# dumb voltage question

Discussion in 'General Electronics Chat' started by shortbus, Mar 5, 2015.

1. ### shortbus Thread Starter AAC Fanatic!

Sep 30, 2009
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OK, you guy's know I ask dumb questions, so here's my latest. If you have two separate voltage sources, A and B that are joined at common/ground, and at positive in a diode "or" situation what is the voltage coming out of the "or" to the rest of the circuit? Example -

Source A, +10VDC, to diode A, to circuit

Source B, +20VDC to diode B, to same circuit

Sources A and B negative pole tied together as common ground

What would the voltage be going into the circuit?

2. ### wayneh Expert

Sep 9, 2010
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+20VDC less the diode drop

Jul 18, 2013
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+1, the 10v diode will most likely be in a position to be reverse biased by the 20v supply.
Max.

4. ### wayneh Expert

Sep 9, 2010
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And Source A might as well not be there, until Source B falls below 10V.

5. ### ErnieM AAC Fanatic!

Apr 24, 2011
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The highest voltage always wins.

6. ### WBahn Moderator

Mar 31, 2012
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Things are SO much better with a diagram, even just something quick in Paint.

Is this what you have in mind?

Assuming diodes with a forward voltage of Vd, then if the left diode is conducting

Vba = Vd

while if the right diode is conducting

Vca = Vd

We can have Vba or Vca be less than Vd, that just means the diode is cutoff. But we can't have either of them be greater than Vd otherwise the diode is smoked.

Since we know that

Vba = Vb - Va = 10V - Va

and

Vca = Vc - Va = 20V - Va

we can combine these to get

Va = 10V - Vba = 20V - Vca

and this gives us

Vca = Vba + 10V

This tells us that Vca always has move forward voltage across it that Vba, which means that if Vca = Vd then Vba = 10V+Vd which would smoke it. Since we know that the highest diode voltage is Vd, that must therefore be Vca. Thus we know that

Vca = Vd

and

Vba = Vca - 10V = Vd - 10V

Either of these is enough to give us the voltage at A.

Va = Vb - Vba = 10V - (Vd - 10V) = 20V - Vd

or

Va = Vc - Vca = 20V - Vd

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7. ### MikeML AAC Fanatic!

Oct 2, 2009
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Diodes shown are only rated for 1A continuous.
Schottky diodes will have a lower forward drop.

Last edited: Mar 5, 2015

Jul 18, 2013
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One practical use where this is used for multiplexing multiple switches etc into a single input often known as Steering diodes.

Max.

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9. ### shortbus Thread Starter AAC Fanatic!

Sep 30, 2009
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Thanks guys. one reason I asked is from a thread over at ETO where they were talking about "stacking voltages". A single supply voltage that needed a periodical short time higher voltage, and two supplies were used together to get the higher voltage. Guess I should have asked over there how they did it, and will. Just seem to get better answers here.

Jul 18, 2013
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Stacking one supply on another implies something different than both using the same common reference point.
Max.

Last edited: Mar 6, 2015
11. ### shortbus Thread Starter AAC Fanatic!

Sep 30, 2009
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Why? They would/do need a common somewhere in the circuit, or they are just two circuits. Again this is for my EDM power supply. It needs ~100VDC continuous with a periodic spike up to ~170VDC. So 100VDC with a stacked 70VDC periodic.

So how would the extra voltage be "stacked" on the backround voltage?

12. ### WBahn Moderator

Mar 31, 2012
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You have a 100VDC source in series with a 70V pulsed source. Schematically, they would (could) appear to be stacked on top of each other, hence the name.

13. ### shortbus Thread Starter AAC Fanatic!

Sep 30, 2009
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Not trying to be confrontational, but to understand this.

In my first post I used lower voltages as an example, and was told it wouldn't work. So if my example was wrong how would it be done? Doesn't the need to be some diode involved to keep the voltages separated when the 70VDC is not pulsed on? The pulse only needs to be for ~2 or 3 microseconds.

Jul 18, 2013
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To me stacking supplies means one on top of the other, IOW the zero ref for one is connected to the plus of the other, one on top of the other.
Whereas you mention in your OP that they share the same common, which is not the same thing.
In general for two voltages sharing the same common, the higher one will reverse bias the second.
Max.

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15. ### WBahn Moderator

Mar 31, 2012
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But in your first post you didn't mention that they were stacked. In fact, just the opposite in that you specifically stated that they shared a common ground. You REALLY need to provide some kind of a drawing of what you have in mind.

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