[TenTec] Rsc value for LM723 in 280/255 power supplies?

[Mike Bryce]

pete,

the .68 ohm resistor does in fact monitor current, but not the total current from the supply.

the value is not critical in this application. I've used various low ohm resistors with no adverse effect.

however, if you were to take the output side of the .68 and tie it to the Vsense line, then you'd have a niffy little power supply and in this configuration, the value of .68 would in fact set the current for this application.

to compute the value for the current limiting resistor, use this formula

Ilimit = Vsense/Rsc


the way Ten Tec has it wired, the .68 monitors base current to the two pass transistors. but the current limit of the supply is set by the combined values of R3, R4, R5, and R6; all .15 ohms. as in most linear supplies that use several pass transistors, these resistors also balance the load to the transistors.

The engineer that designed this supply took a rather unique direction as some circuits of the model 280 are differently odd when compared to a standard 723 power supply.

mike, wb8vge


On Jun 2, 2012, at 5:07 PM, Peter Bertini wrote:

> I hope I am replying to the current thread correctly; since I don't see a
> reply feature when reading the archives.
> 
> I'm not concerned about the damage, I am rebuilding the supply with all new
> parts including the semiconductors and old electrolytic caps. I'm aware of
> the need for current limiting for the TenTec PAs for low impedance-load SWR
> protection.
> 
> But, my question should have been a bit more specific: I was curious  how
> the value for Rsc was calculated, and what is this being used for in this
> particular power supply? The 0.62 ohm resistor is between pins two and
> three on the LM-723-- they are the current limit and current sense lines
> for the LM-723. It almost looks like they are providing some method of
> foldback protection for the supply if the pass transistor base current
> exceeds a certain level?? I can't make much sense of the app note info for
> the value of Rsc. I'm curious as to what Rsc actually does.
> 
> Pete
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