Firing up the pdt11. Dec put 5 volts on an LED?????

Doug Jackson doug at
Sun Oct 11 22:50:17 CDT 2020

Could it have been a 5V LED with integral current limit?

That would explain the odd behaviour.

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Doug Jackson

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On Mon, 12 Oct 2020 at 14:33, Chris Zach via cctalk <cctalk at>

> Ok, this is weirder: I put the "bad" floppy drive on the bench and
> started to take a look at it. First I checked the LED (yes, it's an
> LED). With a bench voltage of 1.5 volts and a 100ma draw it lit up
> nicely in the IR (detected by phone camera, so nice they can see the
> light) and the photo transistor also seemed to work fine (at the sector
> hole resistance went from infinite down to about 500 ohms). That's good,
> so what is wrong?
> I noticed I could crank the LED higher current-wise to 150 ma and the
> voltage was still <2 volts. Interesting. Then I hooked a break-out
> harness to the pdt11 to see what kind of voltage it was putting out to
> the LED.
> It's putting out +5v whenever the unit is on. Maybe it's current
> limited? To check I hooked up the drive's plug to the breakout to see
> what the LED was seeing.
> +5v. And even weirder, the LED was not lit.
> What the heck is going on here?
> So I put the LED on the bench for a bit of a destructive test.
> Disconnected the PDT11 from the breakout cable, hooked up the power
> supply, turned up the voltage and the LED came on, then went *off* at
> around 3v. At 5v it was dead off, no IR light as measured by the camera.
> Turn the voltage down, and it comes on again. Up and it goes off.
> And unfortunately at 9v it died (CRAP!) as I turned up the current limit
> Yes, I forgot to set the voltage limit on the power supply, my bad, I am
> boo boo the fool...
> But this is weird: It looks like DEC put an LED in there with no current
> limiting, and a straight +5 volts. And the LED is always on at this high
> voltage? With no current limiting resistor? This does not make sense,
> but the volt meter don't lie. I'm going to check the working drive to
> see if it is limiting the voltage somehow. I'd say there was a resistor
> in the LED assembly limiting the current, but if that's true my cranking
> the voltage to 9v should not have blown it up, and it should not turn on
> at low voltages then off at 5v.
> Maybe the solution is to insert a resistor in series with the second
> drive at around r=e/i or r=5/.1 (100ma) or 50 ohms.
> Does this make any sense?
> C

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