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From: Fred Bloggs
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Subject: Re: Limiting total wattage for a constant-current dummy load?
References: <firstname.lastname@example.org> <3DCD43BB.email@example.com> <firstname.lastname@example.org>
Date: Sun, 10 Nov 2002 15:42:01 GMT
NNTP-Posting-Date: Sun, 10 Nov 2002 07:42:01 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
John Muchow wrote:
>>>>See GMR Sensors Manage Batteries, Ramirez&Pelegri at :
> That link lead me to an article on amplitude modulation with DDS'. I
> did find the article here though:
The design idea is in the original PDF link if you take the time to
scroll down to it.
> Neat idea but the circuit delivers a constant-power load and I needed
> a constant-current load,
Yeah right- well the circuit in the article becomes your outer control
loop that takes over from your additional current control loop. The
point is that it finds the voltage threshold for whatever battery
voltage is present corresponding to your maximum power limit, and then
uses that to clamp your current control voltage. So the overall circuit
regulates current up to the maximum power threshold and switches to
constant power mode.
> but with a "wattage warning" to alert me if I
> was about to start running a load that would exceed the power
> dissipation limits of the device/heat sink/fan combination I have.
Since Power = I * V then Log(P)=Log(I)+Log(V) or Log(I)=Log(P)-Log(V).
This means that a Log-Log plot of I vs V is a straight line of slope -1
and intercept Log(P) for V=1Volt. So use the printer to make a simple
chart which is pasted and varnished on the front panel. This simple
technique seems to have worked for time immemorial for normal people. It
would look something like this:
View in a fixed-width font such as
| \ <--- 50Watt Line
I + *
|1 10 * 100
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