如何制作简单的毫欧元测试仪电路

I wanted a milliohm tester circuit that could be used to measure resistance on printed circuit boards to track down shorted components. I looked at several designs and combined several ideas into this project.

By Henry Bowman

Circuit Operation

Referring to the schematic, the milliohm tester is powered by two 9 volt dry cells. The power is connected to the circuit by a double pole, single throw switch S1. Since the voltage was pure dc, I did not add filter capacitors. I didn't add a led to indicate power on because the meter will move to the right as soon as power is applied.

7805稳压器和R1在Q1基部提供恒定电流和电压。有些设计使用齐纳二极管进行此功能，但7805也很棒。较大的电压+9与RH1串联串联到发射器，底座上的电压看起来为负极，允许发射器，基础，集电极电流。RH1通过Q1＆R2提供毫安的电流调整，以测试铅A.

The current will not exceed the constant current at the base of Q1. R2 was also added to the collector side to provide some temperature compensation for Q1. When a resistance load is connected to the test lead terminals A&B, the voltage at terminal A is connected to R3 and the input pin 2 of the 741 IC.

R3和R4的组合确定了opamp的电压增益（R4 / R3 = 1000）。opamp的引脚2是反相输入，因此引脚6处的输出是负的。RH2提供将仪表归零到左侧。负电压通过RH3到1 mA全尺度模拟计量计。RH3提供校准仪表到右侧（满量程）。D1＆D2提供一些过电压保护。C2是可选的。

I added C2 to slow down my meter movement. As the resistance is lowered across test points A & B, the voltage will also be lowered to the input of the opamp. The meter operates just opposite of an analog ohm meter. With only the ten 1 ohm resistors in parallel across the test leads, the meter will be at full scale to the right, indicating 0.1 ohm. When a zero ohm resistance is connected to the test leads, the meter will move to the extreme left for zero ohms. If you want more sensitivity to resistance, increase the parallel one ohm resistors from ten to twelve. This will make the full scale resistance .08 ohms instead of .1.

Construction Details

You need the largest 1mA, or 750uA meter, that you can find. I found one from an old automotive engine analyzer that was 5-3/4” wide and 4-1/4” tall (14.6 X 10.8CM). It has a large spread from full scale to zero. Resistors can be 1/8, or ¼ watt due to the low current.

组件可以安装在通用型PC板上或在穿孔板上的指点接线。我用晶体管和IC的插座，使它们更容易更换。“死虫”也可以使用，其中IC在直接焊接到IC引脚的电路板和电线上倒置。

If you solder the ic and transistor, be sure to grip each lead with needle nose pliers to provide a heat sink for the pins. Be sure that you place the negative side of the meter to the RH3 potentiometer. The postive side of the meter connects to ground. RH1 and RH3 pots need their center connection pin strapped to the right pin. The potentiometer connections are viewed with the pot shaft facing you.

RH2 has wires connected to all three connections. I cannot over emphasize the need for perfect soldered joints in this project. The tester is very sensitive to very small changes in resistance. The three potentiometers and power switch should be mounted externally with the meter. Provide a two terminal mounting post for the test leads A & B and the two connecting wires from the pc board.

Provide some additional strain relief for the test cords by using a cable tie or cable clamp to secure the ends inside the enclosure. The test leads should be insulated copper stranded wires and sized #12--#14 gauge. I used a piece of power cord from an old electric saw. The soldering must thorougly melt on the test leads to assure a good connection. Test leads should extend 16” (41CM) from the chassis. Install the ten (or 12) 1 ohm resistors on the test leads about 8” (20CM) from the chassis.

The number of resistors you choose depends upon the full scale reading your require. Ten will provide a 0.1 ohm full scale and 12 will provide .08 ohm full scale. The resistors can be 1/4 or 1/8 watt rated. The resistors can be pigtailed together and each side soldered before placing on the test leads.

再次，确保在电阻器上有热的铁和良好的焊料流，导致测试引线上的铜线。在校准测试仪之前，不要使电阻器隔热，并满足您的焊接连接很好。完成安装电阻后，移动到测试引线的最终。从每个测试铅端​​部关闭大约1/2“（1.3cm）的绝缘。一旦准备好开机，请转到校准并按照逐步遵循措施以避免损坏仪表。

校准

这里假设您拥有连接到测试引线的1欧姆电阻，并且末端已被剥离。确保您允许足够的时间让电阻从焊接中冷却。采取两端的测试引线并将它们扭曲在一起。

前驱动,形容词,卡尔的设置为零。potentiometers to mid range. Set the ma adj. potentiometer to fully clockwise position. Remember before you power up that zero ohms is to the left and 0.1 (or 0.08) is to the right. Switch on the power to the tester and observe the meter. If it deflects to the left, below zero ohms, adjust the zero pot clockwise until the pointer is on zero.

如果它右转，则零点逆时针调整零盆，直到它处于零。拆下短路末端，仪表应移动到右侧。您将不得不调整Cal锅以使仪表右侧满量程。现在将短回到引线上，看看是否需要额外的零调整。如果您必须再次重新调整零，再次移除短路并重新调整Cal锅。重复一遍直到短路和删除短暂需要进一步调整。现在你有球场校准。

预校准后施工

Now that you have the pre-calibration completed, you need to add some sharp pointed metal ends to the test leads. These can be sharpened copper nails, or sharp test probe ends removed from junk equipment. These sharpened ends should be about an inch (2.5CM) in length. The stranded copper on the test lead ends should be wrapped and soldered around the opposite end of the metal pins. Again, the solder must melt thoroughly so that it adheres to the stranded copper and pins.

您需要在测试销的焊接端提供收缩管或带。由于我们现在增加了引脚的阻力，因此我们需要再次重新校准。您需要使用良好的导电表面来将引脚放在校准上。

You can use a printed circuit solder run, a copper coin, or several layers of tin foil for the conductor. Try to avoid touching the pins while testing as small ac voltages from your skin contact could effect meter readings. Place the test pins as close together as possible on the conductor.

将电源打开到测试仪上，然后调整零罐，直到它寄存零欧姆（在左侧）。测试引脚可能需要一些压力以获得零欧姆。从导体中取出引脚，并检查仪表针以右侧的全尺寸。如果Cal罐需要调整，您必须再次重复导线并重新检查零。

校准will be completed when no adjustment is required by shorting, or removing the short. There should be no movement of the meter pointer when the test wires are wiggled or moved around. If you have this problem, it is due to a bad solder connection. Reheat all soldered joints on the test leads, mid point resistors , points A & B and the problem should be corrected.

Some type of insulation can now be installed on the test cord resistors. Now you will need to mark your meter face plate with as many graduations as possible.

For a .1 full scale, ¾ scale is .075, mid scale is .05, ¼ scale is .025. If you have room on your meter to provide 1/8 scale, it will be .012 ohm. With my meter being so large, I was able to use 12 resistors and .08 as full scale, .04 half scale, .02 as ¼ scale and .01 as 1/8 scale .

How to Test

要使用该毫欧米仪表电路进行耐电阻，我花了2“（5cm）的焊料长度，并用钳子扁平到末端。我将测试探测器放在每个末端，仪表指针在零和.01之间的中途，测量.005欧姆。使用我的测试仪，我可以检测到电阻下降到.002-.003欧姆。

Now you're ready to run down shorts on printed circuit boards on various electronic items. I was able to narrow down a power board short to two surface mounted power transistors that were mounted side by side. There were several components that could have been the problem, but through resistance testing, I narrowed the problem down to two components.

我在一个剩下的时间内剪下了发射器，克服了第二个，短暂的发射器。每次使用之前，启动并让测试仪热身几分钟。快速检查全尺度和零欧姆校准，您已准备好拍摄。+9上的电流漏极约为30mA。-9上的电流漏极为2-3 mA。

Prototype Image