Re: LX200: DEC Motor Study
| Subject: | Re: LX200: DEC Motor Study |
| From: | Martin Tom Brown |
| Date: | Wed Apr 24 02:18:55 1996 |
> Martin Tom Brown (Martin@nezumi.demon.co.uk) wrote:
> >78L05 regulator seems plausible rated to 100mA and 360mW nominal power.
> >A quick back of the envelope calculation suggests that if this circuit
> >draws more than 30mA at 18v it may cause the regulator to overheat.
360 mW is from memory for a TO92 package without heatsink, and
probably includes a design safety margin. Dont have refs to hand.
Spec is strictly <100mA and temperature <125 C operating range.
Note anything >70C will be too hot to touch.
> I measured the drop across the 56 ohm resistor (in series with the LEDs)
> at 2.6 volts. Thus, the current from the 78L05 is 2.6V/56ohm=0.046amp, since
> the other loads on the 78L05 are negligible compared to the LED branch.
According to ????s sketch circuit there are *two* LEDs in the chain
so Im surprised how large the voltage drop is across the 56R.
> Thats 50% "more than 30mA", so it looks like this thing is overloaded.
> The output from my 78L05 is 5.3 volts,
56R
5.3v ---/\/\/\-----o------>|-----o----->|-------- OV
^ 2.7v ^ W
This leaves me puzzled about the voltage drops across the LEDs.
Even if they are IR LEDs Id have expected a forward voltage of
at least 1.6v each - I wonder if one of the LEDs is dodgy?
Can you easily measure the point W to see if the LEDs are
taking equal shares of the voltage drop?
> means 360mW maximum power, then the 78L05 is surviving by virtue of its
> internal protective circuitry, which may put the dec. circuits at risk.
100 mA or overheating are the hard limits. You can run it at higher power
simply by placing the case in contact with a lump of metal or heatsink.
Hence the copper foil clip suggestion.
> Despite having had two dec. motor replacements since 1994, this
> scope has been operating well for months, despite the hot 78L05.
If it gives 5 ish volts output then its working OK.
Can you reproduce the motor fault on the bench where you can measure
voltages for good and bad motor specimens - that might shed some light.
Good luck,
-- Martin Brown <martin@nezumi.demon.co.uk> __ CIS: 71651,470 Scientific Software Consultancy /^,,)__/
