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  148. <!-- Copyright (C) 2009 Lawrence P. Glaister. All rights reserved. -->
  149. <p align="CENTER"><img src="Capacitor%20ESR%20Tester_files/lpglogo.jpg" alt="Logo" height="184" width="277"><strong>
  150. <font size="7"><i>Capacitor ESR Tester</i></font>
  151. </strong></p><strong>
  152. <p align="CENTER"><img src="Capacitor%20ESR%20Tester_files/spectbar.gif" height="7" width="573"></p>
  153. <p align="CENTER"><img src="Capacitor%20ESR%20Tester_files/esrproto03.jpg" alt="protoype esr meter" border="5"></p>
  154. <strong>
  155. <h1>Project Description</h1>
  156. This project was conceived as a way to enhance the collection of test equipment on my test bench.
  157. I buy a lot of older HP test gear off ebay as well as older radios. Most of this gear is 25-60 years old
  158. and needless to say, the condition of the electrolytic capacitors is somewhat suspect. I needed a way to
  159. quickly weed out bad caps with an in circuit tester.<br>
  160. At the present time, there are no plans to market circuit boards, kits, parts or complete units
  161. for sale.
  162. <h2>What is ESR?</h2>
  163. "ESR" stands for equivalent series resistance. ESR is one of the
  164. characteristics that defines the performance
  165. of an electrolytic capacitor. Low ESR is highly desirable in a capacitor
  166. as any ripple current through the
  167. capacitor causes the capacitor to heat up due to the resistive loses.
  168. This heating accelerates
  169. the demise of the capacitor by drying out the electrolyte at an ever
  170. increasing rate.
  171. Over the lifetime of a capacitor, it is not uncommon for the ESR to
  172. increase by a factor of 10 to 30 times
  173. or even go open circuit. Typical lifetime ratings for electrolytics are
  174. 2000-15000 hours and are very dependant on ambient operating
  175. temperature. As the ESR increases, the filtering operation of the
  176. capacitor becomes impaired
  177. and eventually the circuit fails to operate correctly.
  178. <h2>Why are ESR Meters so Useful?</h2>
  179. A typical capacitor checker measures the capacity (usually in micro
  180. farads) of the test capacitor.
  181. Some advanced units also test for leakage current. Most of these testers
  182. require that the capacitor be
  183. removed from the circuit. Unless the capacitor has totally failed, they
  184. will not detect a high ESR value.
  185. In a typical circuit, there may be 10's or 100's of capacitors. Having
  186. to remove each one for testing is very
  187. tedious and there is a great risk of damaging circuit boards. This
  188. tester uses a low voltage ( 250mv )
  189. high frequency (150khz) A/C current to read the ESR of a capacitor in
  190. the circuit. The in circuit testing
  191. is possible because of the low voltage used for obtaining the
  192. measurement. The voltage is low enough that
  193. solid state devices in the surrounding circuitry are not activated and
  194. do not affect the low resistance reading we are attempting to obtain. A
  195. lot of capacitor checkers will be damaged if you happen to test a
  196. charged capacitor.
  197. This circuit is A/C coupled and will withstand up to 400vdc of charge on
  198. a capacitor (but watch your fingers!).
  199. The ESR checker will not detect shorted capacitors as they will read
  200. with a very low ESR value. If you are trouble shooting a circuit, you
  201. will have to use several instruments including your nose, voltmeter and
  202. oscilloscope to locate all the possible failure modes. My experience has
  203. found that the ESR meter catches about 95% of capacitor problems and
  204. potential problems.
  205. <h2>Features</h2>
  206. <ul>
  207. <li>Tests electrolytic capacitors &gt; 1uf in circuit.</li>
  208. <li>Caps may be tested in circuit or by bridging them across the terminal posts or using test leads.</li>
  209. <li>Polarity insensitive testing.</li>
  210. <li>Tolerates charged capacitors up to 400vdc.</li>
  211. <li>Low battery draw (approx 25ma) resulting in about 20 hours of battery time when using 4 cheap AA nicads.</li>
  212. <li>Easy to read analog meter.</li>
  213. <li>Measures ESR range from 0-75ohms with an expanded scale A/C ohmmeter.</li>
  214. </ul>
  215. <p align="CENTER"><img src="Capacitor%20ESR%20Tester_files/esrproto01.jpg" border="5" height="480" width="640"></p>
  216. <h2>Circuit Description</h2>
  217. See the schematic for component designations.
  218. The circuit starts with a 150khz oscillator using one gate of a 74hc14.
  219. The rest of the gates are used as
  220. buffers to increase the current drive to the low pass filter. The single
  221. pole low pass filter is necessary because the square wave signal
  222. contains a lot of energy in high frequency odd order harmonics. The
  223. output of the filter is applied across a 10 ohm load resistor that
  224. provides the low impedance drive signal to the test capacitor. Diodes D5
  225. and D6 protect the circuit from discharge spikes if you happen to test a
  226. charged capacitor. R18 is used to discharge C5 so that you don't blow
  227. up anything if you alternately test charged high and low voltage caps.
  228. C5 isolates the test circuit from DC voltages up to 400 volts.
  229. <br>
  230. <br>Be careful if you are testing high voltage capacitors... the safest
  231. way to work is to make sure the test capacitors are discharged <b>before</b>
  232. testing them. Be aware that high voltage capacitors can hold a lethal
  233. charge for several days depending on the circuit design. I learned this
  234. first hand in high school electronics class. Students (not to be named!)
  235. used to charge the caps and put them back on the shelf for the next
  236. unsuspecting victim to pick up.
  237. <br>
  238. <br>The rest of the circuit is a very straight forward transistor
  239. amplifier with a gain of about 10.5. This amplifies the
  240. A/C signal passed through the test capacitor up to several volts in
  241. amplitude. The signal is then coupled to a full
  242. wave bridge rectifier that has the meter as its load. The threshold
  243. voltage of the bridge rectifier is used to an advantage and provides the
  244. expanded scale function of the ohm meter. The amplified voltage from
  245. the test capacitor must be great enough to overcome 2 diode drops before
  246. the meter will start to respond. This sets the high resistance
  247. threshold for the meter at somewhere between 75-100 ohms. The meter is
  248. zeroed by shorting the test leads together and adjusting the pot in the
  249. meter circuit for a full scale ( 0 ohms )reading on the meter. Proper
  250. operation of the circuit can be verified by checking several values of
  251. resistors with the meter. Shorted leads should indicate full scale, a 1
  252. ohm resistor should read about 90% of full scale, a 10 ohm resistor
  253. should read about 40% of full scale and a 47 ohm resistor should barely
  254. move the needle to about 10% of FSD. The absolute readings will vary
  255. with temperature and battery voltage, but the idea is that most ESR
  256. values should be much less than 10 ohms which means good caps test at
  257. very close to full scale and bad caps test at little or no deflection.
  258. <br>
  259. The board below shows my second prototype. The top 2 wires run to the
  260. front panel banana test jacks, the middle 2 wires are the incoming
  261. switched 5 volts from the 4 AA nicads, and the bottom 2 wires run to the
  262. series combination of the zero adjust pot and the meter on the front
  263. panel.
  264. <p align="CENTER"><img src="Capacitor%20ESR%20Tester_files/esrproto02.jpg" border="5" height="480" width="640"></p>
  265. <h3>Project Files</h3>
  266. Schematic Drawing - <a href="http://web.archive.org/web/20120406084010/http://members.shaw.ca/swstuff/esrschematic.png">esrschematic.png(94.7K)</a> <br>
  267. Parts Layout - <a href="http://web.archive.org/web/20120406084010/http://members.shaw.ca/swstuff/esrbuildit.png">esrbuildit.png(13.4K)</a> <br>
  268. PCB bottom view - <a href="http://web.archive.org/web/20120406084010/http://members.shaw.ca/swstuff/esrpcb.png">esrpcb.png(4.8K)</a> <br>
  269. PCB Xray View - <a href="http://web.archive.org/web/20120406084010/http://members.shaw.ca/swstuff/esrxray.png">esrxray.png(4.8K)</a> <br>
  270. Eagle PCB files - <a href="http://web.archive.org/web/20120406084010/http://members.shaw.ca/swstuff/esrmeter-9mar05.tgz">esrmeter-9mar05.tgz(173.2K)</a> <br>
  271. LTSpice/SWCadiii - <a href="http://web.archive.org/web/20120406084010/http://members.shaw.ca/swstuff/esr.asc">Linear Technology Spice model esr.asc(5.8k) </a> <br>
  272. <h1>System Requirements</h1>
  273. This circuit was first prototyped using a homebuilt prototyping panel.
  274. As you can see by the picture above,
  275. it was ugly but it worked. My final version was hand built on a "pad per
  276. hole" prototyping board with 0.1" spacing.
  277. I used the PCB layout as a guide for placing and wiring the components
  278. (printed and glued to proto board). I did the PCB design as part of the
  279. project
  280. so that I could optimize the board size and parts placement. The pcb is
  281. roughly 2x3" and is single sided with no
  282. special or critical parts required.
  283. <h1>Updates:</h1>
  284. September 2009<br>
  285. The zero of the meter tends to drift with changes in the battery
  286. voltage. Several builders have emailed me with modifications to
  287. use a low dropout regulator to keep the voltage more constant. Since my
  288. unit runs on 4 nicad cells, there is not enough headroom for a 5volt low
  289. dropout
  290. regulator. The charged voltage of my nicad pack is 5.6volts, but most of
  291. the packs useful life is down around 4.8volts. I added a
  292. 15 ohm 1/2w resistor in series from the battery pack and a 4.7volt 1
  293. watt zener to ground. This modification really helps keep the readings
  294. more stable as the
  295. battery discharges. With the lower supply voltage, I had an issue with
  296. driving the meter to full scale. I ended up reducing R17 from 10k down
  297. to 4k7.
  298. The latest success story using the ESR meter was finding an open
  299. 330uF/25v cap on the output of a wall wart supply used with a wireless
  300. router. Replacing
  301. the $0.25 part saved me buying a $100.00 router.
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  304. <img src="Capacitor%20ESR%20Tester_files/home.jpg" alt="home.jpg" border="0" height="29" width="90">
  305. Back to Lawrence's Software Stuff Page</a>
  306. </strong>
  307. </strong></body></html>
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