed.
**broken link removed**
That's how it should be connected. Is your relay only a 20ma coil?
 
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Yes your DAQ cannot sink nearly enough current to drive a relay. The FET is simply a switch but the DAQ output is teeny weeny.
Plus why would you want to drive a relay with a DAQ?
Here is a transistor version.
**broken link removed**
Notice they both need a GND.
 
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Well, we don't drive the relay with the DAQ under normal circumstances. This is just for testing. What you've drawn is the basic circuit. When that LED/resistor isn't there (along with the coil not there) then none of the outputs work.
 

The model for the current source/sink capability of a logic output is a bent V-I curve. Once you have it you're in a much better position to interface to it. Some data books show it, in the app. notes section.

Measure the open circuit voltage with a low state and a high state.
With a low output, find the min value pullup resistor that makes the output at the high end of a valid "0."
With a hi output find the min value pulldown resistor that makes the output at the low end of a valid "1."

Now you have the piecewise linear equivalent circuit. It's pretty useful.
 
It would help if we knew what this was supposed to do? Is it part of your video switcher? Why is the function of the DAQ, why PWM the relay drive?
 
I guess it's useful to mention that the circuit I posted in locked in. It can not change. It is the testing apparatus (where I replace the normal drive with a DAQ digital out), where i get into issues. In the real circuit, the Source is sank with a open Drain of yet another FET. That works like a champ. When I replace that with the DAQ, things get iffy.
Note that in my case, the PWM is simply a power-saving measure. rather than hit the solenoid with a constant voltage, it is PWM'd. Saves heat and power.
So, that schematic I posted is pretty much it, but there's 10 of those circuits interfaced to 10 DAQ digital outputs. The PWM is not done with the DAQ.
 
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The DAQ can supply 24mA sink or source. It's an outboard USB device.
Again, the DAQ is used as a test fixture, it's not in the actual "product". I just wanted a simple way to test the solenoids without hooking up the other control board, and I wanted to talk to it easily with VB.

The DAQ can obviously work, beacuse when I populate 10 solenoids, I can turn on 1 at a time all day long.

edit: The DAQ may or may not have a current boosting buffer attached between it and the cirtcuit, I just found out. I don't have the test jig here, just the circuit, sorry. Lets say for sake of argument that the DAQ has plenty of current drive.
 
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Still makes little sense the point of the DAQ, this will only give the solenoids less power. Why are you using the DAQ? What's the point? Is is a secret?
 
Indeed it is a secret. But the circuit I posted certainly isn't. I guess the core question is: Why would a current-capable DAQ require the loads to be present? Check out the pull-up. It's 22k to 3.3V. The DAQ is a 0 and 5V device. If the pullup to 3.3V wasn't hard enough, do you think it may have issues?
BTW I appreciate your responses! BTWx2: I can't get the blueroom site to work.
 
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