16 Relay Drivers with 6501

[maurosanjo]

I used to drive 4 5v relays with a resistor (10K) and a transistor (bc548), using a diode in the relay coil.

But now I was trying to drive 16 relays using 16 D.O. of my 6501. The problem is when i have like 9 or 10 relays in high state I can't turn another relay on.

All lines are at default (open drain).

I'm not sure if I reached some limit of the device (overall max current in open-drain?)

[crossrulz]

This is acting very much like you reached a current limit.  From what I'm finding, the total current that can go out of the 6501's digital lines is 65mA.  If you had all lines at the 8.5mA maximum, you could only have 7 lines high.

I highly recommend using a digital buffer chip to give you more current capacity.

[maurosanjo]

Thanks for your quick reply! Well I'm working some tests and I'm not sure about that, I now that limit of 65mA in active drive.

But I'm using open drain, so with an ammeter I get 0.3mA per line using a 10K resistor, by reducing the resistance I can increase the current up to 0.8mA more or less per line. I should be able to put 16 lines at high level theoretically, shouldn't I?

For testing purposes, I reduced all resistors to like 500ohms, achieving a current of 0,78mA and the behaviour is more or less the same, when I reach a certain amount of high level lines (13 lines) the overall output voltage suddenly comes down to less than 1.5V, and I can't switch the transistors anymore.

Not sure if I was clear enough...

Best regards

[AnalogKid2DigitalMan]

What is the current limit of the source used to power the relay coils?

-AK2DM

[maurosanjo]

The relay coils are powered up using the 5V output of the NI-6501. The transistor is used to be switched by the digital outputs and is directly connected to the relay.

[maurosanjo]

I'm not sure, but after thinking about AnalogKid2DigitalMan question... The ni 6501 is connected to a usb 3.0 slot, which can drive up to 900mA, it seems that I've reached that limit.

[maurosanjo]

I'm not sure, but after thinking about AnalogKid2DigitalMan question... The ni 6501 is connected to a usb 3.0 slot, which can drive up to 900mA, it seems that I've reached that limit.

[crossrulz]

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@maurosanjo wrote:

I'm not sure, but after thinking about AnalogKid2DigitalMan question... The ni 6501 is connected to a usb 3.0 slot, which can drive up to 900mA, it seems that I've reached that limit.

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That is likely your culprit.  The spec for the +5V on the 6501 is only 230mA max.  You could be easily hitting that with as many relay coils as you are talking about.

[maurosanjo]

Well, for some reason when connected to a USB 3.0 port, the NI-6501 could drive up to 900mA that is the limit of the USB 3.0 port. I could drive 12 5V relays, and each one of them is 70mA.

Not sure if it is safe though...

[AnalogKid2DigitalMan]

That's running a bit too close for my comfort zone.

I make it a point never to power inductive devices from my DAQ's. Granted, you have snubbing diodes, but I'd suggest using an external supply especially since you need to drive up to 16 relays.

-AK2DM