1 A current source with issues - Page 1
Forum section: electronics / general

author: dutch66
author_rank: Contributor
timestamp: March 23, 2022, 11:39:07 pm
content: Hello everybody,I am trying to build a 1 A current source with 1 mA precision to check sensors. I thought it would be a small side project but it turned into a one of these unexpected time sinks.Find the schematics attached. It basically consists out of two parts a power supply and a part to generate the 1A current. The regulation part was modeled before I build it and indicates that the circuit should be more accurate than it actually is.An ADR441BRZ 2.5V voltage reference is used as reference element. The 2.5V can be precisely trimmed using RV1. A multi turn low tempco Vishay pot is used. Then the voltage is scaled down using R6 and R9. Low tempco precise resistors are used and the voltage can be checked at TP7. The current regulation is done using an AD8538 chopper opamp with low offset and a mosfet. Resistor 1 is a 8-W 1Ohm Kelvin connected resistor from Vishay.I connected a cable between J2 and J3 adjusted the voltage at TP7 to 1.00000V and measured at TP9. The result is 1.034 V, more than 30 times the expected errorAny ideas what I am overlooking?Thanks

author: maat
author_rank: Regular Contributor
timestamp: March 24, 2022, 12:00:08 am
content: Quote from: dutch66 on March 23, 2022, 11:39:07 pmOscillations? Did you check with an oscilloscope?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: March 24, 2022, 12:17:07 am
content: Hi,You need to use the Sense- connection on the resistor as a star point for all the small signal grounds:This will help get you good accuracy.The OP-amp V- can be connected to ground.Regards,Jay_Diddy_B

author: guenthert
author_rank: Frequent Contributor
timestamp: March 24, 2022, 12:22:58 am
content: Is R11 exactly 1Ohm? Even when heated by 1W? That seems like a rather large amount of power for a precision application to me. Since you tolerate 0.1% error, I'd think (and you have to verify the spec sheet) that the OpAmp IC4 offset voltage drift is small enough to allow for a smaller voltage drop across R11, i.e. a smaller value of resistance for R11.(said that, there are 1Ohm resistors of Julie research available on FleeBay for an almost reasonable amount which seem perfect for this)

author: dutch66
author_rank: Contributor
timestamp: March 24, 2022, 12:32:56 am
content: Quote from: maat on March 24, 2022, 12:00:08 amJust did and the voltages at the output of the resistor as well as the gate voltage of the mosfet show indeed oscillations. Input voltage is clean.I will grab some caps and start to fight them.Thanks

author: dutch66
author_rank: Contributor
timestamp: March 24, 2022, 12:44:13 am
content: Quote from: Jay_Diddy_B on March 24, 2022, 12:17:07 am

author: dutch66
author_rank: Contributor
timestamp: March 24, 2022, 01:25:01 am
content: Here is a quick update.As suggested by maat I checked for oscillations and the feedback was heavily oscillating. Adding a capacitor between TP8 and GND turned R7 into a low pass filter and stopped the oscillations. I now get a stable 1.0006 AAlso with a external capacitor soldered to a IC pin and a test point the PCB now looks like a real prototype.If there are more suggestions on how to improve the circuit please let me know.

author: mawyatt
author_rank: Super Contributor
timestamp: March 24, 2022, 01:38:57 am
content: You want to make the IC4 op-amp "see" a bypassed FET since it has a large gate capacitance and large phase shift. Insert a resistor (1~10K) between TP9 (- input of the op-amp) and R11 so the feedback bypass capacitor has some reasonable impedance to work into, then a cap of 0.01uF~0.1uF from TP9 to pin 6 the output of op-amp IC4.Also make sure the FET can supply the necessary max current (1a) since the supply voltage is only 5 volts and the gate voltage needs to be Vgs +1 volt at the max 1a current, but can't be greater than Vcc or 5 volts.Best,

author: maat
author_rank: Regular Contributor
timestamp: March 24, 2022, 02:42:00 am
content: Quote from: mawyatt on March 24, 2022, 01:38:57 amI second that. You did not say much about the DUT, but there seems to be binding posts. Do make sure there are no oscillations with the maximum cable length you intend to use. Your opamp feedback loop includes the cable (twice). Using a feedback resistor and capacitor you can tune the loop to make it slow enough for a long cable.Also adding a resistor to the In- pin adds a little protection for the opamp, because it is likely exposed to esd events through the binding posts. I would go for something > 100R.Star gounding could be done, but a solid ground plane with 35 µm copper should keep you within that 0.1% budget without special layout. Thermal issues will likely be the driving factor.Beware the rabbit hole.

author: pqass
author_rank: Super Contributor
timestamp: March 24, 2022, 03:58:13 am
content: With regards to oscillations, see C1, R2 below. Also, designs typically use 100R for a gate resistor.And why place the binding posts between source and shunt? Again, typically they're between Vcc and drain (if your device provides the power source).See Kerry Wong's many CC loads here:http://www.kerrywong.com/2013/03/08/constant-current-dummy-load-in-an-hdd-cooler/

author: HighVoltage
author_rank: Super Contributor
timestamp: March 24, 2022, 08:07:20 am
content: Why did you choose 1 Ohm for R11 and not smaller. At 1Watt you might have a large TC and problem with linearity?What is the DUT and how long are your cables?Picture of your setup?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 24, 2022, 08:21:38 am
content: Ther version with the output between the source and shunt resistor can have problems with a limited gate voltage. The voltage drop over the exterma DUT would reduce the available voltage. So the maxium voltage there would be quite limited.For stability against oscillations in case of a rather inductive load, and also as some ESD protection it would be good to have some RC series element (e.g. 100 nF and 100 ohms) in parallel to the terminals. This applies to both forms.1 W or power is quite a higher power level for a precision resistos. So it would be more suitable to use a smaller resistor, more like in the 0.1 ohms range. The connections at a low value resistor are critical, not to include too much wire or even PCB trace. With 0.1 Ohms a 4 wire resistor is likely needed.

author: mawyatt
author_rank: Super Contributor
timestamp: March 24, 2022, 01:08:52 pm
content: Quote from: pqass on March 24, 2022, 03:58:13 amThat circuit generally won't work with passive loads simply because the FET Drain needs to be supplied with a positive voltage. The external load should be between the FET drain and a positive supply voltage.Moving the load to the FET drain is a good idea IF the load can FLOAT well above ground and have one terminal connected to a positive supply voltage.Best,

author: pqass
author_rank: Super Contributor
timestamp: March 24, 2022, 02:26:54 pm
content: Quote from: mawyatt on March 24, 2022, 01:08:52 pmYes, you are correct. The schematic is of a simple constant current load intended to be an alternate design for the CC portion of his (voltage source + CC load = CC source). I have updated the schematic below to reflect my words.Not mentioned thus far in this thread, +V can be unregulated and far higher than circuit Vcc.Also, I would add a switch (or a third, GND binding post) such that the user can decide if they want a source or sink.And a 1.5A fuse on the drain just in case user connects load in reverse.QuoteYes. I would also recommend to isolate the CC source by using an old-school transformer+linear regulator power supply. It's far easier than flipping the shunt to the source rail, using a P-MOSFET, etc.

author: dutch66
author_rank: Contributor
timestamp: March 24, 2022, 06:28:43 pm
content: Hello everybody,first of all let me thank all of you for your comments. It is great to have so much helpful input.Quote from: maat on March 24, 2022, 02:42:00 amThe DUT is very simple, it is nothing else than a laboratory cable with banana plugs on both ends. The cable I am using in the tests is about 1.1 m (43") long. The unit is used for quick sanity checks on current sensors where the cable is fed through the sensor.I agree that there should be a resistor to protect IN-. I connect the cable an switch the unit on afterwards but the next guy may not and kill the opamp. Also the capacitor in the feedback path makes sense to me. I added a capacitor between TP8 and GND because it was easy to do on the PCB without cutting traces. It killed the oscillations but I like the mentioned cap between IN- and OUT of the opamp.As I have to do a revision anyhow I may try the star grounding. I'm not 100% sure as the unit is within spec right now and I don't have much experience with it. You have shown allQuote from: HighVoltage on March 24, 2022, 08:07:20 amTo be honest, the 1 Ohm resistor was chosen for simplicity and for the sake of a higher voltage. It seems to be OK. Here is a graph showing the stability over a 1h period. The unit was run for 1h then it was connected to the current measurement of a 34401A and set to 1A. I should have connected it from the beginning to avoid settling of the multimeter but I did not. Nevertheless the results are not bad.During the test the temperature of the resistor and of the MOSFET were measured with an IR thermometer (Fluke 62 MAX+). The thermometer is not ideal for such measurements but I don't have a thermal camera. The resistor settled at about 50 deg C as well as the MOSFET. After the test the unit was kept running and temperature at the resistor was measured with a thermocouple. It measure 67 deg C.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: March 25, 2022, 02:15:42 am
content: Hi,You could consider placing one or two diodes in series between J2 and J3 like this:The diodes will provide an alternative path if the banana - banana lead is open circuit. The leakage current should have very little effect because the voltage across the diode should be very small.Use 1-3 A silicon rectifier diodes, like 1n4002 or 1n5402 or the SMD equivalents.Jay_Diddy_B

author: macaba
author_rank: Regular Contributor
timestamp: March 25, 2022, 08:37:01 am
content: I came here to say the same thing as mawyatt - add resistor on the sense line, add capacitor over the op amp (exact connections stated by mawyatt). These two + gate resistor lets you tune the response of the circuit in steady state and step change. Ltspice is useful here, I simulated something similar last week for a 4W voltage output buffer on PWM DAC.