Analog output current

[acewong7]

Hi! ALL! I am using NI UBS-6356 to generate a DC voltage analog output to control a relay. I check the datasheet of 6356(http://www.ni.com/pdf/manuals/370082b.pdf),the output current drivce is -5mA to +5mA (why it is so small), even the overdrive current is only 10mA. The output DC voltage range is -10V to +10V. So for the common relays that are commencial available, suppose I am using a 6V DC output voltage to control the relay, according to Ohms Law, the coil resistance of the relay should be 6V/0.005A=1.2k Ohms. But I talked with most of the relay suppliers, the coil resistace for a 6V DC relay is usually 40 Ohms! Therefore, I will have to let the current output from the analog output channel be 150mA, that is much bigger than the normal output current drive on USB-6356. Can anyone provide me any help on this issue? Thanks a lot!!!!

[crossrulz]

You will need an current amplifier of some sort.  A simple voltage follower will probably be able to do it.  The other option is to use a DIO with a digital buffer circuit that can supply enough current.

[acewong7]

Hi  crossrulz, can you say something more  about the digital circuit buffer ?how  to build such a circuit? Thanks a lot for help_ 

[johnsold]

Look at the ULN200x family of drivers.

Lynn

[acewong7]

Hi, Lynn. thanks for your reply. Actually I saw you answered another similar question with the same answer. I did take a look at the ULN 200x before I post this question. I didn't understand the ULN 200x very clearly. So I post again. I am thinking maybe a Zener diode works. Cause in the reverse-bias of a Zener Diode, the voltage will not change a lot, but a big current will go through. So i am thinking the 6V dc voltge generated from the AO0 channel can still keep at the same level so that the voltage on the relay will still be 6V DC. The only different is that after the 5mA(orignal current from AO0 channel) will be boost up to 150mA or more. so that the relay works as the way I want it to be. Can you take look at this Zener Diode before I palce the order? Thanks! (the Zener voltage is 6.2V and Zener current is 155mA, both are very close to the relay's reqirements)

http://www.mouser.com/Semiconductors/Discrete-Semiconductors/Diodes-Rectifiers/Zener-Diodes/_/N-ax1m... 

[SnowMule]

A zener diode isn't going to source any power.
Something has to source it ... either a power supply capable of the voltage/current required by the relay, or the DAQ.
The DAQs can't usually source a lot of power from the digital outputs. They can sink more than they can source, so use that to your advantage.
The ULN200x's can sink a lot of power, and they're controllable by the DAQ.

It sounds like your relays require a fair amount of power, you'll more than likely need an external power supply to drive them.

[johnsold]

acewong7,

SnowMule is right. The analog outtputs can only supply a few milliwatts.  Relays require hundreds of milliwatts to a few watts.  So you need two things in addition to the DAQ device - a power supply rated for the voltage and current required by the relay coils and an amplifier or driver.

You should probably get some help with basic electricity.  Your lack of knowledge will probably result in damaged or destroyed devices and equipment and may injure someone.

Lynn

[Henrik_Volkers]

I recommend a SSR with opto coupler input

Something like this:

You can get them from Weidmüller, Phoenix contact or others keyword: opto SSR .

They come in different tastes 😉  to switch AC or DC and different control input voltage ranges...

This question comes up once a month??   People asking this (I assume) have not enough electrical background to solder a PCB with IC. Otherwise why should they ask?