[time-nuts] HP 105B Modification

[Bruce Griffiths]

Gerhard Hoffmann wrote:
> On Wed, 09 Dec 2009 15:08:15 +1300, you wrote:
>
>
>    
>> Another point is the LTSpice LED model isn't particularly accurate for
>> simulating the effects of temperature variations.
>> Does anyone knows of more accurate LTSpice compatible LED models?
>>
>> The LED model voltage drop increases with temperature even at low
>> current, whilst the voltage drop across a real LED at low currents
>> decreases with temperature.
>> In practice the variation in the LED forward drop tracks the variation
>> in the pnp Vbe quite well.
>>      
> I have used this model and also this diode:
>
> * from OSRAM TOPLED library
> *
> *
> .MODEL LH_T676_typ D
> + IS=5.5621E-24
> + N=1.3448
> + RS=4.3842
> + IKF=17.186E-3
>
> Comparing the current with a BC860 PNP and a 15 Ohm emitter resistor
> worked quite nicely, so I had no reason to investigate the model.
> (as you wrote, too)
>
> I also tried to operate a power LED with the base bias divider current
> of all 6 distribution channels combined(70 mA tot.) to recycle the current
> but it seemed that the power LED had more Ohmic resistance than
> 6 times LHT676. Could be just my sample, and it was close to the max. specs.
>
> Gerhard
>    
Gerhard

That LED model has the same problem (at least in LTSpice):
The LED forward drop increases (instead of decreasing as does the 
forward voltage drop of a real LED) with temperature.
Its not a great issue as long as one is aware of the problem.

Bruce