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constant current sink understanding (10-50mA)

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Kalvin:
You can build a floating constant current load by using a [micropower] RRIO op amp, a MOSFET, a current sense resistor, a battery or a cheap isolated 5V dc-dc converter module. The external power supply will provide sufficient voltage so that the current sink will operate very close to the zero voltage. Typical topology would be something like this:


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Just make sure that the MOSFET use select can be operated at the maximum load voltage (32V) and can dissipate the maximum power 50mA * 30V = 1.5W..

Alex Nikitin:
For this application a JFET of better still a depletion type MOSFET (BSP129 for example) and a resistor between source and gate would be the simplest and best choice, as it will work as a current load down to a certain voltage (~1V for the BSP129) and as a resistive load below that point.

Cheers

Alex

SiliconWizard:

--- Quote from: Zero999 on February 05, 2019, 09:41:32 am ---
--- Quote from: SiliconWizard on February 04, 2019, 11:44:06 pm ---
--- Quote from: Atom on February 04, 2019, 10:18:47 pm ---
--- Quote from: Benta on February 04, 2019, 09:33:14 pm ---An important piece of information is missing: what's the output voltage of your PSU?


--- End quote ---

sorry the output voltage is from 0 to 32 V .

--- End quote ---

Then it's not going to work down to 0V.

--- End quote ---
No current sink will work down to 0V. Not without an external power supply. A J-FET will work closer to 0V, but the current will still fall, once the voltage drops below a certain point.

--- End quote ---

Exactly, but that had to be pointed out to them as they might have not realized it. If that was obvious to them, that's all good. But that could definitely not fit their "0 to 32V" requirement.

With a single JFET, it'll still be at least 1V depending on the JFET, and there will be a wide dispersion, so a trimmer would be necessary.
There are also not many usual JFETs with a saturation current of at least 50mA, which was one of the requirements, and especially at low VDS voltages.

Actually, unless I've missed something, their approach with an NPN transistor should allow to get a lower operating voltage than with most JFETs as long as you use a low enough reference voltage IMO. You should get around 400mV or so minimum op. voltage with a reference voltage (base voltage) of 1V for instance (which is enough to ensure it will be above the max VBEsat), with an ADR510 for instance (not expensive). Current will also have to be adjusted of course, with a typical resistor value of 5ohm to 25ohm.

Benta:

--- Quote from: SiliconWizard on February 05, 2019, 06:17:50 pm ---
There are also not many usual JFETs with a saturation current of at least 50mA, which was one of the requirements, and especially at low VDS voltages.


--- End quote ---

Actually, the title of this thread is "10-50 mA", so where you got that from is anybody's guess.

Two 2SK170BL in parallel would do the job beautifully, and with much lower minimum voltage than the other solutions presented here.

SiliconWizard:

--- Quote from: Benta on February 05, 2019, 06:50:59 pm ---
--- Quote from: SiliconWizard on February 05, 2019, 06:17:50 pm ---
There are also not many usual JFETs with a saturation current of at least 50mA, which was one of the requirements, and especially at low VDS voltages.


--- End quote ---

Actually, the title of this thread is "10-50 mA", so where you got that from is anybody's guess.

--- End quote ---

From the title. "10-50 mA" means from 10mA to 50mA to me, so potentially up to 50mA. Now if it meant anything in between as we see fit, nevermind.

(With this range, you'd need a JFET with at least 50mA saturation current to ensure a 50mA current sink. You can only adjust the current lower with a source-to-gate resistor, which is the common way of adjusting a JFET-based current source/sink. That was my point. You can also parallel several JFETs as you suggested, but that's more parts and more dispersion.)


--- Quote from: Benta on February 05, 2019, 06:50:59 pm ---Two 2SK170BL in parallel would do the job beautifully, and with much lower minimum voltage than the other solutions presented here.

--- End quote ---

Well, the 2SK170BL seems to have a ID vs VDS knee (@VGS = 0V) at a much lower VDS than with a lot of other common discrete JFETs, so that would be a decent fit regarding the operating voltage indeed.
And yes you can parallel several JFETs to sink more current. You'll also get more dispersion (and a lower output impedance if I'm not mistaken.) This transistor seems to be obsolete though and available only through old stocks. Maybe it has a more recent equivalent. Else the OP will have to resort to eBay.

The NPN version is more accurate as a current sink (this JFET's current will still vary up to at least 2-3mA over the whole voltage range, which may not be a concern here, I don't know), and as I said above, can be made to work at pretty low voltages given you use a low enough reference voltage. Yes, it will need more components. It will have some sensitivity to temperature, but the JFET as well. Not sure which of the two will be worse in that respect.

Yes the JFET solution is probably fine here if all the OP wants is to "bleed" some relatively constant current. That would not make a precision constant current sink, but I guess this is not what the OP is after.

In both cases, if they want to be able to set a defined current (even if not very accurately), they will need to make it trimmable. That's with an adjustable resistor between emitter and ground in the NPN case (and/or adjusting the reference voltage), or between the source and ground/gate for the JFET case. JFETs present a relatively large dispersion in their characteristics. If this is just a one-time project (or just a few), they can always hand-select them for a given current.

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