switching between two outputs - Page 1
Forum section: electronics / projects

author: mingasss
author_rank: Newbie
timestamp: March 22, 2015, 07:58:38 pm
content: Hi guys, I would really appreciate if someone could give me advice on how to build circuit for this case. On my circuit input I have slowly arising voltage from 0V to 10V, and on my circuit I have 2 outputs. When input has 0V-5V, first output has to be active, but when voltage rises from 5V-10V, first output has to deactivate and second output become active. And vice versa, when voltage decreasing to less than 5V, second output has to deactivate and first become active.5V is the point where output changes, depending on voltage.I was thinking to use 5V or 6V relay which gets active when voltage gets close to 5V. But relay coil will always use energy, which is not good in my case, as I need as much as possible voltage and current on outputs. Can you help to project switching between two outputs using transistors or comparators circuit? Thanks in advance.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 22, 2015, 08:32:22 pm
content: use/make/buy 5v voltage reference, put it along with the your Vinp to comparator, when input > 5V, comparator hi, comparator output brached to two path, one is yes logic one is not logic, yes logic goes to your output 2, not logic goes to your output 1, end of story. i'm sleepy i cant make you a schematic, you need to do that on your own.

author: electron_misfire
author_rank: Contributor
timestamp: March 23, 2015, 07:00:57 am
content: You could use a 5v Zener with two transistors: one turning the first output off and one turning the second on.Sent from my iPhone using Tapatalk

author: flynwill
author_rank: Regular Contributor
timestamp: March 23, 2015, 07:40:10 pm
content: Bunch of questions:How accurate does the "5 Volts" need to be?How fast must it react?How exactly do you want the circuit to behave when the input is very near the 5V decision point?--- Should there be hysteresis? (typically yes), and if so How much?--- When switching from output to another -- should it be guaranteed that both are NOT on at the same time?--- Or should it be guaranteed that there is a brief time when both ARE on?--- If there is such an overlap or non-overlap of the outputs how long should it be?When you say "outputs" what sort of outputs? (ie what are they driving/activating?)

author: mingasss
author_rank: Newbie
timestamp: March 25, 2015, 09:23:35 pm
content: Hi, thanks for your reply. To make more clear I have drawn a plan of my circuit and hosted this picture here: postimg.org/image/hr0xq8jthVoltage doesn't have to be very accurate, it can be from 4.5V to 5.3VI need it to react as fast as it can, so I won't loose voltage on outputWhen switching from one to another output, it has to be guaranteed that both are not on at the same timeNon-overlap can occur, but for very short time,( maybe for only 100ms)My picture should clarify a bit more about how I need to use it. If you have any ideas would you be able to draw a quick scheme?I appreciate your help, thanks

author: ABroodryk
author_rank: Newbie
timestamp: March 26, 2015, 12:06:13 am
content: If you can handle some contact bounce I would go with a normally closed single pole double throw relay and a op-amp with hysteresis and transistor/mosfet to drive the relay... it should guarantee that only one output is on at a time. Just remember the free-weeling diode across the relay coil as you don't want the magic smoke to escape from the mosfet/transistor.Or you can do it with with a complementary pair of transistors/mosfets but you will need to work in dead time between the two to ensure only one output is on at a time.

author: flynwill
author_rank: Regular Contributor
timestamp: March 26, 2015, 12:43:40 am
content: Is this boost converter something you already have, something you're planning to buy, or something you plan to design?If it is something you already have you should look close as it's specification sheet. It might already do what you need all by itself. If indeed it's input cannot go much higher than 5 Volts you will need to design carefully -- more on that in a bit.If it is something you plan to buy then I would look to see if there are any units that would effectively do what you want without additional circuitry. Would a buck-boost regulator that gave you 5 Volts with input anywhere from the 0.5 to 10V serve your needs? I'm pretty sure those exist.Finally if it is something you plan to design, then I would design the capability need into the coverter.But sticking with your drawn topology, your original idea of a relay that is energized when the source voltage reaches 5 Volts might be the best, it could be a "make before break" relay (meaning both outputs would be on at the same time briefly) but with your isolation diodes that should cause no harm. A comparator IC and voltage reference (which might just be a zener diode) and a mosfet, or bipolar transistor to drive the relay.My other thought would be to put a low-dropout regulator between the voltage source and the boost converter. Don't bother with switching other than the isolation diodes. When the input voltage rises above 5.7 volts, the direct path through the diode will effectively take over and your boost converter will keep running but be unloaded. Some Low-dropout regulators also have a "shutdown" input, so you could add a transistor or comparator to use that connection to shutdown the boost converter.

author: flynwill
author_rank: Regular Contributor
timestamp: March 26, 2015, 01:31:39 am
content: On second thought most LDOs are going to shutdown entirely well above your hoped for minimum of 0.9 Volts in.I would suggest a depletion mode mosfet for the pass element (between the input an the boost converter) and a circuit that pulls it's gate to ground once the input voltage gets above 5 volts. But you may have to do some searching to find a device that is sufficiently on for your needs with Vgs = 0. (You didn't indicate how much current your converter needs).