[amsat-bb] Re: G-5500 7806 regulator question

[SV1BSX]

Hi Mark, Hi all,

another almost unknown but better and much more accurate combination in 
order to
increase the output
of any 78xx regulator is possible by making a divider in the output of  78xx
regulator according to the formula:


R1=VR/0.02 ( where VR the nomimal output of  78xx regulator)


R2=VB/0.025 (where VB the difference in Volts between "nomimal" output
Voltage of 78xx and "request" output Voltage)


In  -----------
---|  78xx   |-----o---OUT
    -----------      |
          |            R1
         o----------|
          |
        R2
          |
-------o-------------- 0V (GND)



Therefore, in  case we have a 7805 and we need 6 Volts output:

VR=5 => R1= 5/0.02 = 250 Ohms


VB (Differential Voltage):  6 - 5 = 1

R2 = 1/0.025 =  40 Ohms

You can replace R1, R2 with standard value-resistors:
R1=220+27 Ohms
R2 = 39 Ohms


The R1 is connected between + OUT and GND-Pin of 78xx. The R2 between
GND-Pin of 78xx
and GROUND (see my figure above).

*** HOWEVER, in this case the Hotsink (metal plate) of 78xx MUST BE ISOLATED
from the 0V !!
Just keep in mind that,  because the Pin2 is internally  connected
to metal-plate of 78xx ! So, if the GND(chassis) is equal to 0V and the 
metal plate of
78xx is directly connected to chassis for better cooling, then the R2 is 
by-passed
through the internal connection of 78xx. (and the R1 is now just a... 
bleeder !)

Finally, in this combination by using divider (or diodes as mentioned by 
others)
 the 78xx regulators lose the "short-circuit" output protection. That is 
available
only in case where the Pin2 of 78xx is directly connected to 0V (normal 
connection).

Hope this helps.

73, Mak SV1BSX



----- Original Message ----- 
From: "Mark L. Hammond" <n8mh at embarqmail.com>
To: <amsat-bb at amsat.org>
Sent: Sunday, August 17, 2008 2:54 AM
Subject: [amsat-bb] G-5500 7806 regulator question


> Hello All,
>
> It appears that the 7806 voltage regulator in my Yaesu G-5500 control box
> has gone bad; the output is measuring more like 10.8 Volts (!).   This
> explains why both position indicators were off by a factor of 2 (e.g.,
> elevation meter showing 90 deg, array pointing 45; azimuth meter showing
> 180 deg, but array pointing 90 deg., and so on).
>
> Question---has anybody used a 7805 to replace the 7806 regulator in this
> control box?
>
> I don't have a 7806....so this brings the next question--anybody have a
> 7806 (or 2) that they want to sell?)
>
> Thanks,
>
> Mark L. Hammond  [N8MH]
>
>
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