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A simple workshop generator m.cz.

żarówka rtęciowa 14463 37

TL;DR

  • A homemade workshop sine-wave generator provides 100 Hz, 1 kHz, and 10 kHz outputs for servicing amplifiers and receivers.
  • It uses a Wien bridge oscillator with a dual TL082 op-amp, a 60V/20mA telephone bulb for amplitude stabilization, and a buffer stage.
  • The power supply runs from 230 V mains, uses a TS2/14 transformer with a Delon doubler, and delivers about 14 V DC plus 11 V AC.
  • The output level is adjustable from zero to about 4 Vrms, and the project cost only a dozen zlotys.
  • Recycled materials kept the price low, but missing parts stretched the build to about one year.
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  • #31 20654171
    dipol
    Level 34  
    Posts: 2598
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    Mateusz_konstruktor .... that's a question for me ????, what bulbs are we talking about.
    I would like to add that I currently have two more RC PG-23 generators that came to me for repair and the customers have not reported so far.
    In the service, the above-mentioned generators in a soap dish housing with a 4.5 volt flat battery worked reliably due to their handyness and simplicity.
    73!
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  • #32 20654945
    Mateusz_konstruktor
    Level 37  
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    @dipole
    What bulb did your colleague use, in terms of rated voltage or a specific type?
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  • #33 20656920
    ^ToM^
    Level 42  
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    Bulb usually 12 V with a current of 5-20 mA.
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  • #34 20678044
    Krzysztof Kamienski
    Level 43  
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    @Mateusz_konstruktor, for example, from manual telephone exchanges. 24V/20mA.
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  • #35 20770695
    CHOPIN66
    Level 15  
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    Gismot wrote:
    CosteC wrote:
    pawlik118 wrote:
    A fuse at transformers up to ~2.5VA is not required, from what I remember. Perhaps this is because the very thin wire of the primary winding is not capable of carrying the current that would give off the power that could burn through the insulation between the primary and secondary sides.
    e.g. Breve on its products
    I know of no such provision.... It would be very strange, besides it assumes that magically nowhere else in the device there will be a failure that can lead to a fire.
    The fuse has two roles - protection against fire and protection against shock, of which it protects against shock only in specific cases - because depending on the position of the plug, the fuse may be on the neutral line.


    As a curiosity I'll add in the factory equipment of Polish production (metal housing) at 10VA transformers, also was not always used the mains fuse on the primary side of the transformer. The secondary windings of the transformer were always protected by fuses. E.g. ELTRA T8010, T 3015 tuner, ZRK T7010, EQ DIORA FS 042.

    On the other hand, the V40.28 power supply in the Meratronik V640 multimeter does not have any fuse. (It is very common for the transformer to fail in them)

    In the case of devices in wooden housings or plastic (table radios), the transformer always had a fuse on the primary and secondary sides.

    Modern antenna power supply for continuous operation, which does not have any fuses.
    Power adapter with a cable and plug on a wooden surface. Transformer with wires and a capacitor on a wooden table.





    Probably has a thermal fuse between the primary and secondary windings of the transformer if the manufacturer used one. I now have a Sencore radio alarm clock from about 15 years ago and there is no trace of a fuse or even a place for a fuse on the PCB. The only protection is the aforementioned thermal fuse if there is one at all in the transformer because the transformer looks Chinese like from Aliexspress. They simply saved money on materials. From what I remember, thermal fuses originally based on bi metal were introduced in radio receivers by Philips as early as the 1930s until almost the end of the era of popularity of electron tubes c
    and somewhere until the early 1950s when the first TVs with power supply without a transformer began to appear so they began to replace thermal fuses with normal ones. Po ta to go back to thermal fuses 1970s I have a Philips GA209S Electronic turntable from 74 and also on the primary side only thermal fuse fused with transformer( Philips didn't even specify its type). On the secondary side already a normal 1.25A fuse but only on the secondary side.
  • #36 20772121
    pawlik118
    Level 34  
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    There is no thermal fuse, you can see that the windings are bare.
  • #37 20772124
    ^ToM^
    Level 42  
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    pawlik118 wrote:
    There is no thermal fuse, you can see that the windings are bare.


    But I have met similar versions where the thermal fuse was. Apparently, it depends on the manufacturer and his vision. One will put it on, because he prefers to blow cold, while others believe that in the event of a short circuit, one end of the winding, which is wound with 0.02 mm wire, will get hot anyway. Then the lead from the winding to the solder tip "does" as a fuse. :D .
  • #38 21910720
    chisel
    Level 27  
    Posts: 809
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    Gismot wrote:
    Modern antenna power supply for continuous operation that has no fuses.

    And yes and no. Its winding on the primary is thin enough that it will get hot from the winding area to the soldering point in the event of an overload :-)
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Topic summary

✨ A user is constructing a low-frequency sine wave generator for servicing amplifiers and receivers, featuring three frequencies: 100 Hz, 1 kHz, and 10 kHz, powered by 230 V mains. The design includes a ground loop switch to prevent interference when connecting to an oscilloscope. Discussions revolve around the necessity of a mains fuse, with various opinions on safety and design practices. The user plans to share a schematic to clarify the circuit design. Participants also discuss materials used for the front panel and the importance of thermal protection in transformers.
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FAQ

TL;DR: This DIY workshop generator delivers about 4 Vrms and uses a TL082 Wien bridge; as one commenter put it, "the bulb is correct." It is for amplifier and receiver servicing when commercial low-frequency generators cost too much. The practical issues are grounding, output hum, and whether to add a primary-side mains fuse. [#20642550]

Why it matters: This thread shows how to build a very low-cost audio test source that is useful in real bench work, while also exposing the safety and grounding decisions that matter most.

Option Waveforms Frequency approach Complexity Best use from thread
TL082 Wien bridge Sine only 100 Hz / 1 kHz / 10 kHz Very simple Low-distortion servicing source
ICL8038 Three waveforms Wider range Slightly more featured More versatile bench generator

Key insight: In a mains-powered metal generator, the enclosure should stay bonded to PE. The only switch that helps measurement problems is the one that interrupts the signal return path to reduce a ground loop, not the protective-earth bond.

Quick Facts

  • Output frequencies are fixed at 100 Hz, 1 kHz, and 10 kHz, selected by an isostat switch with RC elements. [#20641443]
  • The power supply uses a TS2/14 transformer and a Delon doubler, giving about 14 V DC plus about 11 V AC for the indicator lamp. [#20641443]
  • Output level adjusts from 0 to about 4 Vrms with a Telpod SP 1.2 potentiometer; amplitude is stabilized by a 60 V / 20 mA telephone bulb. [#20641443]
  • The enclosure cost was about a dozen zlotys, thanks to recycled sheet metal, reused mains cable, scrap plastic, and stock parts. [#20641443]
  • Bought parts explicitly named in the thread were 2 × 470 µF / 25 V / 105 °C Panasonic electrolytics and one TL082 dual op-amp. [#20641443]

How does the output ground switch work in a mains-powered signal generator with a non-isolated BNC socket and PE-connected metal chassis?

It works by interrupting the generator’s output return path, not by disconnecting protective earth from the metal case. The author said the chassis is permanently connected to PE, while the rear switch is used when the generator, tested circuit, and analog oscilloscope would otherwise create a loop through their grounded connections. With a standard BNC and metal case, that switch only makes sense in the signal ground path. [#20641606]

Why can a ground loop appear when a workshop generator and an analog oscilloscope are both connected to protective earth?

A ground loop appears because both devices can reference the same circuit through separate PE-linked paths. In the thread, the analog oscilloscope input socket was described as permanently connected to PE, and the generator chassis was also bonded to PE. When both then connect to the tested amplifier, loop current can flow in the signal return and add hum or misleading measurements. [#20641606]

What is a Wien bridge generator and why is it used to generate low-distortion sine waves at 100 Hz, 1 kHz, and 10 kHz?

A Wien bridge generator is an RC sine-wave oscillator that uses a frequency-selective bridge and nonlinear amplitude control, giving low distortion without complex circuitry. Here it was chosen because the EP 1/95 design is simple, uses a TL082, and provides three practical service frequencies: 100 Hz, 1 kHz, and 10 kHz. One commenter called its distortion value “almost immeasurable,” which captures why this topology is popular for audio servicing. [#20642995]

What is a Delon voltage doubler and how does it work in a simple transformer power supply?

A Delon voltage doubler is a rectifier circuit that uses two diodes and two capacitors to stack charging peaks, producing a higher DC output from an AC secondary. In this build, it follows a TS2/14 transformer and, with two filter capacitors, provides about 14 V DC for the generator while a separate roughly 11 V AC path feeds the indicator lamp. [#20641443]

How do you build a simple low-frequency sine wave generator based on the Elektronika Praktyczna 1/95 circuit with a TL082 op-amp?

Build it as a two-board mains device with a TL082 Wien bridge oscillator, a buffer stage, and a simple transformer supply. 1. Assemble the TS2/14 transformer supply with the Delon doubler. 2. Build the TL082 board with RC selection for 100 Hz, 1 kHz, and 10 kHz plus the lamp stabilizer. 3. Add the output level pot, BNC socket, and PE-bonded metal enclosure. The author completed the project over about one year because of parts shortages. [#20641443]

What role does a 60V/20mA telephone bulb play in amplitude stabilization in a Wien bridge oscillator?

It provides automatic gain control by changing resistance as the filament warms. In this generator, the 60 V / 20 mA telephone bulb sits in the Wien bridge feedback path, so a rising output increases lamp temperature and resistance, which reduces loop gain and stabilizes amplitude. The result is a cleaner sine wave than a fixed-gain RC oscillator usually gives. [#20641443]

Which bulbs work best for amplitude stabilization in RC audio generators, and which types should be avoided?

Use low-current incandescent bulbs, typically about 5–20 mA, because the thread stresses current more than rated voltage. Examples given were 12 V bulbs at 5–20 mA and telephone bulbs such as 24 V / 20 mA or 60 V / 20 mA. Avoid high-current lamps, because they are less sensitive in this role and make amplitude control poorer. As one reply put it, “the key parameter is its current, not voltage.” [#20656920]

Why are people concerned about the lack of a mains fuse in a generator powered by a TS2/14 transformer?

They are concerned because a fault in the transformer or wiring can overheat the device before anything external trips. Several replies challenged the author’s no-fuse choice and explicitly raised fire risk, including a case where a controller board burned until the installation protection operated. The concern is not only transformer failure, but any internal short on the 230 V side. [#20642060]

What are the fire and shock protection arguments for adding a primary-side fuse to a small mains transformer device?

A primary-side fuse limits fault energy and can reduce both fire risk and some shock scenarios. One commenter stated that a fuse has two roles: fire protection and, in specific cases, shock protection, though that second benefit depends on plug orientation because the fuse may end up in the neutral line. Even when small transformers sometimes omit a fuse, the thread repeatedly argues that adding one improves safety margin. [#20642060]

How does a plastic front panel made from ABS affect grounding and safety when the enclosure itself is metal and connected to PE?

It keeps the user-facing front surface non-conductive while the metal enclosure remains safely bonded to PE. The author later confirmed the front panel material is ordinary ABS plastic, and the original post states the box is permanently connected to the protective-earth conductor. That combination reduces accidental contact with grounded metal at the controls without removing chassis earthing. [#20647191]

What output level can you expect from this TL082-based generator, and how do you adjust it from zero to about 4 Vrms?

You can expect an adjustable output from 0 to about 4 Vrms. The thread says the level is set with a Telpod SP 1.2 carbon potentiometer, while a separate Telpod CN-15 cermet potentiometer sets the stabilized amplitude inside the oscillator. In practice, one control trims oscillator behavior and the other acts as the front-end output level control. [#20641443]

ICL8038 vs a Wien bridge TL082 generator — which is better for a simple workshop signal source for amplifier and receiver servicing?

The TL082 Wien bridge is better when you want a simple, low-distortion sine source for audio servicing. The ICL8038 was suggested because it offers three waveforms, wider range, and even offset adjustment, but that is a different priority. For amplifier and receiver alignment at 100 Hz, 1 kHz, and 10 kHz, the thread favors the Wien design’s simplicity and sine quality over broader feature count. [#20648594]

How can you wire the generator output and oscilloscope connections to avoid hum and grounding problems during amplifier testing?

Use one clear signal-return path and avoid duplicating the earth-referenced return through both instruments. 1. Keep the metal generator enclosure bonded to PE. 2. Connect the oscilloscope and generator grounds to the tested circuit carefully, watching for duplicate return paths. 3. If hum appears, use the generator’s output-ground switch to open the signal-return loop, not the chassis PE bond. This method addresses the exact loop problem discussed for analog oscilloscopes with PE-linked inputs. [#20641606]

Why is bulb current considered more important than bulb voltage in amplitude-stabilized sine wave generators?

Current matters more because the stabilization depends on how easily the filament’s resistance changes with heating. The thread explicitly says to choose a bulb with “as little current as possible,” and explains that its nonlinearity is what stabilizes the oscillator. That is why bulbs such as 20 mA telephone types are preferred even when their printed voltages differ, such as 24 V or 60 V. [#20642550]

What low-cost recycled materials and parts can be used to make a DIY workshop signal generator enclosure and power supply?

Use scrap steel sheet for the case, reused three-core mains cable, recycled plastic for the front panel, and stock components wherever possible. The author built the chassis from hand-cut and bent sheet metal, used a locally bought scrap sheet, and reused a mains cord from electronic waste. That kept the total cost to about a dozen zlotys, with only the transformer, TL082, and two 470 µF capacitors clearly bought new. [#20641443]
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