build a FET voltmeter (KP303E,BF245,2N3823)

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Update 2021 Have changed the resistor values (across Base-Cathode) from 2.7k to 6.8k. This increased the sensitivity and lowerded the current consumption by 1/3. Now the full scale range has improved. Sensitivity 50mV - full scale, no additional resistor. Does it this make sense having a tiny small range? Want using the meter for measuring on rather high voltages on tube amps. Initial build made. Testsetup runs in DC mode ok. Have used slightly different parts. 2 * FET transistor BF245C 2 * NPN transistor BC172 4 * diode BAT48 Amperemeter 100uA For making the circuit working some resistor values had to be changed. Reason being: -have used a 100uA meter instead of the 1mA meter the original maker used -transistors used have different parameters. No problem. Before making the final build do some investigations, Replace the 2.5k trim pots with external potentiometer. Find the resistor value that makes the full scale then. Write down the found resistor value, put on an additional resistor (lowered by 2.5k) with the 2.5k trim pot. For the different meters used the additional resistor value: 18-24kOhm. 40th anniversary of publishing the original guide. Creator: Dipl.Phys. Frank Tuengler, Berlin (GdR). Have resumed working on the old voltmeter project New layout files. Have made small modifications, rearanged, changed assignment of wiring connecting the pcb with the big rotary switch. The refreshed article from electronics magazine, orig. 1981 (German language) Construction of a FET-voltmeter It gives you a very good understanding how those circuits work. . This thing is primarily about making a MOSFET voltmeter. The shortest possible description after a proposal given by reader Greg: DIY MOSFET Voltmeter with PCB Design Layout (double sided) uses KP-303E (USSR) or 2N3823 or BF245-247 Siemens/Philips Source : article by author Dipl.Phys. Frank Tuengler "Ein universelles FET-Voltmeter" (A universal FET-voltmeter) The book: "Elektronisches Jahrbuch 1981", Militärverlag Berlin, GdR The FET transistor KP303 According to the authors recomendation the FET transistor should have high gain. If you are looking for the KP303 the appendix E should be given. datasheet online: https://rudatasheet.ru/transistors/kp303/ Now the files section has some files for download, mostly russian language One file has a replacement table in just in case the datasheet website vanishes. Reed Relais (general) It has a contact inside of the tube. The tube has the air evacuated or it is filled with protective gas. Outside sits a coil that agitates the contact inside the tube magnetically. You are on your own finding a suitable reed relais. The contact should be closed normaly (when the coil is not activated). If it has the contacts normally open the default mode seen in the schematics will be reversed. (AC versus DC) The relais of my interest RGK series Amateurs rule No.1: take what you have first! Some GdR made reed relais families varying in mechanical and electrical specs do exist still. I.e. the RGK13 / RGK20 (pin distance) can be found. The RGK20 series has one/two normally open contacts. This does not match n.c. relais contacts as depicted in the schematic. Anyway, I have no problem with reversed AC/DC mode when not activated. Literatur: Online description: http://www.ddr-rft-elektronik.de/relais/rgk-relais-20-1-und-rgk-20-2-relais-mit-schutzrohrkontaktsystem-tgl-32441/ The PDF file has the GdR made "DIL/SIL RGK ReedRelais series" description taken from the former GdR based "Funkamateur" universal electronics magazin I had started buying and reading 1973. Source: Funkamateur, Heft 2-1988 (abbreviation: FA) The "FA" is now a western owned magazine, exists still. It provides now HAM operators mostly. If they do complain I could find the sources from the manufacturer published in the "radio fernsehen electronic" as well. F'*** IP shit (German word: Kacke). The RGK relais do exist still. Can be had on ebay. a few hits below: RGK20 9volt version https://www.ebay.de/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=251127931070&_sacat=0 RGK20 12volt version https://www.ebay.de/itm/12V-DDR-REED-RELAIS-RGK-20-1-1-112-01/181855073688 RGK20 18volt version https://www.ebay.de/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=230837259141&_sacat=0 Take care on the coil voltage, there do exist many different versions. I will use two 6volt relais in series with a resistor to fit the 20 volt from the psu. The 24volt relais (or 2x12volt in series) is possibly a bit over to much, do not know. Making of the pcb: Transfering to both sides concurrently was a one shot only job. Made a bag from the two sides sheets. Put the copper clad in, fed it through the laminater. Unfortunaly the bag has gotten ripped off by feeding through. But the toner had grip enough to not fall off. Did a number of following passes using freshly cut to size blank baking paper sheets then. If it would work like in laserprinter this could be done by simply feeding the pcb trough. But I wanted not to put risk on the work done before. It worked anyway. Tonertransfer works good enough, helps avoiding using much chemicals or making noise and dust on the cnc mill. The files Some graphics can be read better by selecting the expanded full view. Tick the 45 degree tilted cross overlay in the upper right corner of a picture. Having opened the expanded view there is a line "View Original". Ticking this should bring up the saved file in full size to the screen. Stupid, but if it works for you this way, go with it. Oh my. improved pcb black/white files The ZIP-file has the individual layer files, 1200dpi Some pictures from the book are realy poor to view or even missing. Have made new design files using "Sprint Layout" (Abacom Germany) I'm not about to build the case as shown by the pictures. The MOSFET voltmeter build: -- This build is not intended to compete with digital voltmeters regarding accuracy. You might own a bunch of them. It is about rebuilding a very well made DIY project from the late 70s. Get back the benefit of using analog gauges, perfectly showing trends. It is dead easy to see if a value is inside a specified range. Hard to see from digital multimeters. To keep the input impedance high and reduce leakage some industrial grade parts are required. 1. The input selektor should be made from ceramic 2. The mode switch relais should have the contacts perfectly separated when opened. Look for "Reed contact Relais". The name depends on your language a bit. The author gave instructions how to make one DIY. The required power supply can be made using any stable linear voltage regulator with +20volt output. Avoid using switch mode psu. Specs as of the authors build. DC mode Input resistance 10MOhm Linearity deviation less 1% max value error less 2% zero point shift less 0.1% AC mode Input resistance 10MOhm/20pF Linearity deviation (readout is nonlinear) max value error less 2% zero point shift less 0.1% The measuring ranges: AC/DC (AC up to 110kHz) 0.6 Volt 1.2 Volt 3.0 Volt 6.0 Volt 12 Volt 30 Volt Understand the schematics -- Feel free to modify the input devider resistor network. The schematics could be simplified if you want DC mode only. Some explanations on the parts used in the schematics: - The KP303 FET-transistors (made in the USSR) can be replaced with western BF245/BF247 or 2N3823. - The other transistors are very similar to BC547/BC548 and many other. - Germanium diodes could be replaced with low power shottky diodes probably. - The universal switching diode SAY17 (Ureverse=50 volts, If= max 175mA, switch delay time =4 nanoseconds) probably has a suitable replacement: Diode 1N4148 (Ureverse=100 volts, If= max 200mA , switch delay time =4 nanoseconds) I have no info about the reverse current and I will not go into the matter further. Find a data sheet and check it out. If looking for something else take care on leakage current in reverse mode. Should be as low as possible. The diodes are grouped in antiparellel series 4 each. This builds the input overload protection. Why not using Z-diodes? Silly question? Z-diodes would reduce the impedance because of leaking current. more to come The PCB -- - The PCB size is 80x50mm - fiber glass epoxy material, FR4 recommended I will put the basic instructions regarding some construction details and the inital adjustment here later. The chassis/case -- The input impedance of the circuit is high. - Consider using a matall chassis of the same type as used in tube amplifier builds. See the pictures of the completed build made by the author. But details of the mechanical build are hard to see. Hints -- - Take transistors from the same production lot. - If you have the tools select the transistors by gain and current. The authors specs are pretty good. But matched pairs can improve stability. For best result thermal coupling of the transistor pairs seems to be ok. I made a simple build many years ago unshielded. The meter had the frontplate made from a sheet of aluminium only. It was extremely sensitive against changes of the near field. It moved the pointer in the instrument even by moving my hand toward the meter. Same effect can be seen with radio frequency circuits. Make a good looking representation of your designs and skills ;-) Show off your design work (not limited to pcb desing) even animated in 3D Method 1 making a representation of your design artwork the traditional way, processing pictures in pixelgraphics software only. Have repaired the graphics using the layer mode in Gimp. Pictures of both sides of the pcb loaded into Tinkercad. A screenshot makes the picture suitable for presentation purpose. For some details see Method 2 below. Method 2 making a representation of your design artwork using old pictures as template loaded into PCB CAD software and Gimp 1. Scan in the pictures using a good old flatbed scanner or take pictures from digital source 2. Do some postprocessing using free Opensource "Gimp". Cleanup , remove unwanted parts , remove colours, increase the contrast, convert it into 1bit depth black and white picture. 3. Take a picture from a selected layer only in the PCB CAD you are using. Many have an "export layer to picture" function. The green top layer picture comes from the new pcb design. I have made it using "Sprint Layout" made in Germany. It is round about 60 Euro. The beginner friendly software has been pirated in Russia and Asia often. I know. 4. have converted both pictures to 1bit bmp graphics 5. load the pictures as objects into "Tinkercad" 6. stack them up with some clearance 7. set the objects hight to 0.1mm gives a pleasing good result Big benefit of using Tinkercad: -the objects can be displayed easily with several colors and translucency effect -Tinkercad gives them a light drop shadow. 8. make a screenshot to get the final picture. The Tincercad built in "export to picture" does not satisfy my needs. 9. On Tincercad.com a 3D animated visulization of the 3D object you have created can be watched through 3D capable webbrowsers. On request the full article can be provided for individual NO COMMERCIAL use only. If it gives you some joy and inspiration, then all is good. Worth the effort. Thats what it is what I'm about. All the time.

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