What do we have ?
Cheap.
For current around 5mA
No need for accuracy.
Voltage drop down to 1.5V
Cheap.
For current around 5mA
No need for accuracy.
Voltage drop down to 1.5V
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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.
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.
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.
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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 ).
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.
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.
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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.
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.
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.
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.
Need adjustment? Add 1 resistor between gate and source (gate is the low side of the CS). 2 components in total max.
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.
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.
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I see there is no way to beat the basic CCS when there no high performance required.
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
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
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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.
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.
No simply the gate to drain needs to be pinched off. Any FET with Idss >= 5mA ad Vp < 1.5 V will work.
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.
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.
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I looked at Mouser and Digikey. I was enable to figure out how to find these with their product selector.