Electronics > Projects, Designs, and Technical Stuff
1A current source but still not stable
dmm2018:
--- Quote from: Doctorandus_P on January 19, 2019, 11:14:52 pm ---I find it a bit strange that you're "complaining" about what looks like a bit of drift in the last 3 digits of a 7 1/2 digit EUR 1500 meter with such a simple circuit.
From memory:
I think Davey Jones put some beefy capacitors over the shunt resistors to improve stability. But because the shunt resistors are so small you would need some relatively big caps (Few 100 uF) to get a usable time constant.
Talking about specifications:
What kind of figures are you expecting for?
Such a wide open circuit will probably drift a few miles if you touch it or blow in its direction if you measure it with a 7 1/2 digit DMMM.
(P.S. Don't take it badly, i'm jealous, but please learn to interpret what you see on such a meter more accurately).
--- End quote ---
Thank you very much and we really appreciate for the comments/suggestions. We're impressed with the stability of Dave Jones Precision 1A Current Source (shown in Part 2) which was constructed on a breadboard and without heatsinks. We hope to achieve similar stability. :D
Our aim is to have a stable current source that will complement the 34470A in low ohmic resistance (~1mΩ) measurement. We note that the Keysight 34470A does not do so well in low ohmic resistance measurement, e.g. for 1mΩ, the uncertainty is 400%. If not mistaken, DMM6500 can do better than 34470A in low ohmic resistance measurement (e.g. for 1mΩ, the uncertainty is 20%).
--- Quote from: Doctorandus_P on January 19, 2019, 11:14:52 pm ---Is your MOSfet designed for lineair applications, or is it optimised for fast switching, which most are?
This can be a source of instability (local heating in the FET) and even destruction.
--- End quote ---
The power MOSFET we used is RFP12N10L, which is not designed for linear applications. We used the same power MOSFET in a DC electronic load project (5A max) and several were damaged so far... :-[
dmm2018:
We have done several tests to evaluate the thermal distribution of the current source prototype. PROSKIT MT-4612 infrared thermometer was used.
A small heatsink was placed approximately 1m away from the current source prototype. The temperature of the small heatsink serves as the ambient temperature (Tamb).
The temperatures of seven (7) points on the current source prototype were measured (see figure below). The temperatures were recorded every 5 minutes, and it took approximately 1 min to record all the Tamb and T1~T7.
Several observations:
(a) The ambient temperature Tamb is quite consistent over the 60-min duration.
(b) The temperatures of AD780 and voltage divider resistive network (R1~R5) are very close to each other.
(c) The temperature of AD708 is consistently approximately 2°C higher than the temperature of AD780 and R1~R5.
(d) The temperature difference between the VPR221 (0.5Ω) and the voltage reference AD780 is approximately 3°C to 4°C.
(e) With VS = 12V, the temperature near the power MOSFET (T1) rose to 40.7°C after 60 mins, almost 10°C higher than that when VS = 6V.
The results suggest that the heat generated by the power MOSFET @ VS = 12V has indirectly caused the temperature of other components to raise about 2°C to 3°C. This probably explains why the drift when VS = 12V is more significant than that when VS = 6V.
Still figuring out how these temperature variation "contribute" to such a huge 100ppm drift when VS = 12V...
Kindly advise.
dmm2018:
Just took some thermal images of the current source prototype using Testo Thermal Imager. The current source prototype was tested under three different VS (3V, 6V, 12V, VS is the main voltage source that supplying the ~1A current, see first thread). Thermal image was taken every 2 mins for a duration of 1 hour.
Kleinstein:
When just using it to test meters there is no need to have so much voltage turned to heat at the MOSFETs. This also makes the selection of the MOSFET easier. At low voltage it is less important to have a MOSFET that is specified for linear operation.
If the FET gets really hot, the gate current might get significant, and this could add to the current. To avoid this one could use a small FET and a BJT combined as a kind of IGBT replacement circuit. So the FET driving the BJT base current.
The adjustment of the 1 A value is probably better done at the divider, not at the shunt.
For less higher frequency noise if could help to add some filtering cap to the voltage divider.
The AD708 may not be the best choice: it's high precision for relatively high impedance source - the shunt is more like low impedance. So other OPs may perform better. If only DC is important one could consider an ADA4828 or similar.
For the stability one must also take into account that the DMM is not that good at 1 A - the DMM internal shunt and amplifier may be lower quality.
dmm2018:
Plan to replace the AD780AN with LTC6655BHMS8-1.25. However, found out that the short term drift of the LTC6655BHMS8 is quite bad and is worse than the AD780AN.
A basic circuit of LTC6655BHMS8 was built (BasicConnectionDonutBoard.png). The output voltage of the LTC6655BHMS8 was logged (15 sec after the circuit was powered up) for 1 hour (PLC=10, sampling interval 500ms). The test was repeated several times and the short-term drift was quite consistent and was worse than the AD780AN. For example, as shown in 6655DB1.png, the output drifted 46uV. The trend shows that the output would have drifted further after 1 hour. :-//
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