Xiaofan Chen wrote:
> You can still do that (regulating the voltage down). To measure the input
> voltage, use a resistor divider and feed that to your voltage monitoring
> circuit or an extra circuit.
>   
Let me see if I can clarify this...

I have an input on a device, which isn't really able to be changed at 
this point.

It can handle 9-32V input or thereabouts.   This two pin input is *both* 
the source of power for the device, and the spot which is metered.  
Again, this is not able to be changed.   Designed, and in-production 
device.   And the current draw varies quite a bit in some configurations 
(relays being turned on and off, etc.), so it might be 1/4A at one 
moment, and a full amp or more the next.

So... a simple resistor divider isn't really going to work to take say 
18-64V and divide it to stick it into this device.  Plus, as the current 
draw changes, the voltage would change, which is not good.   Secondly, 
the up to 32V drop at an amp or so is 32 watts.  Not pretty either.   
The voltage drop/current thing also applies to *any* linear regulator 
style device on the front end.  I can't be wasting 32watts not doing 
useful work, not to mention the heat issues.   32W is a lot of heat to 
dissipate.  

I realize I could regulate this down to a fixed 24VDC with a switching 
regulator.  I do have this option, and will probably end up doing that, 
and measuring the voltage in some other way.  Just not as 'neat' of a 
solution from the end-user's standpoint.

I would much rather put a 'switching voltage divider' on the front 
end.   Then I don't need any intelligence at all, or any special 
connections, or anything like that.   Just say 'plug your 48VDC in here, 
and connect this to your existing device'.   Voltages get halved, and 
simple.   If it wasn't for the heat issue, I'd just do this with some 
sort of linear solution.  Instead I need a switcher  due to the currents 
involved.

-forrest
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