Constant current source 1

[alansimpson]

I wish to supply hall probes with a constant current. Could be 1 mA or 100 mA. Wish to make sure the hall voltage isn't affected by changes in internal resistance due to temperature. Could make up a current source with op-amp but I don't wish to have to do electronic design or build as I am a mechanical engineer and will probably get it wrong. Are there any products out there ready made to do the job? I have found some expensive programmable supplies for lab applications but this is over kill. Supply voltage 6-12V dc.

 

[VEBill]

Constant current sources can be made with a couple of bipolar transistors, or one plus a diode or two, and some resistors.

 

[IRstuff]

What exactly is your definition of expensive? Keithley's 236 would do the job. Nor sure about the pricing

http://www.keithley.com/products/currentvoltage/?path=236/Documents#4

TTFN

FAQ731-376

 

[Noway2]

What about a JFET with the gate shorted? It wouldn't provide good regulation or consistency from device to device, but would give you a semi regulated current in the range you are talking.

 

[drillz]

In line with what VE1BLL said....here is a circuit for a current mirror....

http://en.wikipedia.org/wiki/Current_mirror

The current mirror will give you constant current of any value set by the "so called programming resistor".

 

[VEBill]

Or maybe...

http://en.wikipedia.org/wiki/Current_source

 

[alansimpson]

Does anybody know of a ready built product? I do not wish do electronics work as I am rather bad at it.

 

[IRstuff]

Was there a problem with the Keithley 236?

TTFN

FAQ731-376

 

[Comcokid]

Simple current sources can be made with just a couple of electronic parts.

For 1 to 10 mA, the LM334 can be set with a single resistor as a current source. Add a couple more components and get some temperature compensation. Go to

http://www.national.com

and search LM334 for datasheet with examples.

For 10 mA to 100 mA, a LM317 type voltage regulator can be setup with a single resistor to act as a current source. Go to

http://www.national.com

and search for application note AN-178. Many other three-terminal voltage regulators will act like a current source if connected as descirbed in this note.

There are current source diodes - a single two-lead component that will act like a current source. A little more expensive and somewhat harder to find. According to the book "Current Sources and Voltage Regulators" by Harrison there is a CIL series of current source diodes made by Crystalonics with values from 5.1 to 51 mA. To get 100 mA, put two 51mA devices in parallel (OK, it's actually 102 mA then). The book mentions other manufacturers such as Central Semiconductor, InterFET, Linear Integrated, Solitron, Vishay, but most of their current source diode offerings have a max of around 2 to 5 mA.

Of course the book referenced has plenty of more complex designs with higher accuracy and stability, but these require more components.

 

[alansimpson]

Keithley havn't got back to me with price and delivery.

 

[ScottyUK]

About US$4000 for a reconditioned unit, about US$8000 for a new one. Does that fall in to the category "expensive programmable supplies for lab applications but this is over kill"?


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If we learn from our mistakes I'm getting a great education!

 

[IRstuff]

This one runs about $1800 to cover the range you specified:

http://www.ietlabs.com/IET/VI-700.html

TTFN

FAQ731-376

 

[VEBill]

For only $1700 I'll build one using an LM317...

;-)

 

[alansimpson]

I would consider $100 expensive for a product I could make up with a few electronic components costing $20. I do not require the flexibility and features that will be present in a lab product. Keithley says their 236 is a meter and not a current source.

 

[IRstuff]

Then, you are going to be building something. There's already been $100 of NRE invested in your project from other people. At standard rates, it would cost close to $1500 to build a repeatably usable device.

Whomever you're talking to at Keithley is a nitwit. The 236 is a SOURCE/MEASUREMENT unit and is designed specifically for Hall, Vander Pauw, other similar Kelvin measurements. It can your specific measurement as a standalone system:

http://www.keithley.com/data?asset=5512

TTFN

FAQ731-376