Precision current source single Mosfet or twin transistor setup? - Page 1
Forum section: electronics / general

author: Overspeed
author_rank: Frequent Contributor
timestamp: December 28, 2024, 10:10:35 am
content: HelloIn the aim to built a precision current source 100 mA I have two solutions ( I try to learn )one Mosfet as the circuit linked ( from Eevblog forum ) this circuit is a 1mA circuittwo transistors ( PNP / NPN ) as the circuit linked ( from M. S. Mrozowski design )This question is a general question on the circuit design so what is the advantage of the both solutionboth are voltage driven Op Amp source tested working with proven results.Purpose is to built a 100 mA circuit for low resistance measurement , I agree than BS170 can be not enough , perhaps a paralleling Mosfet setup can solve the problem or a bigger Mosfet ? Mosfet do not need gate current tuning too.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 28, 2024, 12:23:40 pm
content: Both ways can work. Which one works better also depends on how the voltage at the resistor is measured as they have a different common mode voltage for the DUT. Another point could be if one needs extra protection in case there is an external voltage - this could be tricky in the 2nd version.With 100 mA the leakage current at the MOSFET gate is usually not that critical.One the other hand 2.5 V would be a bit high for the reference resistor to handle the power.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 28, 2024, 08:30:33 pm
content: Quote from: Overspeed on December 28, 2024, 10:10:35 amThe second design has the unknown part in the feedback, both terminals floating, and is less ideal for measurement.The first design is a current sink, not source, but maybe you are ok with +ve voltage to one terminal ?I’d suggest using a power Pmos, to allow grounded load, and a power spread resistor if you must operate from the 14V shown. Add some clamp diodes, in case the unknown element has some inductance.

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 28, 2024, 08:36:09 pm
content: Here's my Precision Current Source recycling project:https://www.eevblog.com/forum/metrology/recycling-of-precision-current-source-noise-reduction-for-low-burden-shunts/msg1432410/#msg1432410I use a BS170 for its low bias current with an NPN power transistor in a Darlington configuration, up to 100mA.Frank

author: Overspeed
author_rank: Frequent Contributor
timestamp: December 29, 2024, 10:24:31 am
content: Quote from: Kleinstein on December 28, 2024, 12:23:40 pmHelloThanks for your answerEevblog YT video 577 1 amp source use a RFP3055 which is equivalent to the new IRLZ14 I link a screen print with a drain to source resistance at 0.2 OhmOn the Mosfet circuit 1 mA there is no resistor on the gate , normally there is resistor to control ( limit) the current gate ) I have added one on my PCB in processRegarding the twin NPN/PNP I link the pdf with the complete schematic , the potentiometer are used because that not matched pairRegarding the 2.5V in the ref resistor , Vishay propose 8 W foil resistor as the VPR220 with low tempco , could be possible to adjust the ref voltage to match the real resistance value ? VPR from Vishay even with 0.5 tolerance are quite accurate in value .RegardsOS

author: Overspeed
author_rank: Frequent Contributor
timestamp: December 29, 2024, 10:33:06 am
content: Quote from: PCB.Wiz on December 28, 2024, 08:30:33 pmhelloI have linked a complete schematic in PDF in my Kleinstein answerthe designer of this circuit claims impressive performances but unmatched transistor request a lot of adjustements compare to a MosfetRegardsOSRegardsOS

author: Overspeed
author_rank: Frequent Contributor
timestamp: December 29, 2024, 10:39:16 am
content: Quote from: Dr. Frank on December 28, 2024, 08:36:09 pmHelloThanks for the link , use a small Mosfet to drive a bigger transistor is an elegant solution and I suppose by using a quite big transistor ( 1.35 amp) for a quit low current shall allow a negligible thermal problem .My goal is to built an affordable circuit to compete with a costly Keithley 6220 current source as I have no budget to purchase a $$$$ Keithley and also I need only a DC 100 mA sourceRegardsOS

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 03, 2025, 07:54:47 pm
content: I've always been partial to the MOSFET design. I first saw it in an early Siliconix MOSFET data book but it might go back further than that.

author: trobbins
author_rank: Frequent Contributor
timestamp: January 03, 2025, 09:40:55 pm
content: I'm sure you are on a better path than what I used, but fwiw:I used the power fet path for 100mA and 1A stable switched CCS. My requirements were a little different as I was after a transfer CCS reference, or simple bench top reference, which could operate from a 5V USB battery bank, and use an LM336-2.5 that I have on some spare pcbs, and an opamp I had available. The current was stable enough for 6-digit meter transfer, but needed measurement and maintaining a constant burdon voltage, especially for the 1A mode, in order to maintain transfer conditions. I didn't use low tempco resistors, but selected two 10W ceramic block for lowish tempco for the reference R (not as shown in schem below). I used a bypass resistor for the 1A mode to alleviate the FET temp rise.Regulation from burdon voltage variation was influenced/limited by the FET on-voltage being in the relatively high-gain but non-linear analog region. Thermal stability was influenced by various part tempcos, including FET on-voltage. FET heatsink was live so regulation can be sensitive to external leakage/contact.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 03, 2025, 09:59:10 pm
content: If a large current range like with the Keithley6220 is wanted, it would make sense to use largely separate current sources for small and large currents. The regulating transistor (JFET or MOSFET and maybe with an BJT for more current) is the least problem. The resistor switching, ref. voltage and the amplifier for the regulation have different requirements. The higher currents like 100 mA to 1 mA have the keep heating of the resistor in mind and want a relatively small voltage and low voltage noise / drift. The small currents in the low µA range no longer care about the heat, but leakage and current noise become relevant.A big question is also if one need protection against applied voltage. With the higher currents and high external voltage (e.g. 300 V) the SOA of the transistor can become an issue. Instead of high power parts one can consider a turn off if external voltage is detected.

author: trobbins
author_rank: Frequent Contributor
timestamp: January 03, 2025, 10:12:17 pm
content: Quote from: Dr. Frank on December 28, 2024, 08:36:09 pmFrank, what bridge rectifier did you use for B1 and B2 for the dc rails in post #2 of that thread? The scope image suggested to me that the cause of the transients could relate to diode commutation and leakage inductance in the transformer secondaries, which could be mitigated by using fast soft-recovery diodes and a tuned C-RC snubber across each secondary winding (the 100nF shown may not be optimum).

author: Dr. Frank
author_rank: Super Contributor
timestamp: January 04, 2025, 11:08:56 am
content: Quote from: trobbins on January 03, 2025, 10:12:17 pmYou might probably be correct .. those are very simple, old bridge rectifier components, but I don't know the type any more.A snubber network, or 100nF capacitors across these diodes might help to improve the noise further.I already mitigated the transformer coupling by adding those 470nF capacitors from mains earth to case GND, but a properly shielded transformer would be better.As said, I simply re-used all the 35 y/o components.Frank

author: Overspeed
author_rank: Frequent Contributor
timestamp: January 04, 2025, 01:15:15 pm
content: Quote from: trobbins on January 03, 2025, 09:40:55 pmHelloThanks for tour circuitI have a question : what is the 10K//100K resistor as ratio is 1/10 between 100 mA and 1 AmpRegardsOS

author: Kleinstein
author_rank: Super Contributor
timestamp: January 04, 2025, 01:54:02 pm
content: 10K//100K stands for 10 K and 100 K in parallel. Ideally they want 9 K to get a divider to 1/10 of the voltage.

author: Zoli
author_rank: Frequent Contributor
timestamp: January 04, 2025, 04:08:20 pm
content: Quote from: Kleinstein on January 04, 2025, 01:54:02 pmAnother trick: parallel two 18kΩ for 9kΩ.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 04, 2025, 04:35:14 pm
content: Another way for a 1/10 voltage divider is using 6 equal resistors as 3 in series and 3 in parallel. With some matching this can give a rather good accuracy and low TC even with not so good resistors to start with. There are now also ready made dividers in SOT23 and similar small size.

author: iMo
author_rank: Super Contributor
timestamp: January 04, 2025, 04:38:14 pm
content: What "precision" do you actually target with that above circuit?

author: Overspeed
author_rank: Frequent Contributor
timestamp: January 04, 2025, 06:49:55 pm
content: HelloQuestion is for who ? in my case I need to run 4 points resistivity measurement so I need to be ''better'' than Keithley 6220 , but as not a programmable value bu a fixed value better can be reachable .I also don't want to fall under 10 Volt to reach acceptable power level .tests / measurements are 10 minutes so i don t need 1 year bomb proof stability too as far I am stable .I link the Keithley 6220 data table for information 100 mA is not amazingI have not the budget for new/calibrated 6220 and I don't need all functions of the 6220 too , just a robust accurate ( stable and precise ) 100 mA source for low to very low resistance material , primary idea was to built a zlymex or a Eevblog current source ( 1 amp) tuned to 100 mA , current source recipe don t look to be NASA rocket science as first is to have a stable and accurate voltage ref as ref54 / ref80 or other with a precision op amp and precision low tempco resistor.RegardsOS

author: Kleinstein
author_rank: Super Contributor
timestamp: January 04, 2025, 07:32:13 pm
content: For a resistance measurement one likely would not need the high settling speed of the 6220 and not much current programming other than maybe on / off / reverse polarity. For a 100 mA range only, the main critical part is the current setting resistor. One would likely want a 4 wire resistor or at least seprate traces directly from the resistor.Depending on the meter used for the voltage reading, one could also consider the variant with the DUT and ref. resistor directly in series and not on different sides of the FET. This version would allow easier switching between the voltage at the DUT and ref. resistor and also an easy polarity switching. However it kind of needs a separate supply, and a ratiometric measurement gets a bit more tricky. Depening on the resistance range a true ratiometric measurement may not be needed - very low resistors (e.g. < 10 mohm) may well have other limitations.

author: iMo
author_rank: Super Contributor
timestamp: January 04, 2025, 09:50:55 pm
content: Quote from: Overspeed on January 04, 2025, 06:49:55 pmThe Q is for you. You want a precision current source, but except the "100mA" there is none other indication what you actually want (provided this is the Metrology section people wait on some indication). Based on some specific parameters (meaning numbers) people may judge on the actual circuit and parts to be used.

author: Overspeed
author_rank: Frequent Contributor
timestamp: January 05, 2025, 09:28:50 am
content: Quote from: Kleinstein on January 04, 2025, 07:32:13 pmHelloThanksI agree on the simple need compared to the possibilities o 6220 .for the 4 wires resistor I agree that in use on the eevblog 577 amp and also on Zlymex ( eevblog post ) I link the picture of the circuit and the current values.architecture looks similar , a precision voltage source Zlymez use a AD780 and a VR4 resistor and Eev use a ltc6655 and a VPR221Z with an OPA376both voltage source are 2.5VA concern is the resistor heating as the sense resistor handle the power so a direct link wih the voltage and the current .RegardsOS

author: Overspeed
author_rank: Frequent Contributor
timestamp: January 06, 2025, 08:24:46 pm
content: Quote from: iMo on January 04, 2025, 09:50:55 pmHelloI agree , base on data provided by Zlymex on his 1ma source ( see pic linked ) and the table linked or the 6220concern is more the current level increase , more difficult to reach tight specs .Times Electronic 1021 DC Current Source is advertised for ± 0.02 % of setting ± 0.02 % of range, ± 0.2 μA and 25 ppm/hr stability which sound acceptableTimes Electronic 1024 is accuracy ± 0.02 % of setting + ± 0.005 % of range + ± 0.2 nAAs I measure low resistivity alloy I need 10 mA and 100 mA , problem with 100 mA is the Keithley 6220 is not wonderful on he 100mA range with 0.1 % accuracy and no real value regarding the noise or the stability under loadI need something better than the performance of the 6220 without any other option as that a 100 mA only DC Times Electronic 1021 looks better for my purposes.I will try the Trobbins circuit with selected resistor and a ref55xx , perhaps set two Mosfet in // can improved the thermal concern of the Mosfet ? in the aim to built a better 1024 time Electronic modelRegardsOS

author: zlymex
author_rank: Frequent Contributor
timestamp: April 22, 2025, 03:17:10 am
content: I=V/R，for 100mA the good choice is around 0.25V/2R5.However, the best V lies in 2.5V to 10V, therefore there are two approaches:one, 10:1 voltage divider, twin or array;two, 1m/100m current booster, and 1mA=2V5/2k5The later one also needs a resistor pair but can be assembled by 10p/10s combination.

author: Overspeed
author_rank: Frequent Contributor
timestamp: April 22, 2025, 12:47:48 pm
content: Quote from: zlymex on April 22, 2025, 03:17:10 amHelloConcern quite high current as 1 Amp is to get 0.25 V that request 0.25 Ohm and lower is the resistor value higher is the TempcoCould be an assembly of 1 Ohm resistor as 1 Ohm have a better Tempco and resistor in // increase wattage too .Other concern is to match the voltage value at the amp op input so voltage ref divider shall provide perfect value or use DAC ref ? in 20 bits ?RegardsOS

author: zlymex
author_rank: Frequent Contributor
timestamp: April 23, 2025, 04:17:50 am
content: We have relative less noise and smaller thermal EMF for higher shunt voltage, but more power dissipate create more temperature rise thus more thermal EMF. The common way to solve this is to use lower thermal resistance shunt, and the OP's multi-parallel-shunt just did this.The target of a large current is more on short time stability rather than accuracy. Yes DAC can provide fine adjustment of the current or even make it programmable.

author: Overspeed
author_rank: Frequent Contributor
timestamp: April 24, 2025, 07:11:54 am
content: Quote from: zlymex on April 23, 2025, 04:17:50 amHelloOk for multi // set up as that the only solution regarding TC and precisionFor thermal management that a concern as Vishay clearly provide dataI plan to use VPR221 in // , just try to optimise the size of the heatsink as the Mil spec ref one of Vishay is BIGRegardsOS

author: zlymex
author_rank: Frequent Contributor
timestamp: April 25, 2025, 02:39:23 pm
content: There maybe two criteria for short time stability of a shunt:One is the warm-up drift from cold start.drift1 = (element temp rise)*TCR + Power*PCR + (Thermal EMF effect etc.)However, if we skip the first 15 seconds or so, the second term can be omitted.The other is the drift of say ten minutes or 1 hour duration after completed warm-up.drift2 = (temp variation)*TCR + (Thermal EMF effect etc.)Temp rise(and temp variation) refer to that of the resistive element, which depends very much on the power and thermal factors.temp rise = power*(thermal resistance element to ambient)thermal resistance element to ambient = element to sink + sink to ambientBy multi-parallel we reduce element to sinkBy proper heat-sinked we reduce sink to ambient.Therefore, five things can be done to reduce short time drift:1. Choose shunts of small thermal resistance such as AE FNP(0.1K/W), Isa RUG-Z(0.1K/W), AE PSB(0.3K/W), Powertron UNR 4-3425(2.1K/W). For VPR221, the thermal resistance is about 10K/WThis 3.4K/W is from AE PC at its best but is till not enough for strict requirement2. If one shunt is still not good, parallel more to reduce thermal resistance3. Proper heat-sink such as large, greased, fanned.4. Choose low TCR shunt elements.5. Matching and compensation.Low TCR shunt elements maybe classified into 4 or 5 grades(30-50℃):1st, <=1ppm/K,AE PNP-ZandVishay CSNG2nd, <=2ppm/K, selected VFP4Z and VCS331Z or alike3rd, <=3ppm/K, VCS331Z, AE PC Y, AE PD Y, VPR221Z, USR UNR TK3, JRL CH48, good RUG-Z4th, <=5ppm/K, , VCS301, USR UNR TK5, PBV-Z,AE PSB X, AE PC X, average RUG-Z5th, >5ppm/K, VPR221, VHP-4, AE PC W.

author: Overspeed
author_rank: Frequent Contributor
timestamp: April 27, 2025, 09:08:47 am
content: Quote from: zlymex on April 23, 2025, 04:17:50 amHelloThat why I work on resistor in // with a thermal management ( heat sink + air flow in a controlled temp enclosure ) , VPR have low TC good accuracy in the range of 0.02% for 2 Ohm value so 4 VPR 221 will allow 4x8 = 32 watts and 0.5 Ohm value with an acceptable costVPR221 Z is a way to improve they are available in 1 Ohm to reach 0.25 Ohm by using 4 in //RegardsOS

author: zlymex
author_rank: Frequent Contributor
timestamp: April 27, 2025, 12:03:48 pm
content: Quote from: Overspeed on April 27, 2025, 09:08:47 amWell, 8 watts is the max (certain heat-sink but no fan， temperature rise is about 80degreee) which should be downgraded for precision use.

author: Overspeed
author_rank: Frequent Contributor
timestamp: April 27, 2025, 02:50:15 pm
content: Quote from: zlymex on April 27, 2025, 12:03:48 pmHelloI agree as 8 watts is the maximum under dedicated BIG heat sink setup , as I use 5 V PSU under 1 Amp I have 5 Watts by using 2 or 4 resistor I keep the thermal stress low .and also try to improve stability , in the Dave EEVblog #577 - Precision 1A Current Source he use a single VPR 221 Z in 1.25 Ohm with no heat sink ?RegardsOS

author: zlymex
author_rank: Frequent Contributor
timestamp: April 27, 2025, 03:02:53 pm
content: Quote from: Overspeed on April 27, 2025, 02:50:15 pmA perfect negative example.

author: Overspeed
author_rank: Frequent Contributor
timestamp: April 27, 2025, 03:18:34 pm
content: Quote from: zlymex on April 27, 2025, 03:02:53 pmHelloThe problem for measure and metrology purpose or goal is to reach stability and repeatably , current source shall deliver accuracy and stability , heat sink design and setup is a crucial point in design.Dave circuit works as it but need next step as robust PCB design and thermal management .RegardsOS

author: Overspeed
author_rank: Frequent Contributor
timestamp: April 28, 2025, 07:52:18 pm
content: Quote from: zlymex on April 25, 2025, 02:39:23 pmHellomy idea is to use warm up load to draw current through the Ref resistor , stabilize the enclosure temp / heat sink , switch the load and use the current source after , with a temperature control to avoid any unwanted drift .For my measurement I don't need hours of current supply as the test in the 5 minutes duration time but I need repeatability day after day as I need to repeat and compare resultsRegardsOS