Talk:HP 200A

Thermal mass

The lower thermal mass should have improved the high frequency response characteristics but it seems to not have been an issue.

From the patent [1]

Since the lamp itself responds relatively slowly to current changes it tends to average out the voltage peaks and does not cut off the peaks of the generated waves, maintaining the amplitude constant without producing distortions in wave form.

The low thermal mass of the filament would therefore have contributed to fast settling times to range changes or other supply or frequency changes but seemingly not been affected by the oscillation in the 20 to 40000 Hz range which it would have averaged out. It is possible that the low thermal mass would have introduced non-linearity to the low frequency end of the unit where the light current and temperature could have started to modulate at the output frequency.

I think the statement about the thermal mass should perhaps be reworded to indicate that the low thermal mass would have improved amplitude settling times to range changes compared to a thermally more massive composite thermistor.

Idyllic press (talk) 11:34, 4 November 2013 (UTC)

Copied form User Talk: Constant314

How did they use the light bulb if not as a thermistor?

Reading the patent it sounds very much like this is exactly what they did, the choice of using a light filament was what allowed for fast settling times in the thermistor controlled amplitude regulation circuit.

Idyllic press (talk) 12:23, 7 November 2013 (UTC)

Hewlett used the light bulb as a metallic resister. Like most such resisters, its resistance increases as it gets hot. Not all devices that have a resistance that changes with temperature are thermistors. Intentionally using a device that changes resistance as a function of temperature does not constitute using the device as a thermistor. Hewlett designed from the beginning using a light bulb. He did not design it with a thermistor and then change to a light bulb because he could not get a thermistor (if he even knew that that they existed). If you want to say that any specific device that is not a thermistor is used as a thermistor, you will need a reliable source. It is very unlikely that you will find a source that says in effect, “We designed for a thermistor but then used an xxx device instead for that function”. I am not saying that a thermistor could not be used to stabilize an oscillator. I think, but do not know, that either PTC device could be used in the same leg of the bridge as the light bulb or an NTC device could be used in the other leg of the bridge for the same purpose. I would still want a reliable source saying that it was so. Thermistors and light bulbs have different resistance vs. temperature characteristics, so they are not always interchangeable. This article is about a specific instrument that used a light bulb and not a general article on oscillators. If you want to go to the Wien bridge oscillator page or thermistor page and you have a reliable source that says that a thermistor could be or is used to stabilize an oscillator, I would not dispute that. Hewlett’s big breakthrough was to stabilize the amplitude of his oscillator by servoing the gain to keep the amplifier in its linear range. Hewlett’s use of a light bulb was an inspired example of lateral thinking. The light bulb served as a detector, low pass filter and a gain-controlling element and you can say that. However, those functions do not require a thermally sensitive component. A rectifier, op-amp and a jfet are commonly used today.64.129.61.50 (talk) 13:40, 7 November 2013 (UTC)

The change of mine you reverted was to the Self-heating effects and uses on the Thermistor page. It was not on the HP200A instrument page where the connection to thermistor is stated or the Wien bridge oscillator page where the use of thermistors and other devices is addressed in the Amplitude Stabilization section.

I accept that Hewlett likely had not had much experience with thermistors in 1939 as they had first been marketed in 1930's and he compares his lamp to a contemporary equivalent device called 'ballast lamps' that were used inter alia as crude series voltage regulators in other equipment of the day.

However it is clear that his innovation was to use the self-heating caused resistance change to affect the loop gain and so stabilise the amplitude which is why I added it to that section in where it was most pertinent.

The thing I cannot answer is whether or not a hot filament can be called a thermistor simply because it behaves in a similar manner in certain conditions. The wiki page for thermistor indicates some things that are thermistors but does not say what cannot be a thermistor. If it is functional characteristics alone then a filament might comply, if it is some other material or non-linearity that is needed then it is as you say only a substitute and not an equivalent. The wiki page says the resistance changes significantly with temperature compared to a regular resistor (which tries to remain constant) and in this respect a lamp does behave like a thermistor. So if a lamp is like a thermistor (and I have seen tiny filament lamps used inside telephone instruments in the 1980's that were not visible from the outside acting like thermistors) then the HP200A comment is valid as a self-heating use of a lamp. The wien-bridge page also implies that thermistors can be used in the amplitude control so at least this use could be mentioned even if the HP200A use cannot be justified on the basis of the lack of equivalence between bulb and thermistor.

Thank you for taking the time to give a considered response to my enquiry.

Idyllic press (talk) 17:05, 13 November 2013 (UTC)

Yes, I did get the pages mixed up. But it is all the same discussion whether it goes on the thermistor page, the Wien bridge oscillator page or the HP200 page (which I did not know about until recently). Anyway, we can continue the discussion here on my talk page or any of those three pages if you want to bring other editors into the discussion. It is my intention to expunge all suggestions the Hewlett used the light bulb as a thermistor everywhere on Wikipedia unless someone produces a reliable reference (preferably Hewlett's own writings or HP literature).

Shoot forward 50 years when all light bulbs are replaced by LEDs. Then someone writes that Hewlett used the light bulb as an LED. Well why not, they both emit visible light, they both heat up, they both change resistance when there temperature changes. Right? So, Hewlett used the light bulb not as a thermistor but as an LED because thermistors don't emit visible light. Absurd, right? The thing is, the light bulb came first. We might use an LED to replace the function of a light bulb and we might use a thermistor to replace the function of a light bulb, but Hewlett used the light bulb as a light bulb.

Let me quote the second paragraph on the Thermistor: "Thermistors differ from resistance temperature detectors (RTD) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals." So, is light bulb more like ceramic or polymer or is it more like a pure metal? Clearly, if you are going to say that Hewlett used the light bulb as something other than a light bulb then you should say he used it as an RTD.

Now this is just me speculating, but the light bulb used to stabilize oscillators is usually run at a temperature where it is dim but visible. I'll just make up a number: 2000F. If the air temperature around the bulb is 70F then there is a 1930F temperature difference. If the air around the bulb rises 30 degrees then the temperature difference 1900F. So the change is about 1.5%. Now suppose you had an oscillator that was stabilized with a thermistor. How hot would you run it? Let's say 250F. So at 70F there is a 180F difference. Let the air heat up by 30F and now the difference is 150F. That's about a 20% difference. So, a thermistor would probably make a poor regulator. Of course, it is not microphonic like a light bulb, but in bench equipment that is not so important. Now, I know of an oscillator stabilized by a light bulb, but I don't know of a single commercial design that uses a thermistor for stabilization. So, you really should have a reliable source that says a thermistor can be used to stabilize an oscillator. I think it is probably true that it could be done, so I won't dispute that, but someone else might.

But, have a look at WP:SYN. Let me paraphrase it. If you have a source that asserts "A implies B" and you have another source that asserts "A is true" you might think that you could cite those two sources and then assert "B is true". But that would be your original synthesis and it is not allowed. You must find a source that asserts "A implies B, A is true and therefore B is true" or a source that simply asserts that B is true. So, you cannot take the description of a thermistor and a description of how Hewlett used the light bulb and conclude that he used the light bulb as a thermistor.

So, it comes down to this: find a reliable source that says that Hewlett used the light bulb as a thermistor or it has got to go. By the way, Wikipedia is not a reliable source.Constant314 (talk) 23:42, 13 November 2013 (UTC)

Electronic Principles, Physics, Models and Circuits by Gray and Searle, 1969, page 61 "A thermistor is simply a piece of intrinsic semiconductor ..."Constant314 (talk) 03:21, 14 November 2013 (UTC)

In light of further research I concur with you on removing the HP200A mention from the thermistor page. There is no direct evidence that he was using the bulb instead of a thermistor but he was using it in a similar manner to a PTC thermistor. I made a couple of other small cited changes to the page that you can have a look at and chop at if not up to standard. I also modified the Resettable fuse page to distance that from a thermistor. I think it is correct to say that thermistors are in a class of devices that make use of alterations in charge carriers for their thermal properties so the polymer fuse or humble bulb should not be held as equivalent.

Idyllic press (talk) 09:16, 14 November 2013 (UTC)

I can't understand why this discussion has gone on for so long. First of all thermistors were not widely used when HP got started. It's doubtful that Hewlett or Packard knew what a thermistor was anymore than they knew what a triac was at that time. Secondly most thermistors in use today have a negative temperature coefficient. Go into any electronic parts store and grab some thermistors out of a bin and most, if not all will have a negative tempco. Some companies do sell positive tempco thermistors but it is a very small market. All metals have positive tempcos. HP's 200A oscillator used a lamp with a metal filament to stabilize the Wein bridge oscillator because of its positive tempco. As the bridge current increases the lamp filament gets hotter, increasing its resistance. This limits the current excursions. If the lamp was replaced with a negative tempco thermistor the current would just keep increasing until the unit failed. A positive tempco thermistor might be used to replace the lamp except for the fact the resistance of thermistors are generally much higher. HP did use thermistors later on for the power sensing element of their RF power meters; also a bridge circuit. (circa 1960's).Zen-in (talk) 04:10, 8 December 2013 (UTC)

I agree with everything you say, but some one or more editors have added words to the effect "Hewlett used a lamp as a thermistor" on at least three articles and I remove it when I find it which always causes a long discussion. There seems to be a substantial population that think any temperature dependent resister is a thermistor.Constant314 (talk) 17:14, 8 December 2013 (UTC)

I would check if any editor who adds that might also add "Hewlett used a tube (valve) as a transistor".Zen-in (talk) 18:04, 8 December 2013 (UTC)

PTC thermistors (maybe not sold specifically as such, but components with a PTC behaviour) were in wide use within PSU circuits by this time. Andy Dingley (talk) 12:09, 8 December 2013 (UTC)

No, I don't believe that is the case. If you can produce a Rider's manual that shows a PTC thermistor in a 1930's radio it would add something to this discussion. In my personal experience with pre-WWII vacuum tube radios I never came across an inrush current limiter. They appeared in the 50's in early B&W TVs. It was a large disk-shaped device near the power cord connector that had a high resistance (~ 50 - 100 Ohms) at room temperature and a much lower resistance at a higher temperature. These NTC thermistors have the opposite temperature transfer function of an electric lamp.Zen-in (talk) 18:04, 8 December 2013 (UTC)

The designs I'm thinking of were little more than (and often were) simply a filament lamp, but they were being used specifically for their PTC behaviour. Andy Dingley (talk) 20:34, 8 December 2013 (UTC)

What you may be thinking about is a ballast tube. For example the Boonton 250A Rx bridge, manufactured in the 50's, uses a 6H-6 to regulate the filament voltage. It has a very long filament, no grids or plate and looks like a tube with a black plastic base. They weren't called thermistors. I checked Ghirardis' Modern Radio Servicing (1935) and Radio Physics Course (1937). There are a few pages on the temperature coefficient of resistors but no mention of thermistors. Terman's (A Stanford professor when Hewlett and Packard attended) book, Electronics and Radio Engineering (1955) has a section on semiconductors as well as tempco or resistors but no mention of thermistors. The widespread use of thermistors in electronics (except for surge suppressors in TVs) did not occur until at least the 60's. I remember reading about thermistors in Popular Electronics around that time.Zen-in (talk) 03:24, 9 December 2013 (UTC)