Update to the Milliohm Meter Project
Updated V1.5 board assembly for the Milliohm Meter Project.
The Milliohm Meter board V1.41 is featured in Scullcom’s Milliohm Meter Udpdate YouTube video.
I had already started on the version 1.5 board update a week earlier and was about to send the prototype files out to OSH Park for manufacturing, when I noticed a new Scullcom video thru my YouTube subscription notice. I was pleasantly surprised to see that it was an update to the original Milliohm Meter and it was featuring the V1.41 board that I had designed. Louis had made some similar updates to components, so I quickly updated the V1.5 board file to match before sending it off.
The only major differences in components, are some even larger capacitors in the +/- 10 volt power supply section, and a PTC resettable fuse on the 9V battery supply section.
I had been putting together a BOM (bill Of Materials) for the 1.5 board and found that the 33uF capacitors were the same size and price as the 22uF capacitors that were originally used in the V1.41 board. Using a larger capacitor reduces the output ripple even further and also slightly increases efficiency for no additional cost in size or money, a win — win design change.
I have also been wanting to get some power protection added to the board for a while and should have included it in the 1.41 board. I tend not to include it in the earlier prototype’s until I am happy with the design, and can measure the current used under actual operating conditions. While prototyping I will often use a current limited power supply, or an external fuse.
The V1.5 board now has a PTC fuse to limit current, in case of a catastrophic fault on the board.
For those not familiar with PTC fuses, they are a resetable fuse, sometimes called polyfuse, polyswitch, or polymeric positive temperature coefficient device (PPTC). They protect a circuit by changing from a low resistance at room temperature when operated at or below their rated holding current, to a high resistance if the circuit exceeds the trip current. The excess current causes the device to heat up, greatly increasing it’s resistance under fault conditions and limiting the current. After the fault has been removed, and the PTC fuse has cooled down it will normally return to it’s low resistance state allowing the circuit to operate.
Other updates to the V1.5 board include larger traces in the 100 mA current source circuitry to reduce resistance and trace heating to less than 0.005°C. The small via’s are now masked to reduce the possibility of shorts, and I also included a two pin 5V header for those wanting to power the panel meter directly from the on-board 5 volt supply.
A word of caution, some panel meters can generate significant noise back thru the 5V supply. Additional inline filtering for the display may be necessary.
I created a new front panel for the V1.5 Milliohm Meter using “Front Panel Designer” to fit a standard Hammond 1455N1601 extruded box with metal end plates 6.299″ L x 4.055″ W x 2.087″ H. Link to design file
The panel is made from “Medium bronze” anodized aluminum and is 2.5 mm in thickness. It has a rectangular cutout for the panel meter, D‑holes for the four banana jacks, and countersunk holes for box mounting.
The front panel meter was again purchased from ColdfusionX on Ebay and is a 2 Volt full scale meter that operates from a 5 volt supply.
Bill of Materials for the Version 1.5 board including banana jacks and battery pack.
Part Number, Manufacturer, Manufacturer Part Number, Reference Quantity, Description A105944CT-ND, TE CONNECTIVITY, YR1B10RCC, R11, 1, RES 10.0 OHM 1/4W 0.1% AXIAL BH26AAW-ND, MEMORY PROTECTION DEVICES, BH26AAW, BATTHLDR9V, 1, HOLDER BATT 6-AA CELLS WIRE LDS 493-3717-ND, NICHICON, RR71C151MDN1, C5, 1, CAP ALUM POLY 150UF 20% 16V T/H 3296P-101LF-ND, BOURNS INC, 3296P-1-101LF, R13, 1, TRIMMER 100 OHM 0.5W PC PIN INA106U-ND, TEXAS INSTRUMENTS, INA106U, IC4, 1, IC OPAMP DIFFERENTIAL 1MHZ 8SOIC MAX680CSA+-ND, MAXIM INTEGRATED, MAX680CSA+, IC3, 1, IC REG SWTCHD CAP INV 10MA 8SOIC LT3092EST#PBF-ND, LINEAR TECHNOLOGY, LT3092EST#PBF, IC2, 1, IC CURRENT SOURCE 1% SOT223-3 LT1634BCS8-1.25#PBF-ND, LINEAR TECHNOLOGY, LT1634BCS8-1.25#PBF, IC5, 1, IC VREF SHUNT 1.25V 8SOIC 3266P-1-104LF-ND, BOURNS INC, 3266P-1-104LF, R12, 1, TRIMMER 100K OHM 0.25W PC PIN PPC56.2ZCT-ND, VISHAY, MRS25000C5629FRP00, R1 R3-5, 4, RES 56.2 OHM 0.6W 1% AXIAL PPC60.4ZCT-ND, VISHAY, MRS25000C6049FRP00, R2, 1, RES 60.4 OHM 0.6W 1% AXIAL A105891CT-ND, TE CONNECTIVITY, YR1B499KCC, R10, 1, RES 499K OHM 1/4W 0.1% AXIAL 100ADCT-ND, YAGEO, MFP-25BRD52-100R, R9, 1, RES 100 OHM 1/4W 0.1% AXIAL 501-1081-ND, POMONA ELECTRONICS, 1581-3 PJS+, 1, JACK BANA PANEL MT TIN ORG 501-1079-ND, POMONA ELECTRONICS, 1581-1, PJS- 1, JACK BANA PANEL MT TIN BRN 2269-0-ND, POMONA ELECTRONICS, 2269-0 PJV+-, 1, BANANA JACK DOUBLE BLACK BC1084CT-ND, VISHAY BC COMPONENTS, K104K15X7RF5TL2, C6-10, 5, CAP CER 0.1UF 50V X7R RADIAL 493-14231-ND, NICHICON, RNS1C330MDS1, C1-4, 4, CAP ALUM POLY 33UF 20% 16V T/H LM2940IMP-5.0CT-ND, TEXAS INSTRUMENTS, LM2940IMP-5.0, IC1, 1, IC REG LDO 5V 1A SOT223 BC1078CT-ND, VISHAY BC COMPONENTS, K103K15X7RF5TL2, C11, 1, CAP CER 10000PF 50V X7R RADIAL RXEF025HF-ND, LITTELFUSE INC, RF2628-000, F1, 1, POLYSWITCH PTC RESET 0.25A
I ordered 100 of the 56.2 Ω resistors, and 20 of the 60.4 Ω resistors and then picked the best of the group by measuring the resistance at two different temperatures 15° C apart for the lowest temperature coefficient. The board has an area for adding a copper shield around the dual charge-pump voltage converter +/- 10 volt supply section, which has a switching frequency of around 8 kHz. I used a 1/2″ strip of 26 Gauge copper sheet from Integrity Beads on Amazon to form the shield, and soldered it in place using the thru-hole grounds.
Front panel banana jacks are connected using silver tinned Teflon 22 AWG wire, with a ferrite bead on each wire. I also used ferrite beads on the panel meter connections, along with a 2 pin jack for the panel meter power connection.
Calibration was fairly easy with two trim potentiometer’s to adjust. To calibrate the zero reading, short the Sense (S) + and — jacks and adjust the zero trim pot to read 0.0000 on the panel meter. Then connect a high quality multimeter for measuring milliamps and connect the leads to the Current Source © + and — jacks and adjust the 100 mA trim pot to read 100.00 mA on the multimeter. Disconnect the calibration leads to prevent drain on the battery as soon as the 100 mA calibration is complete, and connect your 4‑wire kelvin leads to the meter and you should be ready to measure a known milliohm resistance as a check.
Be sure and watch Scullcom Hobby Electronics — Milliohm Meter Update on YouTube as he explains the theory of how the meter, and different devices in the circuit work.