switch relais USB-6341

[kurt4916]

Hello,

I'm using an USB-6341 to control solenoid valves through a vi. These valves are switched on and off using relais.

The problem:

some of the relais are working  and some of them are not, even if they are brand new.

The DAQ-card is giving a signal of 5 volt output and I don't understand why some of them are working and some other not.

The relais feature:

G2R-1-E

5 VDC

16A 250VAC

16A 30VDC

Do I need to change relais? Or do I need to change the settings of my DAQ-card? Is possible to give an higher voltage outpu?

Thanks,

kurt

[Buechsenschuetz]

The relays you are using have a coil current of more than 100mA. Are you sure that the outputs can suppy (source) such a current and still maintain 5VDC with such a load?

I suspect that you are overloading the outputs a bit with these relays. Each relay has a "must operate" voltage which is rated at 3.5VDC (70 p.c. of nominal VCC) in this case - but some relays may even operate (although not reliably) at a somewhat lower voltage. So, if the voltage on the outputs drops to, say, 3.4VDC with a relay coil load, some may operate, some may not. 

I would recommend to check voltage levels across the coil terminals when the relays are operated. If the voltage is close or even below the above mentioned "must operate" level you should consider using external relay driver circuitry (which implies an external power supply), or use relays which consume less current.

Also, NEVER forget to place a snubber diode across the coil terminals, the diode must be placed "reversed" to cut off any backlash voltage which may damage your driver device.

BTW many years ago I built some data acquisition multiplexesr with a total of 192 relays (low-power relays to be honest) in each device and we never had any initial relay failures when driving the relays correctly.

[kurt4916]

Hello,

here you can see a block diagram of the settings.

Only Relay 1 is working, and also changing the positions of the relays it remains the only one working in all the different positions. Which kind of relay I need to use? The solenoid valves are working with 24V, now the relays are 5VDC G2R-1-E. The voltage across the coil terminals is 2.85V.

Does the diode needs to be positioned at the coil port of the relay? Or where?

Thanks!

[Buechsenschuetz]

2.85V across the coil teminals is far too low. From the datasheet you can read that these relays need at least 70 p.c. of their nominal voltage, i.e. 3.5VDC for "must operate", i.e. reliable operation.

The snubber diode(s) should be placed directly across the coil terminals. The catode should go to the (+) terminal, the anode to the GND terminal.

BTW, why are you using such "heavy" relays? Maybe the supply current of the valves could be handled by much smaller relays which will need less current for the coils, the driving power of the digital outputs might be sufficient for smaller relays. Check the specifications of the digital outputs of the USB device, usually the maximum current is clearly indicated. As mentioned, I think 100mA or even more are a bit too much for a digital output.

[Buechsenschuetz]

I checked the specs of the USB6341. Although the digital I/O specs are not as detailed as on many other NI products it is recommended not to source or sink more than 24mA from the dig I/O lines in high or low state respectively.

Your relays need more than 100mA for the coil to operate properly so you are operating the I/O lines clearly out of spec.

If you want to go on with these heavy relays you have to consider using a power driver for the relays. You might even need an external power supply. I would strongly suggest to check the supply requirements of your solenoid valves and select a relay the contacts of which can easily handle the supply current (with some headroom, of course), maybe you will find some relays which need much less current to operate properly.

[kurt4916]

Hi,

I found a solid state relay which (my professor said) could solve the problem since it needs less voltage to work.

It has the input (4 and +3) 3.5 - 32V

and the output (1 and +2) 100V 7A

So, how I should wire it?

Thanks!!

[Buechsenschuetz]

We are also using SSR (solid state relays) for driving high voltage load. 

If you are not using an open-collector output configuration, you should wire the (+) input terminal of the relay to the digital output and the (-) output to (digital) GND.

Since SSR relays just need some milli-amps you will probably see even more than 3.5V across the input terminals.

Beware of short circuits on the output of an SSR. Different from contact relays, they cannot handle overloads and the output circuitry will be damaged.