floating current sorce

[Mook Johnson]

Common mode voltages will not be more than 20 volts. Current output would
be about 10mA and load not more than 500 ohms. Frequency must be a <1% THD
sine wave at 45Hz.

Current regulation must be +/-100uA.

How would you guys approach this problem?

 

[John Woodgate]

Common mode voltages will not be more than 20 volts. Current output would
be about 10mA and load not more than 500 ohms. Frequency must be a <1% THD
sine wave at 45Hz.

Current regulation must be +/-100uA.

How would you guys approach this problem?

1. Look in my Little Black Book

2. Look in Art of Electronics

3. Look in data and applications handbooks

2. Google for 'current source circuits' - I get 2.4 million hits.

 

[Harry Dellamano]

(in <[email protected]>) about 'floating current sorce',


1. Look in my Little Black Book

2. Look in Art of Electronics

3. Look in data and applications handbooks

2. Google for 'current source circuits' - I get 2.4 million hits.
--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural
selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Me too.
Harry

 

[Mac]

Common mode voltages will not be more than 20 volts. Current output would
be about 10mA and load not more than 500 ohms. Frequency must be a <1% THD
sine wave at 45Hz.

Current regulation must be +/-100uA.

How would you guys approach this problem?

How are you defining THD? Is that with respect to the current or the
Voltage?

If the latter, then why does the subject of your post say "floating
current source?" The reason I ask is that normally I see THD used in
conjunction with Voltages, not currents.

Anyway, if you had to design a non-floating current source, could you?

So just design a non-floating current source, and feed it into a 1:1
transformer. 45 Hz is not very far from the 50 Hz which many small power
transformers are specified to work at, so you'll probably be OK using one.
If you can't find one with a 1:1 ratio, just use two identical
transformers with their secondaries in parallel.

Or maybe you could find a small power transformer with dual isolated
secondaries and use them, leaving the primary open.

Note that I've never done this, but I don't see why it wouldn't work.

Good luck!

--Mac

 

[[email protected]]

I'd try and find out exactly what you meant by a "floating current
source".

45Hz is essentially DC, +/-100uA in 10mA is +/-1% and 20V of common
mode voltage is within the output swing of an op amp like the Texas
Instruments (was Burr-Brown) OPA452

http://focus.ti.com/docs/prod/folders/print/opa452.html

so you could probably use it to make a Howland current source as
described in Horowitz and Hill's "The Art of Electronics", if your idea
of "floating" allows a common ground.

If you only wanted either only postive or only negative current, then a
P- or N-channel FET would do the same job with fewer components.

 

[[email protected]]

I'd try and find out exactly what you meant by a "floating current
source".

45Hz is essentially DC, +/-100uA in 10mA is +/-1% and 20V of common
mode voltage is within the output swing of an op amp like the Texas
Instruments (was Burr-Brown) OPA452

http://focus.ti.com/docs/prod/folders/print/opa452.html

so you could probably use it to make a Howland current source as
described in Horowitz and Hill's "The Art of Electronics", if your idea
of "floating" allows a common ground.

If you only wanted either only postive or only negative current, then a
P- or N-channel FET would do the same job with fewer components.

 

[Fred Bloggs]

I'd try and find out exactly what you meant by a "floating current
source".

45Hz is essentially DC, +/-100uA in 10mA is +/-1% and 20V of common
mode voltage is within the output swing of an op amp like the Texas
Instruments (was Burr-Brown) OPA452

http://focus.ti.com/docs/prod/folders/print/opa452.html

so you could probably use it to make a Howland current source as
described in Horowitz and Hill's "The Art of Electronics", if your idea
of "floating" allows a common ground.

If you only wanted either only postive or only negative current, then a
P- or N-channel FET would do the same job with fewer components.

View in a fixed-width font such as
Courier.

 

[Fred Bloggs]

How are you defining THD? Is that with respect to the current or the
Voltage?

If the latter, then why does the subject of your post say "floating
current source?" The reason I ask is that normally I see THD used in
conjunction with Voltages, not currents.

Anyway, if you had to design a non-floating current source, could you?

So just design a non-floating current source, and feed it into a 1:1
transformer. 45 Hz is not very far from the 50 Hz which many small power
transformers are specified to work at, so you'll probably be OK using one.
If you can't find one with a 1:1 ratio, just use two identical
transformers with their secondaries in parallel.

Or maybe you could find a small power transformer with dual isolated
secondaries and use them, leaving the primary open.

Note that I've never done this, but I don't see why it wouldn't work.

Current source drive into a transformer is shunted by magnetizing
current. Now you could use a "current transformer"- but the 1% accuracy
requirement would require the primary current to be >100x the load
current- say 1A.

 

[Ban]

Common mode voltages will not be more than 20 volts. Current output
would be about 10mA and load not more than 500 ohms. Frequency must
be a <1% THD sine wave at 45Hz.

Current regulation must be +/-100uA.

How would you guys approach this problem?

We would correct your data first:
floating current source=? probably you meant a bipolar current source. Is
the load grounded or referenced to a voltage rail? A current source is
always "floating" with respect to voltage output.

Frequency must be a <1% THD sine wave at 45Hz? Do you want to control the
current source/s or is that the voltage variation on the output?

Current regulation must be +/-100uA? So you want the 10mA to be +/-1%
exact. You will need some precision resistors or a small trimmer in series
with a resistor to set the current.

Please tell us more about your application, maybe scan the rest of the
circuit, so we can give some better advise

 

[Mac]

Current source drive into a transformer is shunted by magnetizing
current. Now you could use a "current transformer"- but the 1% accuracy
requirement would require the primary current to be >100x the load
current- say 1A.

Hmm. Guess I'll have to actually go open a textbook now to make sure I
understand this. ;-)

--Mac

 

[Mook Johnson]

By floating I mean that the output must not have a DC path between the
current source terminals. It must be galvanicly isolated but the common
mode voltage will not vary much beyond +/-20V from system ground.

By ,1% THD I am impling a clean sine wave if looked at on the scope. No
major distortion products.

Idealy it would look like a voltage controlled current source (as seen in
spice) where the input and output are isolated.

 

[Winfield Hill]

Mook Johnson wrote...

By floating I mean that the output must not have a DC path between
the current source terminals. It must be galvanicly isolated but
the common mode voltage will not vary much beyond +/-20V from
system ground.

The best way to do this is with a single-ended circuit powered from
a completely floating isolated power source. There are plenty of
ways to create the signal in the floating current source circuit,
such as analog isolation amplifiers, etc. These are offered by TI
(Burr-Brown) and Analog Devices.

 

[Marte Schwarz]

Hi ,

By floating I mean that the output must not have a DC path between the
current source terminals. It must be galvanicly isolated but the common
mode voltage will not vary much beyond +/-20V from system ground.

By ,1% THD I am impling a clean sine wave if looked at on the scope. No
major distortion products.

Idealy it would look like a voltage controlled current source (as seen in
spice) where the input and output are isolated.

google around about electro impedance tomographie. These guys needs the same
thing.

Marte