Electronics Switching Design Problem (Please Help)

[hshah8970]

Hello everybody!

I'm working on a project and am facing a problem I simply cannot solve. I'm unable to design the electronic circuit that accomplishes the following task:

I want to couple 2 DC inputs in such a way that when Input 2 reaches certain voltage level (from zero), both inputs become connected in series. Before this threshold voltage level is reached by Input 2, we simply get Input 1 at the output. Check out the attached image to see what I mean.

For voltage values of DC Input 2 higher than the threshold voltage, the series combination is maintained.

I would highly appreciate it if somebody helped me out with this! Thanks! And an explanation to go along the circuit would be nice too. Again, thanks a lot to anybody who responds!

 

[daddles]

I've attached a sketch using a single pole double throw relay that shows something that might work (note I don't bother with switching the - line, as it's the same in both pictures). You'd use a comparator and possibly a transistor to switch the relay. More details about your DC inputs would be needed though. Can they really be connected as you suggest? Also, what happens if the comparison voltage stays right at the switching level? You can have a situation where the relay chatters (switches rapidly) and this isn't good for the relay, obviously, not to mention that it might not be good for the DC inputs. So, adding hysteresis might be needed. Also, the real world comes with noise, so we'd want to know more about the comparator's voltage input. So, can you give us more details?

 

[nepow]

Can you use relays... and use a comparitor circuit to switch them?

 

[hshah8970]

@daddles: Thanks for replying! I saw your sketch and it makes immense sense to me. Unfortunately this would be my first experience with relays and some help in that matter would be highly appreciated. How do I make DC Input 2 the control for the relay? Let's use some sample values to simplify my question. DC Input 1 will be 12V. The threshold voltage of DC Input 2 would be 5V. Once this 5V is reached and our inputs are connected in series, the voltage level DC Input 2 would not border on 5V - it will only increase. I hope this answers the 'chatter' question.

While providing me with your answer and the idea to control the relay through a comparator, keep in mind that any circuit we use will have to utilize only these 2 power supplies. Even for the Vcc of any opamps used, we have to come up with a design to utilize our given inputs i.e. DC Input 1 and DC Input 2.

@nepow: Thank you for replying to my post! My project does not have restriction on the type of equipment I use. So yes, I can use relays and a comparator circuit for switching. The question however is how they should be connected with the DC inputs.

Thanks to all who reply!