NI-USB 6501 Relay

[averagejoe]

i have a 6501 Ni-USB daq.

I have 20 relays that i want to be able to either switch on independetly or seperatrly. The NI-6501 has an output voltage 2.0 V min, 3.5 V max at 8mA. My relays are axicom D2n V23105-A5475-A201. These require 5V and around 140mA to switch on and off.

I have an external power supply that i can use as voltage source for the relays.

I have connected the an output of the 6501 to a pull up resistor to the external supply voltage. I have tied the gnd of the PSU and NI650 together. My thinking was that seeming as i only can get 8mA out of the 6501 i would use the pull up resistor to pull the votlage up to what is required to switch the relay on. Then when i select it to give a low input it would then turn the relay back off.

I just cannot get my head arround how i can switch them on and off using the hi and low digital outputs from the NI6501.

Any help would be great.

[SnowMule]

You're going to need a driver.

Several of them.

ULN2003's work well, use an inverter between the DAQ and driver if the default (line high) condition is unsafe or dangerous in your application.

Power supply to all the relay coils, other side of the relay coils to a line on the driver, driver to digital output. 

[JamesKent]

Hi averagejoe,

Do you know if the Axicon D2N's are drive high or low? As in, do you need to switch them on the positive or grounded side of the relay. As discussed above, you will need to have a driver of some sort that will switch the relays. 

The ULN2003 is a valid option for driving the circuit but bear in mind that they can dissipate a significant amount of heat by virtue of being a transistor, and this problem is compounded if you use multiple ULN2003's in a DIP package for example.

A MOSFET is another alternative which will dissipate significantly less heat due to their extremely low resistance when switched on, however also bear in mind that driving the MOSFET is another concern as you will need to be able to drive the gate properly relative to the source pin, however there a number of MOSFET's that are TTL logic compatible such as a 2N7000 MOSFET.

Best Regards,

James Kent

[averagejoe]

thanks for your replies. I got it working yesterday. I connected the I/O outputs of the DAQ to a ULN2803 inputs. I Connected all my relays to 5Vdc, and connected the GND terminal of the relay to the output of the ULN2803. Then connected teh ULN2803 to the same gnd. Each time i send a hi signal it gnd the corrosponding terminal and allows current to flow thus switching the relay.

I will be using a protection diode across the relay to stop any EMI spikes when the relay switches.
Each relay draws 100-120mA of current. Maximum amount of relays i will have on at any time is 7, This means it will draw 0.8A.

I checked the temperature of ULN2803 with 0.8A and there was no increase over an hour period.

[averagejoe]

thanks for your replies. I got it working yesterday. I connected the I/O outputs of the DAQ to a ULN2803 inputs. I Connected all my relays to 5Vdc, and connected the GND terminal of the relay to the output of the ULN2803. Then connected teh ULN2803 to the same gnd. Each time i send a hi signal it gnd the corrosponding terminal and allows current to flow thus switching the relay.

I will be using a protection diode across the relay to stop any EMI spikes when the relay switches.
Each relay draws 100-120mA of current. Maximum amount of relays i will have on at any time is 7, This means it will draw 0.8A.

I checked the temperature of ULN2803 with 0.8A and there was no increase over an hour period.

[Paul_Hanley]

Hi, I use the exact same way for switching relays as well. Some of the axicom relays have diodes inside them so external diodes are not necessary. I'd check the specification sheet. Paul.