Identify part - UK Vintage Radio Repair and Restoration Discussion Forum

Nukeybrown · 16th May 2018, 10:04 pm

What is this part kss6w ?

Nickthedentist · 16th May 2018, 10:04 pm

Thermistor?

Might be KS56W

See for example: http://forum.radios-tv.co.uk/viewtopic.php?f=3&t=4930

Ancient Geek · 16th May 2018, 10:06 pm

It's a thermistor, I believe.

Nukeybrown · 16th May 2018, 10:46 pm

560ohm thermistor i think.

Argus25 · 17th May 2018, 12:22 am

I realize that you didn't ask how it worked, but in trying to figure out what an unknown part is, it is often instructive to see what it is supposed to be doing or might be doing.

As you can see the part is placed between the bases of two output transistors. As the transistors heat up, their base-emitter voltage drops, something like -2.1mV/DegC and their base current increases for any fixed applied base voltage and so does their collector current increase. If this is not compensated for there is a risk of thermal runaway and heat damage to the transistors, though the transistor emitter resistors also help by dropping more voltage and reducing the base current through DC negative feedback as the currents increase with heating.But they are not enough on their own to thermally stabilize the transistors.

In any case it means that the part in question must be a negative temperature coefficient resistor (as noted called a Thermistor). It lowers its resistance (and therefore the voltage drop across it) as its heated, lowering the base currents to compensate the transistor's increase in base current with heating. Ideally that thermistor should be thermally coupled/bonded to the transistor's heat sink (if there is one) but some manufacturers for low power output stages just put them nearby to the output transistors on the pcb.

Often you will see one diode and resistor or two diodes used instead of a thermistor in this circuit position because the diode's voltage drop decreases with heating, much like the transistor's base-emitter junction, or even a transistor and potentiometer wired as a vBE multiplier to adjust the inter-base voltage and the transistor's quiescent collector current and thermally stabilize it.

16th May 2018, 10:04 pm	#1
Nukeybrown Heptode Join Date: Jan 2016 Location: Gateshead, Tyne and Wear, UK. Posts: 504	Identify part What is this part kss6w ? Attached Thumbnails

16th May 2018, 10:04 pm	#2
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,844	Re: Identify part Thermistor? Might be KS56W See for example: http://forum.radios-tv.co.uk/viewtopic.php?f=3&t=4930

16th May 2018, 10:06 pm	#3
Ancient Geek Retired Dormant Member Join Date: Aug 2017 Location: Bristol, UK. Posts: 136	Re: Identify part It's a thermistor, I believe.

16th May 2018, 10:46 pm	#4
Nukeybrown Heptode Join Date: Jan 2016 Location: Gateshead, Tyne and Wear, UK. Posts: 504	Re: Identify part 560ohm thermistor i think.

17th May 2018, 12:22 am	#5
Argus25 No Longer a Member Join Date: Oct 2016 Location: Maroochydore, Queensland, Australia. Posts: 2,679	Re: Identify part I realize that you didn't ask how it worked, but in trying to figure out what an unknown part is, it is often instructive to see what it is supposed to be doing or might be doing. As you can see the part is placed between the bases of two output transistors. As the transistors heat up, their base-emitter voltage drops, something like -2.1mV/DegC and their base current increases for any fixed applied base voltage and so does their collector current increase. If this is not compensated for there is a risk of thermal runaway and heat damage to the transistors, though the transistor emitter resistors also help by dropping more voltage and reducing the base current through DC negative feedback as the currents increase with heating.But they are not enough on their own to thermally stabilize the transistors. In any case it means that the part in question must be a negative temperature coefficient resistor (as noted called a Thermistor). It lowers its resistance (and therefore the voltage drop across it) as its heated, lowering the base currents to compensate the transistor's increase in base current with heating. Ideally that thermistor should be thermally coupled/bonded to the transistor's heat sink (if there is one) but some manufacturers for low power output stages just put them nearby to the output transistors on the pcb. Often you will see one diode and resistor or two diodes used instead of a thermistor in this circuit position because the diode's voltage drop decreases with heating, much like the transistor's base-emitter junction, or even a transistor and potentiometer wired as a vBE multiplier to adjust the inter-base voltage and the transistor's quiescent collector current and thermally stabilize it.