# Floating Dc Current Source

Discussion in 'General Electronics Discussion' started by Rajinder, Nov 24, 2016.

1. ### Rajinder

531
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Jan 30, 2016
Hi all,
I was wondering if someone coulf help me, I am looking to build a floating current source (dc). this will be used to supply a current of between 1uA to 300uA.
I was thinking of the best way to go about this,, perhaps an isolation amplifier or opto device. Any ideas?
The current will need to be varied from 1uA to 300uA, perhaps from a DAC output or resistor combination.
Any help would be appreciated.
Best regards,
Raj

2. ### Alec_t

2,950
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Jul 7, 2015
What will power this current source?
Is this for some medical purpose?

3. ### Rajinder

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Jan 30, 2016
No it's not for medical equipment.

4. ### Rajinder

531
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Jan 30, 2016
The power is going to be either a DAC or LDO voltage regulator.

5. ### BobK

7,682
1,688
Jan 5, 2010
Niether DACs nor voltage regulators are sources of power. A power source might be a battery (probably best for your project) or a linear or switching supply powered by mains.

What you want is probably neither of those. If you want a constant current, you use a constant current source. Google this to see some circuits.

You told us how much current it must supply, but not the voltage. Anything you design will have a maxium voltage that it can output. You can determine this voltage by multiplying your max current by the max load resistance.

For example. If you need 300uA into a 1K load you need 1000 * (300 / 1000000) or 0.3V If you need 300uA into 1 Megohm load you need 300V.

Bob

6. ### AnalogKid

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Jun 10, 2015
Bob touched on this - floating from what? Any normal wall wart makes a 5 V or `12 V or whatever output that is galvanically isolated from the AC powerline - floating. If what you are asking about is a part of a larger system, then what is the overall power source?

Also, if the current level is determined by a DAC, that means that somewhere there is a computer or microcontroller spitting out digital codes. Is the signal ground from the digital word source the same as the output current, or is that what you want to "float".

Usually, a floating current source means one that can drive a load that does not have one end tied to the current source circuit ground. But that does not imply or require galvanic isolation. Can you go into more detail about what it is you are trying to avoid or solve?

ak

Last edited: Nov 25, 2016
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7. ### Rajinder

531
9
Jan 30, 2016
Thanks for the replies.
I have a DAC outputting voltage, this is fed into an opamp. The opamp has negative feedback that includes a FET and sense resistor that is fed into the other end of the opamp input.
So for example the voltage from the DAC divided by the sense resistor gives me the current out. For example 3V/1M gives 3uA. D However this is grounded on one side.
I need the output current to be floated so it can connect to another input.
I was thinking of a 9V battery to power a LDO voltage regulator.
Thanks
Raj

8. ### Rajinder

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Jan 30, 2016
It is this current output that I am trying to float.

9. ### TedA

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Sep 26, 2011
Raj,

It is not clear from what you have said so far if you need galvanic isolation for the load, or a different polarity for the current.

I think what you have already is a current source that sinks current to the common, or ground, used by the controlling computer system. It is unipolar, so it can only sink current; pulling the output in a negative direction. The load must be referenced to some positive DC voltage. This positive voltage is referenced to the computer ground.

But this is not what you need. Instead, I need the output current to be floated so it can connect to another input.

Where does the return for the load ("another input") connect? The load for the current source must have two connections, so a circuit for the current can be completed through it.

It might help to show us a typical application circuit for the current source you require.

Ted

10. ### Rajinder

531
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Jan 30, 2016
Hi Ted,
I will draw a schematic. I will post it asap.
Thanks