TO-18 vs TO-92 packages and thermal stability/drift - Page 1
Forum section: electronics / projects

author: MrSlack
author_rank: Frequent Contributor
timestamp: January 18, 2016, 02:29:24 pm
content: Is there any difference in thermal stability between TO-18 and TO-92 packages?I've built a simple differential amp using old Motorola TO-18 packaged 2n2222a's and the same thing with Fairchild TO-92 bc337-25's. None are cheap Chinese transistor value bag cack. They are hfe matched using a crap multimeter. I left the TO-18 packages unbonded and thermally isolated by air. I tried the same with the bc337's but the whole thing was rather unstable at the Q point. If I put my finger on either transistor it would swing violently one way. I suspected a duff bc337 so replaced both with new rematched ones (with different hfe) and it was just as bad. I tried to bond them together with some heatsink compound between the flat faces of the TO-92, some arctic silver and a cable tie (ugly hack). That improved things slightly but the whole thing seems very much more stable with moldy old To-18 packaged 2n2222s.Circuit is a textbook long tail pair diffamp with current mirror on the tail. There is negative feedback and the Q point is set at 2mA. +/-12v across the amp, signals centered on 0v.Any thoughts appreciated.

author: MatthewEveritt
author_rank: Unknown
timestamp: January 18, 2016, 03:23:15 pm
content: I'm really no expert, but a few thoughts occur. The junction-ambient thermal resistance is ~300 C/Wfor the TO-18, and ~200 C/Wfor the TO-92.Combine that with the lower thermal mass of the TO-92 and I would suspect that the die heats up considerably faster when you apply your finger than the TO-18 will, which would explain what you're seeing. The effect of thermal resistance should be reversed when the transistors are thermally coupled as the lower thermal resistance should keep the dies closer in temperature, although this doesn't really tally with your finger test.Possibly the higher thermal conductivity of the metal case has an effect, more uniformly heating the outside of the device. I've no idea if transistors react to temperature gradients over them. It will presumably also slightly reduce the temperature increase due to your finger as compared to the smaller, less conductive plastic case.It also occurs to me that the hollow metal can will probably be more sensitive to where the non-uniform heat source is applied. The closer to the die you put your finger the more strongly the heat will couple through (until the can reaches thermal equilibrium, which may or may not be a noticeable amount of time)I would suspect that if you waited longer with a more realistic heat source (put them in the airflow from a warm power supply or something), or caused one transistor to dissipate more power than the other you'd see a different result, with the theoretically better thermal coupling of the TO-92s making it more stable.I could of course be wildly wrong, but I appreciate the mental exercise, so thank you.

author: MrSlack
author_rank: Frequent Contributor
timestamp: January 18, 2016, 03:30:37 pm
content: Thanks for the insights - interesting indeed. Will play some more with applying different temperature gradients (I have found a can of freezer spray and my wife's hair dryer) and see what happens.Also going to build them separately dead bug style from scratch in case this is the breadboard which is a dick most of time.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 18, 2016, 07:01:38 pm
content: I would be more inclined to suspect the 2N2222s of justplain being bad for the circuit(or bad in general), so that your circuit isn't amplifying the difference that's necessarily present.The Vbe tempco is a material property of silicon, so all diff amps will respond in exactly the same way. Same for old germanium transistors (though at a more pronounced tempco), or for that matter, vacuum tubes at very low currents (though, because they're already hot inside, there isn't much of a tempco with respect to ambient conditions!).Tim

author: MrSlack
author_rank: Frequent Contributor
timestamp: January 18, 2016, 08:51:39 pm
content: Oh so after a few minutes of squirting it and blowing it and seeing predictable behaviour, I poked it with a pen which adds no heat and it went crazy. So I built two on PCB scraps dead bug and solder using 337s and 2222s.Both work identically.Took the breadboard to bits and the damn thing has corroded underneath and has its own resistance between rows. When I put pressure or heat on it. Fuuuuuuuuu... thrown in the bin.Wisher WBU-502L for reference. Only a year old. No idea what the corrosion is.Mystery solved anyway. No more breadboards!

author: Audioguru
author_rank: Super Contributor
timestamp: January 18, 2016, 10:05:38 pm
content: Besides having intermittent contacts, solderless breadboards are perfect for picking up hum and all kinds of interference.

author: MrSlack
author_rank: Frequent Contributor
timestamp: January 18, 2016, 10:25:11 pm
content: Indeed. I should have taken inspiration from Mr Pease or Williams when it comes to this stuff...