Milliohm Meter Version 1.5

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.

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.

Link to orig­i­nal Milliohm Meter Project page.
OSH Park print­ed cir­cuit board Milliohm Meter V1.5 project page.
EagleCAD Milliohm Meter V1.5 board files ZIP