555 Capacitor Tester

This is something I built from a published schematic late in the 1980s. It works very well. I gave away the magazine with the schematic because I believed I would never need it again and we were downsizing.

The circuit is built around a 555 timer. These are very inexpensive and very available. I am always nervous about ruining a semiconductor by applying too much heat while soldering, so I used an 8 pin socket and soldered it into place. Then I pressed the 555 timer chip into the socket when the soldering was finished.

The photo shows my tester. I drilled holes through 1/8 inch Plexiglas to make a circuit board. Just decide where each component should be located and mark the location for the holes. Drill with a small drill. I place the component on top of the Plexiglass and connect leads below the Plexiglass. There is a selector for different resistance arrays. I tapped the Plexiglass for 8-32 brass screws. I soldered leads to the screw heads under the Plexiglass and I attach an alligator clip to the appropriate screw for the desired resistance range on each test. I used hot glue to fasten components to the Plexiglass where necessary. The battery holder is fastened to the Plexiglass with a screw.

Step 1: Removing the Mystery

I know just a little about electronics, but not a lot. For a long time I was in awe of the genius who used a 555 Timer chip to make a capacitor tester. Then I began reading a little more about 555 Timer circuits. According to my rudimentary understanding, they can be configured in different ways, including astable, monostable, and bi-stable. Each works a bit differently with different results for different purposes. After reading about each of these just a little, I decided the capacitor tester I built is a very common monostable multivibrator or “one-shot” configuration.

A monostable multivibrator turns “on” when a momentary contact switch is depressed and released. The multivibrator produces a continuous pulse that lasts until the capacitor in a resistance/capacitance bridge charges up to a particular percentage of a full charge. When that happens, it signals the 555 Timer chip to stop the pulse. In this case, that means an LED came “on” when the momentary contact switch was depressed and released. It continued to be lit until the capacitor charged to its threshold level. Then the 555 Timer turned the LED “off.” If the resistance has been carefully chosen, counting the number of seconds the LED was “on” indicates the value of the capacitor multiplied by 1 or by 10 or by 100 according to the selected test range.

This link at Circuit Digest discusses the resistance/capacitance bridge in a monostable multivibrator circuit using a 555 Timer chip, and it gives the standard formula for calculating the time in seconds an LED will be “on” based on a specified resistance and a specified capacitance. It also provides a schematic for the configuration of a 555 Timer chip to be used. As noted, R1 and C1 are the variables. On my tester, if R1 is 900,000 Ohms the multiplication factor is 1. If R1 is 90,000 Ohms the multiplication factor is 10. If R1 is 9000 Ohms the multiplication factor is 100. In the photo I used for the Introduction I connected a 100 microfarad electrolytic capacitor to the test alligator clips while observing polarity. The LED went out in 10 seconds. The selector was set on the 10x option. 10 x 10 = 100. The capacitor’s value is very close to its specified value. (This tester does not indicate other things, like internal resistance of the capacitor.)

The image is a monostable multivibrator circuit from the link above to Circuit Digest. You could build the circuit as shown. R1 and C1 are conveniently marked. I would add a three-position selector for the resistances mentioned in the paragraph above. It would make the tester easier to use.

Step 2: My Circuit

As I mentioned, I did not save the magazine with the schematic that I built, but gave it away. I have looked, but not found anything exactly like it on the Internet. I believe any monostable multivibrator circuit would work. They seem to vary just a little. Variations are usually a matter of adding very small capacitors for the sake of decoupling one part of the circuit from an influence that might effect functionality.

I did try to trace the circuit from my actual tester. It can be viewed in the photo with this step. I viewed my circuit board from the bottom and tried to trace the connections accurately. There is always the possibility I made an error, although I checked it a few times.*

I am accustomed to pin out diagrams on IC chips that begin with #1 in the upper left corner and progress to pin #2 and so on. See the circuit diagram in the image from the previous step. Pin #1 is on the bottom at the center. What you see in that diagram is now the standard way to show the pin out for a 555 Timer chip. My diagram of what I built is further complicated because the pin out is from the back side of the circuit board.

See the second photo. Notice the shiny round area on the 555 Timer. It indicates pin #1. Pin #2 is below it. The lower right corner is pin #5. Pin #6 is above it. Pin #8 is at the top right corner.

*Even from the underside of my Plexiglas circuit board the wiring looks like a rat’s nest. This circuit tracing was done with the aid of a continuity tester and double checked. Later I did it a second time on a new piece of paper and got the same schematic. I am reasonably confident this is an accurate description of the circuit I used.

Step 3: How to Use the Tester

The magazine that contained the circuit diagram for my tester gave no information on how to use it. I had to work that out by trial and error. This tester is for electrolytic capacitors of a larger size, normally 10 microfarads and larger. It will work for capacitors down to 1 microfarad in size.

Notice the 9 volt battery is connected. I always remove the battery when I am finished and install it when I want to use the tester. An alligator clip has been attached to a brass screw to choose the range. Alligator clips have been connected to the capacitor under test. The LED is “on” and the test is underway.

1. Always discharge the capacitor first.

2. Select the appropriate resistance range. (If you are testing a 4700 microfarad capacitor counting 47 seconds makes more sense than counting 4700 seconds to arrive at the approximate value of the capacitor.)

3. Attach the positive (+) and negative (-) test leads to the capacitor. Be careful to observe the correct polarity.

4. Depress the momentary contact switch and release it.

5. Count the number of seconds until the LED goes out. Multiply by the appropriate factor for the resistance range selected.

Good capacitor—The LED stays “on” for the appropriate number of seconds before turning “off.”

Range set too high—The LED turns “off” as soon as the momentary contact switch is depressed and released.

Capacitor is “open” and must be replaced—The LED turns “off” as soon as the momentary contact switch is depressed and released.

LED stays “on”—The capacitor’s connection to the tester is the wrong polarity, or the capacitor is shorted and needs to be replaced.

Step 4: Do You Need This?

About the time I found the magazine with the capacitor tester circuit I bought a 40 year-old Zenith Trans-oceanic AM-Shortwave radio built with vacuum tubes. Electrolytic capacitors began to blow one by one when I began using the radio, and I used it quite a bit at the time. It was helpful to test suspect capacitors rather than just throwing money and new capacitors at the radio indiscriminately. This tester helped me identify a faulty capacitor and change it. I no longer have that radio, but occasionally I find it very helpful to check a capacitor when I am trying make something work again. I do not use this tester often, but it is very helpful when I need it. I do have a multi-meter with a capacitance scale now, but such meters often do not cover the range I need. The tester I built usually does.

The image is from Monitoring Times by way of the Internet. It is very much like the radio I had, but not a photo of it.