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All,

Here's a real world example of damaging undervoltage:

I once built a RTTY TU ( Terminal Unit ). The teletype had a "loop" consisting of a 120VDC power supply feeding a resistor and going to the selector magnets.

I used a high voltage transistor to switch the current on & off in time to the incoming baudot data. Baudot is like ASCII, only older and more primitive - less bits, and a "shift" code to make the carriage hop into upper case.

The transistor was safe as long as it was hard ON - zero voltage across it. OR hard OFF - zero current through it. Either one of these cases resulted in zero power dissipation.

IF OTOH you turned the transistor on just a little bit - say because of too low base voltage/current - that poor little transistor would let out its smoke with dispatch.

- Jerry KF6VB

On 5/6/21 12:31 PM, jerry@... wrote:

All,
? Here's a real world example of damaging undervoltage:
I once built a RTTY TU ( Terminal Unit ).? The teletype had a "loop" consisting of a 120VDC power supply feeding a resistor and going to the selector magnets.
I used a high voltage transistor to switch the current on & off in time to the incoming baudot data.? Baudot is like ASCII, only older and more primitive - less bits, and a "shift" code to make the carriage hop into upper case.

One small nit... BAUDOT does not have upper and lower case. Uperr
only. The shift weas for non-alpha characters.

bill

The failure mode on your RTTY terminal is similar to that of applying a signal in the threshold region?
to a logic gate:

> Some extremely high powered logic parts (74ASxxx family parts, I'm looking at you) can overheat
> and/or oscillate when an input signal is in the transition region between logic 1 and logic 0.? This is
> why it is good practice to tie unused logic inputs to a solid logic 1 or 0 (typically to ground).
> Many parts have some hysteresis designed into the input buffers, mostly avoiding this issue.

Typically an issue only on really high power parts,
careful design can usually avoid this.

Jerry, KE7ER

On 2021-05-06 09:38, Bill Gunshannon wrote:

One small nit... BAUDOT does not have upper and lower case. Uperr
only. The shift weas for non-alpha characters.

*** Yes, indeed. I had forgotten, it was a long time ago. Around 1970. I had a railroad surplus model 15. Which had no case. To say it was loud would be an understatement.

- Jerry

bill

On 5/6/21 12:48 PM, jerry@... wrote:

On 2021-05-06 09:38, Bill Gunshannon wrote:

One small nit...? BAUDOT does not have upper and lower case.? Uperr
only.? The shift weas for non-alpha characters.

*** Yes, indeed.? I had forgotten, it was a long time ago.? Around 1970. ?I had a railroad surplus model 15. Which had no case.? To say it was loud would be an understatement.

I had a Lorenz LO-15 and really miss the sound. :-)

bill