Different voltage rails connected via a resistor? One derived from the other?

[Chris935]

Attached is a section of the schematic for a Peavey 5150-II guitar amplifier, this part is the channel/effect switching circuitry. It seems to show a 24V rail connected to a 22V rail with only a 47R resistor in between. This doesn't seem to make sense as the 24V isn't connencted to any of the components that aren't also connected to the 22V, so if these were separate PSU rails I can't see any reason for the 24V to be there at all.

Is the 22V derived from the 24V? Do the directions of the arrows indicate this, or have any other significance? If so, how is this voltage being derived?

The relays have a resistance of 720R so all sub-circuits shown have around 1.5K to ground, either a 1.5K resistor, an 820R + 720R relay, or  two 720R relays. The LEDs pass around 13-14mA. I predicted this based on a 22V supply minus the LED forward voltage divided by the 1.5K and confirmed this when I measured it. I notice that the maths works out similarly when using a 24V supply and including the 47R in the calculations...

Is the "22V rail" a virtual construct to make it easier to know what voltages to expect when measuring (virtual in the sense that it would become 24V if disconnected from ground whereas the 24V exists at the PSU)? Does the 47R behave as R1 in a voltage divider with the rest of the circuirty making up R2 and the 22V being Vout?


Thanks,

Chris

[fcb]

I would suggest that the 22V derived from the 24V, and that the 2V difference indicates that the 22V circuitry consumes approx. 42mA (24v-22v/47R).  Also backed up by the 1000uF cap.

[Brumby]

I'd be interested in seeing where that 22V line goes ... but the circuit seems fairly straightforward.

The 47R resistor, together with the 1000uF capacitor provides some supply decoupling.  Basically, the 24V supply would have a little too much ripple on it, so to smooth that out for a sensitive part of the circuit, a 1000uF capacitor is fitted.  However, if that capacitor is connected directly to the 24V supply, then the ripple on that supply can still find its way into the circuit.

By adding the 47R resistor, the voltage across the resistor can vary with the ripple, allowing the capacitor to maintain a much more constant voltage.

Please note that this approach only really works well when the total current drawn by the working circuit (in this case, whatever is connected to the 22V) does not vary too much.

* Edit  The larger this resistor is, the smoother the 22V supply gets ... but it is also much more sensitive to variations in current draw.  Just a little more current and the 22V will start dropping.

[IanMacdonald]

Likely the 22v is a nominal meter reading for faultfinding purposes rather than an accurate voltage. It's basically the 24v supply, decoupled.

[Neganur]

The 22V is definitely derived by the 24V. It's the cap+resistor in the centre of the PCB with the relays and tubes to the right of it:

http://tetsuoelectronics.files.wordpress.com/2010/12/dscf0509.jpg

(Can't attach, too high resolution)

[Chris935]

Thanks, that's more or less what I'd suspected. The three lines exiting the bottom left of the crop go to switches that are used to select various functions by connecting the lines to ground and allowing current to flow (or not) through the relays/transistors and LEDs. There are also transistors in place which can be turned off to remove the ground connections, handing over control to the footswitch jack (switches must be on on the device AND in the footswitch).

Is the 22V arrow pointing right (towards the 24V) a standard way to indicate that one voltage is derived from the other? Is the different positioning of the "+" symbol before or after important?

Chris

[Brumby]

Is the 22V arrow pointing right (towards the 24V) a standard way to indicate that one voltage is derived from the other?

No.

The arrows are simply an indication of a connection to some other point of the overall circuit.

Is the different positioning of the "+" symbol before or after important?

Not that I am aware of.  Just somebody being inconsistent.