What kind of rectifier used in analog AC voltmeter? - Page 1
Forum section: electronics / beginners

author: BeBuLamar
author_rank: Super Contributor
timestamp: February 19, 2025, 12:23:08 pm
content: Old analog VOM like the Simpson or Tripett has recifier to measure AC voltage. I wonder what kind of rectifiers were used in these instrument. A diode needs a forward bias voltage before it conducts and these meter can measure quite low voltage I wonder how they did it?

author: CaptDon
author_rank: Super Contributor
timestamp: February 19, 2025, 01:17:11 pm
content: The really old meters had selenium rectifiers. I believe some used germanium crystal diodes. VTVM meters used a 6AL5 dual diode vacuum tube. Most diodes will begin to conduct early in the forward bias condition but only in the microamp region and therefore you would see a very non-linear A.C. volt scale. With the use of a specialized diode+opamp circuit you can create a 'precision rectifier' circuit and the opamp gain can bring a feeble signal up to detectable levels. Most 'solid state' diodes have a logarithmic initial response to current flow then becoming more linear at higher currents. The logarithmic response is used in things like radar video I.F. strips and detectors, Logarithmic amplifiers and the old DBX systems used on tape recorders to expand the 48 dB signal to noise ratio of the physical tape capability to more like an 80+ dB overall system SNR. They are horrible finicky to keep in adjustment however.

author: BeBuLamar
author_rank: Super Contributor
timestamp: February 19, 2025, 02:01:28 pm
content: I am asking because I have a Simpson 260 series 2 and in AC volt it reads only about 70% of the voltage. For example if I feed it 10V it would only read 7V. I suspect the one of the rectifiers is bad.

author: Andy Watson
author_rank: Super Contributor
timestamp: February 19, 2025, 02:18:34 pm
content: Just checked-out an Avo 8-MK IV. Reads accurately (as near as makes no difference) down to 50mV. It uses germanium diodes ... and a 10:1 transformer.Apparently a Simpson 260-2 uses copper-oxide rectifiers: (page 24 of the manual).https://www.simpson260.com/downloads/downloads.htm

author: Kleinstein
author_rank: Super Contributor
timestamp: February 19, 2025, 02:35:32 pm
content: In my old Russia meter they use germanium diodes in an odd bridge with 2 diodes and 2 resistors. it gets full wave rectification with only 1 diode drop, though extra current.I know that quite some meters use a transformer for AC current.

author: schmitt trigger
author_rank: Super Contributor
timestamp: February 19, 2025, 02:35:53 pm
content: I cannot find the manual for my Sanwa analog meter, which included a schematic diagram.If I recall correctly, it had a Schottky diode.This meter is a fairly recent purchase (2018), older models most likely had germanium diodes.The diode drops is the reason that VOMs with passive rectification the AC function didn’t have the very low voltage ranges of the DC function.

author: BeBuLamar
author_rank: Super Contributor
timestamp: February 19, 2025, 02:42:44 pm
content: I have to desolder the rectifiers to check. I tried to check them in circuit but the readings makes no sense.

author: jonpaul
author_rank: Super Contributor
timestamp: February 19, 2025, 02:48:29 pm
content: FW bridge,Diodes are germainium, 1N34, copper oxide (NOT selenium) for low V drop.Attached is typical audio VU meterAC meters in general need a nonlinear scaleBon chanceJon

author: Zero999
author_rank: Super Contributor
timestamp: February 19, 2025, 02:59:07 pm
content: Didn't any of them bias the rectifier with a resistors and compensating diodes of the same type?

author: bdunham7
author_rank: Super Contributor
timestamp: February 19, 2025, 03:28:08 pm
content: Quote from: BeBuLamar on February 19, 2025, 02:42:44 pmThere are resistors involved, in circuit measurement won't work. What do the rectifiers look like? According to the available manuals yours should have a copper-oxide half-wave bypass with two selected resistors for calibration. The next model (and all after AFAIK) used 1N34 with two bridge resistors and two adjustable resistors for calibration. The old metal rectifiers didn't look anything like a glass 1N34.

author: bdunham7
author_rank: Super Contributor
timestamp: February 19, 2025, 03:30:25 pm
content: Quote from: Kleinstein on February 19, 2025, 02:35:32 pmI believe most of the Simpson line used the same arrangement, but early ones used a half-wave bypass arrangement with metal rectifiers. In the first example, R23 and R24 are selected for calibration, in the latter you have adjustments.

author: BeBuLamar
author_rank: Super Contributor
timestamp: February 19, 2025, 04:03:26 pm
content: I will remove the solder joints and check the rectifiers tonight. The meter reads fine with DC volt, DC amp and resistance so the meter movement is good. Once I remove the rectifiers I can verify if the series resistors from the input to the meter movement is correct. If they are then pretty much the rectifiers are the problem. But if that is the case I wouldn't know what to replace them with.

author: Andy Watson
author_rank: Super Contributor
timestamp: February 19, 2025, 04:17:18 pm
content: Quote from: BeBuLamar on February 19, 2025, 04:03:26 pmGe would be my guess. Simpson must have faced the same problem of replacing the diodes. I would look at the later manuals and schematics and see what they did.Also, remember that excess (soldering) heat will destroy Ge semiconductors - use a heat shunt.

author: MrAl
author_rank: Super Contributor
timestamp: February 19, 2025, 04:25:52 pm
content: Quote from: BeBuLamar on February 19, 2025, 12:23:08 pmOne of the main ideas behind this is that a diode does not behave the way we approximate it to behave sometimes. We often use 0.7 volts as the voltage drop when it conducts, so how could we measure a voltage as low as 0.2 volts.The thing is, this approximation is only good for some circuits where the voltage drop is not that significant relative to the other voltages in the circuit. As soon as it becomes relevant, we have to use a different model of the diode other than the usual 0.7v constant drop model.The better model is the exponential model,and that shows conduction at voltages that are lower than 0.7 volts. The conduction is not as good at lower voltages though so the current will be lower which means the AC voltage scale has to be calibrated to take the different voltage drops into account. That's why we see analog meters with a scale that is not the same as the DC scale. As the voltage gets lower and lower, the scale has to have increments that are closer and closer together. That means the distance between increments from 0.1v to 0.2v would be smaller than from 0.2v to 0.3v. The image I am posting shows this difference down around 1vac.Not all of them are that bad though, and the ones that can use a battery can use a precision rectifier circuit which would be linear all the way down to zero volts AC. They would use an op amp or something like that along with some diodes to do the rectification.

author: TimFox
author_rank: Super Contributor
timestamp: February 19, 2025, 04:49:34 pm
content: Semiconductor diodes used as rectifiers to measure low AC voltages are often approximated as "square-law" devices.Many years ago, my relevant test equipment was limited to a Heathkit RF probe (with a single 1N34 Ge diode), a vacuum-tube Heathkit RF generator (with AM capability), and a Navy-surplus (Daven) audio voltmeter.I measured RF filter response using the AM source and found the square-law response to be quite accurate.See the -hp- application note:https://hpmemoryproject.org/an/pdf/an_986.pdffor further details.

author: BeBuLamar
author_rank: Super Contributor
timestamp: February 20, 2025, 03:53:47 am
content: I found the problem. It was R23 bad. I found it completely open but I wonder why it worked just reading low. So I first attached 2 wires to where R23 was and connect them to my helipot. Making the adjustment and it worked. The value of the resitor was about 900 ohms to get correct reading. But then when I solder in a 1K trim pot I shorted 1 rectifier. So I used the silicon diodes in place. It works but reading of low value reads low. So when I adjusted the pot to get the correct 2.5V full scale the measurement of 1V is about 1.15V. Higher range is better and quite good. I am ordering the germanium diodes and see if it's better.

author: schmitt trigger
author_rank: Super Contributor
timestamp: February 20, 2025, 04:17:58 am
content: Where did you order the Ge diodes from?Since these devices are so scarce, expect to pay a premium.Ge Diodes priced “affordable” may be actual Schottkies.I know, I have been scammed myself.

author: BeBuLamar
author_rank: Super Contributor
timestamp: February 20, 2025, 04:27:46 am
content: I am ordering from Jameco.

author: bdunham7
author_rank: Super Contributor
timestamp: February 20, 2025, 05:10:50 am
content: Quote from: BeBuLamar on February 20, 2025, 04:27:46 amIf you haven't placed the order yet, get a pair or two of these if you don't already have heat sink tweezers.https://www.jameco.com/z/HT-156-Jameco-ValuePro-Clip-On-Heat-Sink-for-Soldering-Sensitive-Devices_159126.htmlhttps://www.jameco.com/z/GT-CH51-Jameco-ValuePro-Aluminum-Clip-On-Heat-Sink-Spring-Loaded_100378.html?CID=MERCH

author: Kleinstein
author_rank: Super Contributor
timestamp: February 20, 2025, 08:33:55 am
content: Schottky diodes should be reasonable similar to a GE diode.The meters usually have a scale that is nonlinear at the low end to compensate for the drop at the diodes. The problem with replacing a diode (or both) is that the type of diode effects the details of the nonlinear scale.Both the Ge diodes and Schottky diodes can deviate a bit from the ideal curve and there is also the current scale from the size of the diode. Larger diodes have naturally more current at the same voltage.If only one diode is bad one could measure the drop at a low current (like 1/3 the meter movement, ~ 20 µA) and look for a diode with a similar forward voltage. If needed parallel diodes could be used for a slightly lower voltage.

author: Poroit
author_rank: Frequent Contributor
timestamp: February 20, 2025, 08:46:41 am
content: At my old company (Paton Electrical) we used 4 x OA91 in a bridge format.https://www.universitypaton.com.au/page/10/our-history-university-paton

author: BeBuLamar
author_rank: Super Contributor
timestamp: February 20, 2025, 02:36:33 pm
content: Quote from: Kleinstein on February 20, 2025, 08:33:55 amThanks but it's too late now. However, when I check it with the diode check function of the Fluke 189 it's 1.4V. Strange.

author: bdunham7
author_rank: Super Contributor
timestamp: February 20, 2025, 02:48:43 pm
content: Quote from: BeBuLamar on February 20, 2025, 02:36:33 pmWhat do the rectifiers look like? Are they the copper-oxide discs in a holder? If so, the test current of the 189 may be too high as they're intended for a 50µA meter circuit.

author: BeBuLamar
author_rank: Super Contributor
timestamp: February 20, 2025, 03:37:59 pm
content: Quote from: bdunham7 on February 20, 2025, 02:48:43 pmThey have wires already attached. Since I saw the wires I solder them without heatsink but I did get one of them shorted. The yellow wire is where the 2 diodes connected together.

author: tggzzz
author_rank: Super Contributor
timestamp: February 20, 2025, 03:46:16 pm
content: Quote from: BeBuLamar on February 19, 2025, 02:01:28 pmFor a sine wave, Vpeak vs Vrms would give that ratio.Also be aware of the reason some meters are marked "true rms".

author: schmitt trigger
author_rank: Super Contributor
timestamp: February 20, 2025, 04:41:07 pm
content: Quote from: Poroit on February 20, 2025, 08:46:41 amLove the factory floor. Those huge ceiling windows must have provided plenty of natural light on the work area

author: MrAl
author_rank: Super Contributor
timestamp: February 20, 2025, 05:40:46 pm
content: Quote from: BeBuLamar on February 20, 2025, 02:36:33 pmHi,If you change the diodes I would recommend recalibrating the meter.I am not sure what the scale looks like on your meter but it may involve changing that if nothing else works.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 20, 2025, 07:17:33 pm
content: One should definitely check the accuracy after a diode change. However by now the old analog meter would no longer be used for high accuracy AC. The AC accuracy was never really good for the analog meters, especially not at the lower end of the range (like < 20%).

author: radiolistener
author_rank: Super Contributor
timestamp: February 20, 2025, 08:37:11 pm
content: Old-school AC voltmeters, such as the В3-48, measured AC voltage by amplifying the alternating signal and feeding it to a thermoconverter -essentially a heating element. The measurement was performed by detecting the temperature in the thermoconverter. And they was pretty precise in AC RMS measurement.With diode rectifiers you can't measure proper AC voltage. You can use diodes for a rough estimate of AC voltage with a known and clean waveform shape, but not for precise AC measurements.These days you can use true-rms DMM or oscilloscope for such kind of measurement.

author: floobydust
author_rank: Super Contributor
timestamp: February 20, 2025, 08:41:58 pm
content: Simpson 1-115970 1N87A gold-bonded Ge diode. That diode goes back to 1958, used as a video detector. Used in 260-2 to 260-5. Then changed to 1N270/276 in 260-6. Then changed to 1N100 in 260-7.Weston Copper Oxide Rectifiers Used in Measuring Instruments 1946source urlAttached Vf curve for it, as old hat.

author: TimFox
author_rank: Super Contributor
timestamp: February 20, 2025, 08:54:56 pm
content: Quote from: radiolistener on February 20, 2025, 08:37:11 pmThermal converters are very accurate, since they use a servo with two matched resistor-thermocouple (or thermistor) combinations in a thermally-insulated enclosure.The servo adjusted the DC voltage across the second resistor until the two thermocouples gave equal outputs: the DC voltage across the second resistor therefore equalled the rms voltage across the first resistor, and could be measured accurately with a DC voltmeter.They can tolerate a large "crest factor", the ratio between peak and rms voltages, compared with solid-state rms converters.

author: BeBuLamar
author_rank: Super Contributor
timestamp: February 23, 2025, 02:12:14 am
content: The 1N34 germanium diode came in today. The forward bias voltage is 0.25V. Since the original R23 was bad so I replaced it with a 1K 10 turn trim pot. With the parts replace and some adjustment of the trim pot it now works very well. Well now I want to see how I can improve the accuracy in the DC voltage measurement. The 260 series 2 doesn't have any adjustable potentiometer like the newer version.

author: floobydust
author_rank: Super Contributor
timestamp: February 23, 2025, 02:32:25 am
content: If the meter movement is out, it affects all functions. I think Simpson later trimmed for the meter movement resistance using associated 50uA full scale calibration, and another trimpot for full scale.But the 260-2 meter is 2kΩ and later models 1.8kΩ and you don't have a 50uA range. So I'm not sure best how to improve the DCV cal.I would consider also adding the two protection clamp-diodes that later models incorporated, across the meter movement. Schottky or Ge as full scale is 100mV.https://www.simpson260.com/downloads/downloads.htm