{"id":7516,"date":"2013-06-06T12:59:58","date_gmt":"2013-06-06T19:59:58","guid":{"rendered":"http:\/\/rustybolt.info\/wordpress\/?p=7516"},"modified":"2013-06-23T09:57:24","modified_gmt":"2013-06-23T16:57:24","slug":"2013-06-06-dc-dc-converter-for-dmms","status":"publish","type":"post","link":"https:\/\/rustybolt.info\/wordpress\/?p=7516","title":{"rendered":"2013-06-06 DC-DC Converter For DMMs"},"content":{"rendered":"

\"DSC_0355S3\"<\/a>I’ve blogged this project more than once in the past<\/a>, (and here<\/a> too), but I didn’t follow through with the circuit completed and installed in a DMM.\u00a0 Well, I finally got my act together and on the road, and finished the first working 1.5V to 9V DMM power supply.<\/p>\n

I thought about it and concluded that since I have a bazillion Joule Thiefs from previous experiments, I could use one of them to be the basis for this DC to DC converter.\u00a0 After considering various circuits, I came to the conclusion that I could use Aki Sadoi’s “Night Joule Thief”<\/a> circuit for the project, after making some modifications to it.\u00a0 Information about the PC board is in the Instructable and on his website<\/a>.\u00a0 I purchased the bare PC board.\u00a0 The parts values I used are shown in the schematic.\u00a0 I did not have to drill any holes in the PC board.\u00a0 Refer to the attached photo (click on it more than once to enlarge).\u00a0 I had to sand down the ends of the PC board to get it to fit into the battery compartment.<\/p>\n

\"DC-DCconv-NIGHTJT-09\"<\/a>This circuit is ideal for the DC-DC converter because it has a third transistor that is used for control of the converter.\u00a0 When the voltage gets to 9V, the Zener conducts and turns this transistor off, and the converter cuts back on battery current, so when there is no load, it draws only about 5 mA from the single 1.5V cell.\u00a0 As the load increases, the 9V drops, the zener stops conducting and this transistor turns on and supplies more current to the converter.\u00a0 At maximum load, which is about 5 or 6 mA at 8.5V, the converter is drawing about 70 mA from the battery.<\/p>\n

The DMM draws only a few mA on most\u00a0 ranges, so the battery only has to deliver about 10 or 20 mA.\u00a0 When the DMM is set to the resistance measuring ranges, especially the 200 ohms range, the 9V current goes up to 5 or so mA, and the battery has to deliver about 60 to 70 mA.\u00a0 The AA cell I’m using should last a long time, but the current is low enough that a AAA cell will work.\u00a0 I did not include an On\/Off switch in the battery lead, because I plan on removing the cell or I will put a small piece of plastic between the cell and the contact when I’m not using it.\u00a0 A small switch is recommended.<\/p>\n

I replaced the ‘upper’ LED with a 1N4148 diode which rectifies the pulses, and the second LED instead is a 470 uF capacitor (black one on the right) that filters out the high frequency pulses.\u00a0 I show an 8.2V zener in the schematic, but I chose to use an old transistor instead.\u00a0 The emitter to base junction on most silicon transistors breaks down or zeners at about 7 to 9V, so it can be used as a zener.\u00a0 I had to try a few transistors to find one that broke down at about 8.4V, but since this can damage the junction, I clipped the collector lead off so that it could not be used as a transistor in the future.\u00a0 I have not tried the 8.2V zener.\u00a0 I did try a 6.8V zener in series with a red LED, which gave an output voltage of about 8.5V, a little on the low side but it worked okay with the meter.<\/p>\n

The CdS photocell and VR1 are not needed, and were left out.\u00a0 I put a 220 uF capacitor across the battery (blue one on the left).\u00a0 I show a 10k for R2, but I used 8.2k.\u00a0 This resistor has to be low enough to turn Q2 fully on, so that it is fully saturated and keeps its collector voltage well below the 0.6V that will turn on Q3.\u00a0 Q3 could also be a BC337-25.\u00a0 Other transistors may work for Q3, but it has to have very low saturation voltage at high currents.\u00a0 The BC547 and 2N3904 are not good choices for Q3.<\/p>\n

The T231212T core was from Surplus Sales<\/a>, and cost about 25 cents U.S.\u00a0 The core can be larger, but I chose a small one so it would easily fit into the battery compartment.\u00a0 I wound 11 turns on it, but the number is not critical, 10 or so will work okay.\u00a0 28 AWG (0.3 mm) wire should work okay, too.\u00a0 Just try out your own hand wound coil.\u00a0 If yours doesn’t seem to put out enough current, remove some turns and try it again, to see if it helps increase the current.<\/p>\n

You don’t have to wind your own core; you can use a 100 uH choke instead.\u00a0 The toroid has less electromagnetic radiation than a choke.\u00a0 I didn’t experience any weirdness in the meter, but it could happen if the meter picks up interference from the converter.\u00a0 That’s why I used the large 470 uF filter capacitor.\u00a0 If interference is a problem, it may require that the 9V is filtered with some RF chokes, and\/or use of an EMI suppressor sleeve.\u00a0 With the parts as shown in the schematic, I measured the frequency at about 150 kHz, but it varies somewhat with the DMM current.<\/p>\n

I drilled two holes through the case and ran the battery holder wires through them, then I glued the holder to the case with clear silicone glue.\u00a0 Hot glue might work okay, too.<\/p>\n

I have a few more bare Night JT PC boards, and I’m going to get another one or two working, maybe this weekend (June 8-9). I think I’ll try to use a 100 uH choke instead of a toroid.\u00a0 I’m just trying to get a feel for what to do and what not to do.\u00a0 After a few builds, I usually find out if and where any weirdness might be.\u00a0 Then I can make modifications to get a circuit that is stable and works good.<\/p>\n

Addendum – One other point about the Night JT circuit is that it uses a NPN for Q2.\u00a0 This has an advantage – or a disadvantage depending on how you look at it – that it limits the maximum current that can be drawn by the base of Q3.\u00a0 I think this is why the circuit would not go above about 70 mA when the load current is at maximum.\u00a0 In the case of the LED, it would limit the maximum LED current and brightness,\u00a0 But in this case, it prevents the circuit from drawing excessive current from the battery.<\/p>\n

If this feature was sacrificed, this Q2 transistor and R2 could be eliminated and the bottom lead of R1 connected directly to the base of Q3.\u00a0 Then C1 would be connected to the base of Q1.\u00a0 This is the typical two transistor “JT” circuit seen in many places on the ‘Net.\u00a0 The R3 10k resistor would then be split into two resistors and the zener diode connected to the ‘center’ or middle leads of these two resistors.\u00a0 I have yet to try this configuration, though.<\/p>\n

Back to experimenting…<\/p>\n

More in my blog on the second one I’ve built.<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

I’ve blogged this project more than once in the past, (and here too), but I didn’t follow through with the circuit completed and installed in a DMM.\u00a0 Well, I finally got my act together and on the road, and finished the first working 1.5V to 9V DMM power supply. I thought about it and concluded <\/p>\n

(Read More…)<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4,17,25],"tags":[],"class_list":["post-7516","post","type-post","status-publish","format-standard","hentry","category-joule-thief-smps-dc-dc","category-meters-and-test-equipment","category-power-supplies"],"_links":{"self":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/7516","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=7516"}],"version-history":[{"count":26,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/7516\/revisions"}],"predecessor-version":[{"id":7518,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/7516\/revisions\/7518"}],"wp:attachment":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=7516"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=7516"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=7516"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}