Re: tektronix 7S14 batteries and time base question
Hi Bruce,toggle quoted messageShow quoted text
Thank you for your post on using clear epoxy to bond LEDs to each other. I chopped off the ends of a white source LED and a red receiving LED with a sharp razor and I glued them together with clear epoxy as he said to do. Preliminary results show an insignificant difference in the output voltage this pair generates. I will be doing more testing to determine this and other things I experiment with in the next few days. I will report my results to the group.
In the original 070-1410-00 7S14 manual I have (Dec, 1973) the resistors in series with the mercury cells (R1 and R2) are 200 ohms each. In the 1985 revision (Apr, 1985) they are 2,000 ohms. The change took place at serial number B030000 and above.
The mercury cells reverse bias the sampling diodes until a strobe pulse forward biases them. Strobe pulses are extremely short. So even under heavy usage the current drain from the cells will have a very small duty cycle. It would be helpful to know how long a strobe pulse lasts and what voltage they go to. Has anyone done this?
The low duty cycle means a very low overall current drain on the cell. But during each strobe the diodes are forward biased and it is important that they get all the forward current they need to conduct. The very low internal resistance of the mercury cell means it can provide any forward biased current the diode draws. The 2K series resistance of R1 (and R2) will limit the current surge to ~500uA which is fine for the cell but that is much more than the output LED of the mercury cell replacement can provide.
A small value low voltage capacitor could store the charge coming from the output LED and provide all the forward bias current needed by the sampling diodes when they are strobed. The voltage it stores is less than 2V. I would suggest it be a low leakage capacitor because of the minute current coming from the LED. Glass capacitors are a good choice since they are cheap and have virtually no leakage. As an example, a 0.033uF glass capacitor (like the ones shown in this link are no bigger than a 1/4W resistor. Unfortunately I could not find surface mount glass capacitors for some reason.
Has anyone mentioned what the sampling diode technology is? If so I missed it. Also, someone mentioned a few months ago the temperature coefficient of the sampling gates. I went looking for where it was mentioned but I couldn't find it. I would think it would depend on the sampling diode technology. If anyone can remind me what the Tc was and what technology the sampling diodes are (I would guess they are silicon Schottky diodes) I would appreciate it. It affect many things we should consider when designing our substitute mercury cell such as turn-on voltage, current drain, strobe pulse duration, temperature coefficient, etc.
At this point in time I am the beneficiary of many experiments and suggestions on replacements for the 7S14 mercury cells. Previous informal random experimentation seems to have recently shifted to detailed discussions of important design considerations. What would help me make a contribution of my own is 1) knowing the technology of the sampling diodes, and 2) documenting the strobe pulse's voltage, duration, and current drain on the battery. I'm hoping someone can help me with #1. I can do #2 myself.
I welcome comments since, as far as I know, some of my thinking hasn't been discussed yet.
Dennis Tillman W7PF
From Bruce Griffiths
Sent: Monday, December 23, 2019 4:17 PM
LED encapsulation is clear epoxy. Clear epoxy is best suited to bonding LEDs end to end. Either mechanical preparation of the mating surfaces or priming is required. Done properly the roughened ends are thoroughly wet by the bonding epoxy and the joint is invisible.
The refractive index of cyanoacrylate adhesives is somewhat lower than that of epoxies.
For a Lambertian source like a LED, butt coupling is effective. Intervening optics doesn't improve the coupling. The dome lens on the LED doesn't collimate the light so that an intermediate lens is required to maximise coupling if this method is chosen.
For maximum coupling when butt coupling the separation of the 2 LED die should be comparable with the source diameter, However reflected light from the cupped lead frame of the LED may relax this requirement a little.
Optocouplers use butt coupling with a thin transparent insulating film between the emitter and the detector.
The peak response of a LED used as a photodetector occurs at a wavelength that differs slightly from its emission peak when used as an LED.
Most of this was covered in the HP optoelectronics Handbook around 1970-80.
On 24 December 2019 at 12:34 Dennis Tillman W7PF <firstname.lastname@example.org> wrote:
Dennis Tillman W7PF