Can't have floating D.O. lines even for unsued pins?

[Broken_Arrow]

I'm using a 6259 DAQ card to sink some FETs, which in turn sink some solenoids. I have 10 of these, and it works fine. However, lets say I just want to test one solenoid. When I just install one solenoid, it will not work. I have to have all the solenoids hooked up, or at least some kind of load, to get just one solenoid to work. Note that when the solenoid is disconnected, the FET is still there, although there's no load for it. I understand why that particular line, with the missing load, may be flaky, but why does that interfere with the line that is loaded?

[AnalogKid2DigitalMan]

Richard:

Can you attach a sketch of how you have things wired up?

-AK2DM

[Broken_Arrow]

Sure AK2DM, attached is the Worst Schematic Ever (don't have system with me). It's typical of a solenoid turn-on, where the 10Khz signal is applied a few mS after the line is sunk to save power. The DAQ just supplies the sinking of the fet, not the PWM.

[Broken_Arrow]

I wonder if the pullup to 3.3V through 22k is too weak for the DAQ which may be expecting a 5V high? Even if so, why would putting the load on the unused channels be necessary?

[AnalogKid2DigitalMan]

Richard:

It's been a long time since I worked with FETS, I couldn't follow your circuit topology (BTW that's not the WSE I have seen :)). I would have expected the DAQ digital output to be wired to the Gate and the 15V common wired to the bottom terminal (Source or Drain, I don't recall). If you need the DAQ and the 10khz signal to control the solenoid, you could logically OR them and apply that to the gate. Do you also have a reverse biased diode across the solenoid to protect the FET?

Is your 15V supply a switching (not linear) supply? Maybe it needs a minimum load to attain the regulated15V output? That's about all I can come up with.

-AK2DM

[Broken_Arrow]

Thanks for checking back in AK2DM. (btw is that a Rush reference?)

The 15V has other loads on it in teh system, so I don't think that's it.

I'm thinking now it may be as simple as the pullup, which is effectively 22k, is too weak AND it's going to 3.3V (not 5V like the DAQ uses) AND the DAQ has a weak pull-DOWN init, which can be as low as 20k. With no buffer between the DAQ's pull-down and my pull-up, there's a voltage divider going on, and we may be in no-man's land in respect to logic level, thus putting the DAQ in latch-up. I can't change the 22k in the system. But I can specifiy that the loads must all be present. I'd just like to have a better idea why.

[AnalogKid2DigitalMan]

It could be the pullup, I see the internal pull down has a pretty broad range (20-50Kohm). You could put a meter on to tell if its in no man's land. Wish I could come up with something more definitive to help out.

Yes, AK2DM is two Rush references rolled into one. I'm a pretty big fan and see them every time they make it out here. Started out in in analog electronics as a kid and now pretty much in programming and computers. Whis I had more hands on time with electronics though.

Good Luck

-Pete

[Broken_Arrow]

Yep, the range of the pull-down is wide, but not very "weak" as they put it, in my opinion, a weak pull-up/down would be on the order of 100k.

Thanks for your time. I'm a huge fan too. NP has been a huge inspiration to me since 1979. Come to think of it, I think you and I had this conversation like 5 years ago! 🙂

Message Edited by Broken Arrow on 11-10-2008 11:42 AM

[Broken_Arrow]

On the same subject, can the internal pull-down resistors in the M-series DAQ's cause a voltage divider with an external pull-up? See attached pic. It's likely the protection circuit has a buffer in it, but I can't find anything specific as to what's in the protection circuit. If you had a weak pull-up....

[S_Hong]

Hi Richard,

The internal pull-down resistor in the M-series DAQ will cause a voltage divider with the external pull-up. Have you tried using 5V as your pull-up voltage?