[sdiy] When to use voltage regulators and when to use resistors?

[Philippe Derenne]

... as I remember, 18.5 minus 15 is 3.5  ;^)

Looking at the 7815 datasheet, minimum Vin is 17.7 V => no problem

Phil.

----- Original Message ----- 
From: "KA4HJH" <ka4hjh at gte.net>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Sunday, May 25, 2008 9:21 AM
Subject: Re: [sdiy] When to use voltage regulators and when to use 
resistors?


> >Probably a pretty silly question but I'm wondering when to consider 
> >voltage
>>regulators versus knocking down voltage with resistors.  In my particular
>>case, I'm adding some modules to my rack mounted SEMs.  The SEMs run off 
>>of
>>18.5v but the additional circuitry I want to add expects 15v.  Should I 
>>put
>>a 15v regulator on or can I just pad down the 18.5 volts coming from the
>>power supply with appropriate resistor values?  I have plenty of spare
>>amperage in the supply to handle the additional modules, I'm just not so
>>sure about getting the voltage down to the 15v. Thanks!
>
> The SEM's have on-board regulation which drops the +/-18.5 to +/-15V
> (there's one spot that actually uses +18.5V, IIRC). The easiest thing to 
> do
> is duplicate this scheme. Resistors are a bad idea and TTR's are cheap.
>
> The one snag is that you have to use a regulator that can operate with an
> input only 2.5V above the output. A 7815 won't work. Tom used a 723 and a
> discrete transistor along with all the extra circuitry. Are there low drop
> 15V TTR's these days? 5V ones seem to be common enough.
>
> How much current do you need? You always swipe a little from the SEM. I 
> get
> the impression  you're doing something which will require more current.
>
> You could always go back to your 18.5V supply and add a 15V section...
>
> Any mistakes in this posting are someone else's fault.
>
> -- 
>
> Terry Bowman, KA4HJH
> "The Mac Doctor"
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