S1E15 Chatshaala with electronics gadgets | Introduction to Amplifier

S1E15 Chatshaala with electronics gadgets | Introduction to Amplifier
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  • Links to recordings:
  • Here is the room link to join: chaatshala
  • Timings: Mo-Fr 11:00AM to 12:30PM

Today we explored the followings:

  • Introduction to Amplifiers and its role
  • BJT as an Amplifier (Basics)

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You can also post your question and discuss in this thread.

Happy Exploring!

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Yesterday our induction heater stopped working.
I opened the gadget.
Inside is a 18V SMPS with a 78L05 series regulator. The 78L05 is a lower output current version of the 7805. 78L05 can supply only 100mA. Ideal for powering a microcontroller.
Some images of the main board and keyboard. The microcontroller is a special device built for induction heater control. I could not locate any specs. However the keypad and LED display is driven by a peripheral chip SM1668 whose specs are available. The SMPS uses AP8012 SMPS controller with a built in power mosfet.
photo_2020-06-29_12-16-09 photo_2020-06-29_12-16-23 photo_2020-06-29_12-16-30

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The symptom: on power on the keypad lights up and then switches off, which is it’s normal behaviour. When one pushes the on/off tactile push button the device turns on. Pushing it again turns it off. However the device fails to turn on when this button is pressed.

I suspect a powersupply overload causing power to microcontroller to shut off.

The mains remains connected whether on or off - a very bad design aimed at cost cutting. A decent circuit will have a relay that cutsoff power. Or atleast a mains switch. One will have to be very careful while trouble shooting to prevent a nasty shock or a fire.

The induction coil drive circuit is intact. No fuse blown. This is a very high powered stage. The IGBT (3rd pic top left black object attached to Al heat sink) and bridge rectifier (right of the same heat sink) are rated at 20 Amps 1200V. If anything goes wrong the fuse is most likely to blow.
So, that is the good news.

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To check without the danger of the always on mains, hook in an external 5v to the board. The microcontroller works. The keypad is intermittent. Pressing on/off starts the gadget after a lot of tries.
Remove the external psu and connect the gadget mains. One must be extremely careful now.
Switch on. Same behaviour. the output voltage from the SMPS is 18V. 78L05 provides 5v. So nothing wrong with SMPS.
Inability of witch to operate could be due to bad push button or a wiring problem.
Checking the switch shows that is is ok.
Check the wires with DMM (digital multi meter). Wires check ok.
However I discover a new issue. Which is that the board works when held in a particular position. This means that the wire cable is a probable intermittent open and makes contact in a particular position. Several hours of manipulating the wire cable fails to show anything conclusive. I decide to change the cable. However the connector on the cable is of a different type which I dont have.

I will also have to replace the connector.

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I replace the existing connector with one I have.
image

The bottom one is the original. The top one is the replacement. Both have the same pitch and foot print.
Notice the missing pins on the original? Desoldering a rigid multi pin component having more than two pins is most likely to damage the PCB. Especially plastic components. With connectors we can insert a small screw driver under the plastic and lift it off the board. We can then desolder one pin at a time. That is what was done, hence the missing pins.

Having replaced the connector and cable we expect the circuit to work.
It did not and continues to display the same intermittent position dependent operation. This time we are 100% sure about our cable and connector.

More trouble shooting required

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What should be the case then? Should the mains remain connected?

Seeing a fuse being blown seems an everyday encounter.
How do we come to know that a fuse has been blown?

What does the fuse do actually?
@jtd

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Definitely not. Apart from a general use safety point, during trouble shooting me must disconnect the mains.

Depends on the type of fuse. Most daily use fuses are in a transparent glass tube. We will see a black shiny deposit inside the tube if the fuse is blown. This is caused by large fault current causing the fuse wire to melt and explode with splatter. However often the fault might cause the fuse to melt much slower and merely break without any splatter. In such a case we have to either try and see the thin wire or use a multimeter’s ohms range to check. A good fuse will show a short.

The fuse prevents a fire being caused under a fault condition. High current flows caused by component failures produces extreme heat. This can set afire wire insulation and anything else that happens to be in contact. A fuse ruptures in a few seconds when it’s rupture current is reached. Note that the operating current is far lower than the rupture current. A 1A fuse does not rupture at 1A but at a substantially higher current, usually at 250%. Also a 1A fuse is operated at 70% i.e. .75A. IF you do operate it at 1A it will blow perhaps after several hours of use.
Fuses are also of different rated speeds- superfast, fast, slowblo, normal, PTC. PTC fuses do not normally rupture but provide a high resistance until the current drops substantially and the PTC fuse cools.
Fuses have a maximum rated voltage. You can operate below the rated voltage, but never above.

See the attached sheets.
Littelfuse_Fuse_322_332_Datasheet.pdf.pdf (805.0 KB)

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