Low current constant current source

[mchambin]

What do we have ?
Cheap.
For current around 5mA
No need for accuracy.
Voltage drop down to 1.5V

 

[wrenchone]

Sounds like a job for a "ring of two" bipolar current source, though 1.5V of headroom is likely barely enough headroom for the job.

 

[rayma]

LM334
https://www.ti.com/product/LM334

You didn't mention whether it has to be floating, or if it will be ground referenced.
A simple discrete CCS will be cheaper but lower performance, if it will be ground referenced.

 

[wrenchone]

Looks like the LM334 will work, too, esp. if you're not all that worried about noise.

 

[mchambin]

LM334
https://www.ti.com/product/LM334

You didn't mention whether it has to be floating, or if it will be ground referenced.
A simple discrete CCS will be cheaper but lower performance, if it will be ground referenced.

I do not need floating.
So I can use a CCS made of 2 transistors and 2 resistors. As said, 1.5V is marginal, but acceptable in my application.
I am looking for cheapest, less components.
Are they cheap current fixed source ? ( No Rset ).

 

[rayma]

Use a 2N3904 transistor for the cheapest CCS.
current source - Google Search

For better performance at a little more cost, sub a Zener or a voltage reference for the R2.
Otherwise go with the LM334.

 

[jackinnj]

You can use a TO92 DMOS device like the LND150 -- see Walt Jung's AX article "Current Sources 101 Part 1" -- archived on the AX site and around the web.

 

[wrenchone]

The LND150 might be marginal for 5mA, esp at low voltage. A BSS126 (Infineon) might be a better choice, but I doubt it would work with 1.5V headroom. If you don't care that much about ultimate accuracy, a 2N3906/2N3906 and 3 resistors could make the grade.

 

[rayma]

Yes, but not cheap.

 

[wrenchone]

A BJT and 3 resistors not cheap? I agree about the MOS/jfet contenders, though, and the 1.5V headroom requirement would probably eliminate them from contention anyway.

 

[U230421]

1 Jfet? Gate and source together. Voila!
Need adjustment? Add 1 resistor between gate and source (gate is the low side of the CS). 2 components in total max.

 

[wrenchone]

The problem is the 1.5V headroom. You would need to carefully select a part with the required Id near its IDSS in order to get that approach to work. If I were to try it, I'd probably reach for some J112s or J113s first thing, as they are relatively cheap and still available with legs. I think you'd need a fair pile of them to get what you want. The BJT approach is a lot easier, and requires no selection. Anyway, you'd need a stopper resistor at the gate to keep the fet from singing, so it's really 3 components.

 

[rayma]

Yes, but not cheap.

This was about the LND150. See post #6.

As I recall, we paid $0.02 for the 2N3904 in quantity.
And for resistors less than that, so maybe $0.05 for a CCS.

 

[wrenchone]

The LND150 is too anemic for 5 ma duty anyway...

 

[mchambin]

This was about the LND150. See post #6.

As I recall, we paid $0.02 for the 2N3904 in quantity.
And for resistors less than that, so maybe $0.05 for a CCS.

I see there is no way to beat the basic CCS when there no high performance required.

 

[rayma]

Set Vemitter = 1V to meet the 1.5V drop spec.

Then Re ~ ( 1V / 0.005A ) = 200R

Then Vbase ~ ( 1V + 0.6V ) = 1.6V

BETA of 2N3904 > 60, so Ibase < ( 0.005A / 60 )

For <2% error, Idivider = 50 x Ibase, so Idivider = 50 x ( 0.005A / 60 ) ~ 4mA

Then R2 = ( Vbase / Idivider ) = ( 1.6V / 0.004A ) = 400R

And for example, if your Vcc = 15V, then R1 = ( 15V - 1.6V ) / 0.004A = 3.35k

Yes, in this case you need to waste 4mA to get a 5mA CCS.

current source - Google Search

 

[John Audio]

A very simple way to generate constant current - is to use a CRD or current regulating diode:

ATC Semitec E-452~E-562 Current Limiting Diode CRD - 4.5mA.

Available in a selection of currents: 4.5 or 5.6mA values are the nearest to what you want.

Not the cheapest solution - but as easy to use as a zener diode.

About £0.84 GBP each in the UK.

 

[scott wurcer]

The problem is the 1.5V headroom. You would need to carefully select a part with the required Id near its IDSS in order to get that approach to work.

No simply the gate to drain needs to be pinched off. Any FET with Idss >= 5mA ad Vp < 1.5 V will work.

 

[mchambin]

ATC Semitec E-452

Pinch off current 4.5mA. Dynamic impedance 0.07M ohms. Constant current voltage 3.7 - 100V. Current tolerance 15%.

Interesting devices....but not for low voltage.
This diode provides 4.5 mA for voltages over 3.7 V up to 100V.
I need 4.5mA down to 1.5V.

Solutions for my applications are:
2 transistors 2 resistors, that give a very constant current.
1 transistor 3 resistors, this gives a not so constant current but acceptable.

 

[mchambin]

No simply the gate to drain needs to be pinched off. Any FET with Idss >= 5mA ad Vp < 1.5 V will work.

I looked at Mouser and Digikey. I was enable to figure out how to find these with their product selector.