Hi,
I'm working on an audio amplifier power supply and one of the components exploded so I can't identify it. It's R1 in this schematic. I'm thinking it's some kind of variable resistor, but I haven't seen a slash symbol quite like that before. Can someone help me out?

 

That was my first impression -- a rheostat. Does the physical part look like it might be (or have been) a variable resistor?

 

I'm thinking it could be a PTC themistor of sorts as a method of managing inrush or some sort of polyswitch?

 

I'm thinking it could be a PTC themistor of sorts as a method of managing inrush or some sort of polyswitch?

That could be -- and it would make sense to have the slash indicating that it's not a fixed value, but not have the usual arrow at the end indicating that it's settable.

 

hi,
Some early valve [tube] equipment used NTC's in their filament/heater circuits, in order to limit high power On currents.
Gradually falling in resistance as the heaters warmed up.
I would go for a NTC device, in that circuit location
E

 

I'm thinking it could be a PTC themistor of sorts as a method of managing inrush or some sort of polyswitch?

I agree with Marc, in this instance R1 looks like it should be a thermistor.. It is also clear that this drawing uses a number of symbols not normally seen. Do you know the country of origin for this drawing? There are several countries that do not follow the IEC standards, and use totally different symbols. It seems that they choose intentionally to be confusing.
Is it possible to show us the complete drawing? Now I am interested, there are some unusual things and circuit variations that it would be educational to understand.

 

I think it is a fusible resistor. I think the exclamation mark inside a triangle is a warning that for safety reasons the exact part number must be used.

Les.

 

Appears to be of European origin, with a direct to 240VAC mains power, hence the exclamation marks.
R1 would be a polyfuse connected to the AC line input (red). Black is AC neutral.

 

I just happened to be watching this Big Clive video

and he drew this schematic:


So, it seems the obvious answer was probably correct.

 

I think it is a fusible resistor. I think the exclamation mark inside a triangle is a warning that for safety reasons the exact part number must be used.

Les.

The triangles with the exclamation points indicate that mains voltage is directly connected to those items and hence there is a shock hazard. Which is true. But a fusable resistor needs to carry a current rating, which is not present on the drawing. That points to it either being a surge limiting thermistor or else one of those thermisters that develop a very high resistance when they heat up, so as to greatly limit fault current. And that device in post #9 looks like an updated version of the ones that used either a 12 volt bulb or a neon bulb and resistor, that were used by some folks for tracing electrical problems by poking the wire insulation full of holes. The users never realized that insulation once punctured never heals, and salt water always corrodes copper wire..

 

Now I am interested, there are some unusual things and circuit variations that it would be educational to understand.

Here's the full schematic. I don't know where it's from.

 

Thanks for all the great responses. I'm going to try a 10 ohm PTC for 2 reasons: 1) The original part was green, and I haven't seen green NTCs. 2) PTC makes the most sense to me at that location.

 

Thanks for all the great responses. I'm going to try a 10 ohm PTC for 2 reasons: 1) The original part was green, and I haven't seen green NTCs. 2) PTC makes the most sense to me at that location.

Indeed, The whole unit is a high-end audio amplifier with some interesting features. It may have come from "The free Information Society" or 'Schematics for free", two of my favorite sites. Are you trying to repair this device? Or just hoping to understand it?

 

hi,
Some early valve [tube] equipment used NTC's in their filament/heater circuits, in order to limit high power On currents.
Gradually falling in resistance as the heaters warmed up.
I would go for a NTC device, in that circuit location
E

Use of an NTC to limit the power-on surge as the bulk capacitor charges is common in modern power supplies too. Unfortunately, the temperature/power and the temperature/time curves are unknown, but it might help to use an NTC the same size as the one that was destroyed.

Those thermistors are pretty rugged, so I suspect a severe problem in the power supply -like something being shorted.

 

Indeed, The whole unit is a high-end audio amplifier with some interesting features. It may have come from "The free Information Society" or 'Schematics for free", two of my favorite sites. Are you trying to repair this device? Or just hoping to understand it?

I'm attempting to resurrect it. It had a blown transistor, which had taken out the rectifier and R1. It looks like it might be good otherwise.

 

I'm attempting to resurrect it. It had a blown transistor, which had taken out the rectifier and R1. It looks like it might be good otherwise.

That is definitely an item worth restoring, even if you had to simply replace the entire power supply. As for the thermistor current limiter specifications, nameplate watts will give you the steady state current, and from that and the needed rectified voltage you can figure out the warm resistance and that should be what you need. Be sure to get the cold resistance high enough, though.

 

According to IEEE Std 315-1975 and ANSI Y32.2-1975, page 23, this line ( / ) is:


EDIT: This is NTC thermistor for inrush current limiting (circuit on Figure 1).

 

Definitely an educational link. I would not choose to use a PTC device for startup surge limiting, but it could serve very well as a circuit protection device, eliminating fuse popping and the probability of replacement with the wrong fuse size. There is a place for each kind of device and those places are different. In the case of the amplifier it is clearly the place for an NTC device. And given the brand name of the manufacturer it is probably from a country that does not use the same symbol set that we are used to seeing.

 

I would not choose to use a PTC device for startup surge limiting,.

Seen it quite a few times in Industrial equipment, usually to mitigate large transformer inrush.
Was also used in the auto switch off circuit of a colour CRT de-gaussing coil.
Max.

 

According to IEEE Std 315-1975 and ANSI Y32.2-1975, page 23, this line ( / ) is:
View attachment 169730

EDIT: This is NTC thermistor for inrush current limiting (circuit on Figure 1).
View attachment 169734

I actually found out from the manufacturer that it is indeed an NTC Thermistor. Obviously for inrush.