VDD mixing idea...

[vttom]

I have a PICAXE project which interfaces 2 different pieces of equipment.

The equipment on either side provides a 5V VDD in addition to input and output signals.

I want the PICAXE to be operational if (1) only 1 of the 2 interfaces is connected and powered-up, if (2) both are connected but only 1 is powered up, or (3) when both are connected and both are powered up.

I can't simply short the VDDs together, because in case (2) above, the powered side would drive power back into the unpowered side.

I could use diodes in a "T" configuration, but I don't want to incur a diode drop on the VDD seen by the PICAXE.

So... attached is my idea for making this happen (each emitter comes from the VDD of a different interface; the "dotted" collectors drive the VDD of the PICAXE).

Does anyone see any gotchyas with this?

 

[BeanieBots]

Do you mean besides the saturation volt-drop being close to that of a diode?
How about using schottky diodes? <0.3v drop. (<0.1v depending on current).

 

[hippy]

I could use diodes in a "T" configuration, but I don't want to incur a diode drop on the VDD seen by the PICAXE.

You could have two relays driven by each side's VDD and some clever routing so when either is unpowered it delivers power from the other side, just from one side when both are powered. A reasonable cap should keep the PICAXE going when the relays switch.

Another trick is to jack-up the power rails on each side by Vdiode, feed Vdd on each side from those elevated Vdd's via diodes, and also diode mix to the PICAXE in the middle. That way all Vdd's will be at the same voltage ( give or take the minor discrepancies on either side elevated Vdd's ).

Perhaps if you could explain why there's such a potential problem (*) with the diode voltage drop there might be alternative solutions available.

(*) is that the first really bad pun of 2009 ?

 

[sghioto]

vttom,

The transistor circuit should work OK with the proper bias resistors. Using a 2N3906 with a 47K resistor the voltage loss was only .05 volt at 5 ma and about .01 volt at 5 ma using a 4.7K resistor.

Steve G.

 

[Ralpht]

What's the difference between a diode drop and the "diode" drop you will get when using the transistors as per your drawing.
Six of one, half a dozen of the other. Give or take minor differences.

Whats the problem with a diode drop anyway? You can do as per Hippy's suggestion and up your VDD a bit to compensate - or just live with the diode drop. The Picaxe will, with no problem.

Either way, your cct should work

 

[vttom]

What's the difference between a diode drop and the "diode" drop you will get when using the transistors as per your drawing.

Look carefully. Those are PNP transistors. They should be able to pull up the collector voltage effectively all the way to the emitter voltage.

My concern, which is mainly why I posted, is whether or not I have to worry about putting a positive collect to emitter voltage across one of the PNPs (which is what will happen in the both-connected/one-powered scenario).

As for why I didn't want a diode drop... I do have other active components besides the PICAXE which are only rated to run @ 4.5 < VDD < 5.5. A typical diode-drop of 0.4-0.6V comes a little too close to putting me "under water".

 

[sghioto]

vttom,

In the scenario of one ON and both connected there is some leakage voltage on the emitter of the other transistor in the schematic as drawn. How much depends on the value of the base resistors and the resistance or load to ground on that side of the circuit. If you put a switch from each base resistor to ground the problem is eliminated.
Another possibility is using two TS5A3159A analog switches.
http://focus.ti.com/lit/ds/symlink/ts5a3159a.pdf

Steve G.

 

[Ralpht]

vttom.

Look carefully. Those are PNP transistors. 
They should be able to pull up the collector voltage effectively all the way to the emitter voltage.

PNP or whatever....

Assuming your transistor(s) are saturated there still will be a voltage loss between the collector and emitter. If you have good tranny's and you didn't specify which ones you are going to use, the loss may be as low as 0.3 of a volt. Could be higher ..... sounds like a diode drop to me !

Transistor emitter base and collector base junctions are often described as and effectively are 'diodes'. Irrespective of wheather they are NPN or PNP.

 

[BeanieBots]

Try it. Measure the "volt-drop".
Then try with a high current schottky diode and measure the volt-drop.
Then please let us know what you observed.

 

[vttom]

Well, I took hippy's suggestion and came up with the attached.

In this case, I have a SPDT relay. One of the VDDs (VDD1 in the drawing) is on the Normally Closed (NC) side, and the other VDD (VDD2 in the drawing) is on the Normally Open (NO) side. The VDD that poweres the PICAXE is on the common terminal.

Since VDD2 also controls the coil, if VDD2 is connected and powered up, it will close the NO side of the relay and power the PICAXE. Otherwise, VDD1 will power the PICAXE.

As long as the relay is of the "break before make" variety, then you don't have to worry about shorting VDD1 to VDD2.

The only drawback is the case where both sides are connected and powered up, and you remove or power down VDD2. In this case, the coil will pull current out of the PICAXE VDD rail until the relay contact switches, causing a momentary droop in the voltage, followed by a spike. Hopefully a capacitor will be able to smooth this out enough to not affect any of the active components.