USB-6008 5V External Power Problem

[Jamie Fraser]

Hi,

Has anyone else seen this problem?

I have a USB-6008 which is supposed to be able to supply 200mA @ 5V from one of the connector terminals. However, if I have a purely resistive load connected to this 5V line that only draws 30mA when the USB-6008 is first connected to the USB, the 2.5V reference fails to start up. This results in all analogue readings being garbage, and no 2.5V reference being output on pin 30.

The only way around this that I have found is to switch the load through a transistor driven from the 2.5V power, so the transistor only switches on once the 2.5V power has started up. Note that the 5V power is always present on pin 31.

Thanks

Jamie Fraser

[Jamie Fraser]

...or is anyone drawing power in this way and NOT seeing this problem?

Thanks

Jamie Fraser

[stilly32]

Hi Jamie,

I have repeated the behavior here with the same results. We're researching possible causes now. Your work-around is a great idea and should work perfectly since applying the load after startup does not cause this behavior. Hopefully the work-around will suffice for now but I will post back once we ascertain the cause and any other solutions.

We really appreciate the heads up - is there anything else I can do for you while we are researching this?

Thanks,

Andrew S.

National Instruments

[Jamie Fraser]

Hi Andrew,

I am pleased you have been able to reproduce the problem. I guess your offer of assistance does not stretch to recompense for the time spent in identifying the problem so I'll sit tight for your findings.

😉

Jamie Fraser

[nealpederson]

Andrew,

Does this same problem affect the USB-6009OEM?

I tried putitng a 100 ohm resistor between +5V and Gnd to draw 50mA. I
plugged in the device and analog measurements appeared to be working
correclty.

I need an answer to this ASAP because I'm working on an application that
will use the USB-6009OEM and a custom circuit that will get power from the
+5V.

Thanks,

Neal Pederson, VI Control Systems

"stilly32" <x@no.email> wrote in message
news:1165009213776-448855@exchange.ni.com...
> Hi Jamie,
> I have repeated the behavior here with the same results. We're researching
> possible causes now. Your work-around is a great idea and should work
> perfectly since applying the load after startup does not cause this
> behavior.&nbsp;Hopefully&nbsp;the work-around will suffice for now
> but&nbsp;I will post back once we ascertain the cause and any other
> solutions.
> We really appreciate the heads up - is there anything else&nbsp;I can do
> for you while we are researching this?
> Thanks,
> Andrew S.
> National Instruments

[nealpederson]

I just tried 50 ohms for 100mA and my analog inputs stopped working.
Apparently the USB-6009OEM has the same problem.

One problem with the OEM unit is that they don't bring the 2.5V ref out to
the connector so I can use this to drive a transistor for the temporary
solution.

Andrew, please contact me ASAP. Neal Pederson: 505-662-1461,
np@vicontrols.com

"Neal" <np@vicontrols.com> wrote in message
news:457754bc$1@PYROS.natinst.com...
> Andrew,
>
> Does this same problem affect the USB-6009OEM?
>
> I tried putitng a 100 ohm resistor between +5V and Gnd to draw 50mA. I
> plugged in the device and analog measurements appeared to be working
> correclty.
>
> I need an answer to this ASAP because I'm working on an application that
> will use the USB-6009OEM and a custom circuit that will get power from the
> +5V.
>
> Thanks,
>
> Neal Pederson, VI Control Systems
>
> "stilly32" <x@no.email> wrote in message
> news:1165009213776-448855@exchange.ni.com...
>> Hi Jamie,
>> I have repeated the behavior here with the same results. We're
>> researching possible causes now. Your work-around is a great idea and
>> should work perfectly since applying the load after startup does not
>> cause this behavior.&nbsp;Hopefully&nbsp;the work-around will suffice for
>> now but&nbsp;I will post back once we ascertain the cause and any other
>> solutions.
>> We really appreciate the heads up - is there anything else&nbsp;I can do
>> for you while we are researching this?
>> Thanks,
>> Andrew S.
>> National Instruments
>
>

[Jamie Fraser]

Neal,

If your I/O count allows you could switch the 5V from one of the digital outputs under software control.

Regards

Jamie Fraser

[nealpederson]

Jamie,

The other idea I had was to use an RC circuit from the +5V so when the USB
is powered it takes a second or so for the RC charge up time before the
transistor is turned on. Do you know how much time it usually takes from
power up before the +2.5V is ready? That would determine my RC timing.

I'm hoping that NI fixes this problem ASAP because I'd rather not have to
use a work around.

"Jamie Fraser" <x@no.email> wrote in message
news:1165484411782-450827@exchange.ni.com...
> Neal,
> If your I/O count allows you could switch the 5V from one of the digital
> outputs under software control.
> Regards
> Jamie Fraser

[Jamie Fraser]

Neal,

Sorry I don't know. If you think that the 6009 and 6008 are similar in this regard, and if you are stuck for access to a 6008 then I can put a scope on the 5V and 2.5V and check the relative timings. Let me know asap though as I am out tomorrow and we are some hours ahead of you and I won't be at work much longer.

Cheers

Jamie

[nealpederson]

Sorry I didn't check back earlier. I'm guessing your gone by now. I do
belive the circuits are very similar between the 6008 and 6009. Please
check it when you get a chance. There's not a huge hurry.

"Jamie Fraser" <x@no.email> wrote in message
news:1165509615710-451040@exchange.ni.com...
> Neal,
> Sorry I don't know. If you think that the 6009 and 6008 are similar in
> this regard, and if you are stuck for access to a 6008 then I can put a
> scope on the 5V and 2.5V and check the relative timings. Let me know asap
> though as I am out tomorrow and we are some hours ahead of you and I won't
> be at work much longer.
> Cheers
> Jamie