How to output analog current more than 2mA

[Henrik_Volkers]

your virtual gnd is to weak

try to add 2-3 green or 4-5 red LEDs   or use another 741 to buffer your gnd

Again realise where the current is flowing!!

Or use an other power supply , or a OP with R2R output or ....

[johnsold]

Ohm's Law. Your (open triangle) ground reference is generated by the voltage divider consisting of the two 430 ohm resistors. Consider the current path through RL. The current comes from the 24 V power supply, goes through the high side output transistor of the op amp, through RL (to the open triangle ground), and through the 430 ohm divider to the 24 V supply ground. If you check the votlage between the two "grounds" you will find that it varies with the current through RL.

When I suggested using the voltage divider I was only thinking of the reference voltage and not the current which would need to flow. You need to create a low impedance (open triangle) ground. One way would be to buffer the voltage divider with another op amp.

Lynn

Edit: Henrik beat me this time!

[Henrik_Volkers]

I had a look in the datasheet of that acient OP 😉

So you really should switch to another one.

http://www.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=opa552&fileType=pdf

if you don't mind using a SMD part:

http://www.ti.com/product/lm8272

Output R2R , Two OP in one housing, so you can buffer you  GND 🙂  AND it's just a click for sample 😉

You can stick with the 741 if you change to a higher heating resistor , change to different schematic (only single sided, for heating that's fine)

[Henrik_Volkers]

What amount of heating power do you want to control?

100Ohm@10mA -> 10mW

100Ohm @ 10V -> 1W

If you just want to control a heating resistor in a loop:

Use a NPN Transistor  (With 10V exitation, 24V supply and 100Ohm hardly every NPN transistor you will find will do the job 😉 )

And you can use the transistor as a heater too 🙂  just use R1 and Q1 together as the heating source , since both will get hot! and you get a nice linear heating power to AOvoltage 😄

BTW V2 is your anaolg out 😉

 As you can see your R1 resistor should have a sufficient power rating (1W for 100Ohm)  and don't burn the poor NPN 😉 

[Henrik_Volkers]

Here is a updated version with added protection for your DAQ, just in case you burn that NPN so it might (Murphy!!)  put 24V into the AO

BTW That software costs you nothing (Well.... time  😉 )

[cli21]

Hi Henrik,

Thanks for your comprehensive solutions. Wonderful work! Yes, my virtual ground is weak. I have proved what johnsold said. On the other hand, I think the capability of 741 also plays an important role. I selected the method of replacing it with a better one. It's a little difficult for me who doesn't have EE background to understand what you post totally, but it's really interesting, I have learnt a lot. Thanks very much.

CJL

[cli21]

Hi Henrik, 

Have you ever used Lock in amplifier? I have a question about Standford Research 830 Lock in Amplifier. Hope you can give me some instruction. I attached one page of SR830 Manual. Would you please explain how does the current flow through whole system as shown in the first picture?Why does different R value(10k&10) make a difference?

Thanks.

CJ

[M.GH]

I am using a 6218 NI USB card for driving a coil with AM modulation  and the current limit is +-2mA too. Is this a peak to peak amount or rms value? I appreciate if anyone let me know! I need about 6 mA is there any method else than an additional module?

All the best,

MG

[johnsold]

MG,

Your question is really unrelated to the discussion in this thread. It would be better to create a new thread and refer to this one if you feel it is relevant.

The 2 mA limit is instantaneous or peak. You will need an external amplifier to drive your coils.

Lynn

[Henrik_Volkers]

I asume your coil is part of a dynamic speaker/vibrator

take any sound amplifier /old active PC speaker and abuse it as an amplifier if only an AC signal in the 20Hz to 20kHz range is needed.

or

open a new thread and provide more information:

data of coil, exitation type and range....

MultiIO devices are made to measure and control stuff, not to drive them.