[SOLVED] Looking for a circuit which can reverse polarity using just one control input line

1) I use a low power device which will switch (and stay) into one state when 3-12V is applied to it and switch (and stay) into another state if 3-12V is applied to it in reverse polarity.

2) I have one control output which will be able to supply 0V (off) or either 1.5/5.5V (on).

Now I need to control that device (1) with the control input (2). The device (1) switches instantly if power is applied to it, so it doesn't matter whether or not the circuit I am looking for will just supply power for a short time when the control (2) is applied.

I found out that DPDT Relays or H-Bridges tend to go into the direction I require, but they need two control inputs in order to work.

So I am looking for something that requires just one control input to work and I prefer this circuit to be an IC.

Any pointers?

You haven't said how much current this circuit has to produce.

If it is just a control signal, you could consider taking the output from the Q and -Q outputs of a flip-flop then using your control signal to change it's state.

Brian.

Please consider me a newcomer to electronics. I know how resistors and capacitors work and that I can switch things with a transistor / mosfet / relay. Everything more complex is not part of my knowledge (yet).

betwixt said:

You haven't said how much current this circuit has to produce.

Click to expand...

The current in play is neglegible: if you look up ebay item 261942451810 you will see what exactly needs to be switched - it's an IR cut filter.

betwixt said:

If it is just a control signal, you could consider taking the output from the Q and -Q outputs of a flip-flop then using your control signal to change it's state.

Click to expand...

What would be "not just a control signal" then? My control signal is just "voltage on" vs "voltage off". What is this flip-flop you talk about labeled? Would this be a bistable flip-flop?
Also: An H-Bridge would be overkill for my application then?

KlausST said:

Why does a DPDT relay need two control inputs.
It is either active or inactive...so one control line.

Click to expand...

Ok so this works similar to the flip-flop which betwixt refers to? I might have misunderstood the information I read about it then.

KlausST said:

Similar with a full bridge:
One control signal, one inverter and a full bridge...

Click to expand...

Since I am new to this field, I don't really understand what you mean. Are you refering to using a full bridge PLUS an inverter?

Hi,

I don't understand:
Why does a DPDT relay need two control inputs.
It is either active or inactive...so one control line.

Similar with a full bridge:
One control signal, one inverter and a full bridge...

Klaus

- - - Updated - - -

Hi,

SPDT:
I don´t know what it it has to do with a flip flop.
It has two COM pins. Just connect one side that at the COM is HIGH when relay is not activated and LOW when activated.
And the other side that at the COM is HIGH when relay is activated and LOW when not activated.

Place your load between both COM pins.

******

For low currents you may use ready to buy "phase reversal analog switches" like MAX4527.


Klaus

The MAX4527 looks promising. I will order it and mark this thread as solved when it works.

Unfortunately, I can not see that Ebay item, the number isn't recognized, even on the ebay.de site. Please post the full page address so we can see it in our own browsers.

The reason for asking about the current is that in many cases, the reversal of voltage is to make a motor rotate one direction or the other and the amount of current depends on the motor size and load.

Brian.

betwixt said:

Unfortunately, I can not see that Ebay item, the number isn't recognized, even on the ebay.de site. Please post the full page address so we can see it in our own browsers.

Click to expand...

I didn't know if I am allowed to post **broken link removed**. But strange, I find the item if I put the number I mentioned above into the search box.

I can see it now using the link you gave. It is similar to filter mechanisms used in astronomy. If I understand it's 'pendulum' control, the filter is free to pivot and has to be held in the 'open' or 'closed' position by passing current through the two wires. In other words, it doesn't return to a home position if the current is removed, it just becomes free to move again.

I can not see any specification for the current it needs either on the ebay site or other sites selling the same module. Can you do a test please, using a testmeter, measure the resistance across the two wires, it will probably be in the region of 50 to 150 Ohms. From that I can calculate the current it needs and then the kind of driver circuit you have to use.

Brian.

It doesn't appear to move when no current is applied and I shake it a little, but maybe I just didn't shake it hard enough (and I won't try to). Anyways, I measured 44.8 Ohms.

OK, so the current needed, if I assume a 5V supply, is 5/44.8 Amps = 110mA.

The least component solution is to use a bridge driver IC such as the SN75441 but the cheapest solution uses five small transistors and 6 resistors. The transistor circuit is still quite simple and cost ~2 Euro, the IC will also need one transistor and two resistors, cost ~8 Euros.

Personally, I would use the transistor version. I visualize a simple H-bridge of four transistors and one as an inverter. It would have an input that changed the output polarity when the voltage changed from zero to supply voltage. For example, from a 5V supply, the output would swap 0V - 5V to 5V - 0V and stay there as long as the control voltage was present.

Sound reasonable?

Brian.

That sounds reasonable, yes. But what about the MAX4527 - wouldn't this work, too? I already ordered it yesterday and wonder what difference it would make in contrast to your suggestions. If I understand its properties correctly, I don't need any external components with that IC, no?

It has the correct function but it can't handle the current you need. It is designed as a signal switch carrying small voltages rather than a relatively high current switch.
If you look at page 4 of the data sheet, find the graph titled "ON-RESISTANCE vs. VA, VB (SINGLE SUPPLY)" and look at the curve for the supply voltage you want to use, then look at the lower scale for the same voltage and read the intersection resistance on the left scale. For example for a 5V supply, the intersection is at 300 Ohms which limits the current to I=V/R = 5/300 = 0.017 Amps (17mA) which is far too low.

You could use a higher voltage (makes the switch more efficient) but that then changes the problem the heat dissipation in the IC. The power it dissipates is W=(V*V)/R where R is the resistance given by the graph and W is the power loss in Watts. You should keep the figure as small as possible but the current needed to operate the pendulum mechanism will push it beyond it's limits and make it run hot (if at all).

Brian.

I have both 5V and 12V available in my circuit. So if I used 12V and extrapolate its curve for "ON-RESISTANCE vs. VA, VB (SINGLE SUPPLY)", I propably end up in the ranges of 170 Ohms. That would allow for a current of 0.07 Amps, which would still be a little too low, or am I wrong?
Heat would be a problem though, since I don't have an efficient cooling policy in place. What kind of temperature difference are we talking about here?
If the MAX4527 is not feasable in the end, isn't there a similar IC which can do the same for higher currents? When I looked for "phase reversal analog switches" like KlausST suggested, I came up with pretty much the MAX4526/MAX4527 being the only one of its kind.
So if cost is not a factor, but using as little components as possible, and given the information that I can use both 5V and 12V, would you still advise on using the SN75441? If so, which transistor/resistors would I need and how would I set them up?

I think Klaus based the design on the assumtion that the current would be much lower, it was before you told us the resistance. You can try the MAX device but I suspect it won't work properly, it is more for routing signals, audio, video etc rather than controlling a small motor mechanism.

I'm about to head out to a meeting so I can't produce a schematic for you right now but what I visualize is two PNP and two NPN transistors in a bridge configuration and one additional NPN transistor working as an inverter. Basically, the mechanism sits across the middle of the bridge and in one mode the top left and bottom right transistors conduct, in the othe rmode, the top right and bottom left transistor conduct. The middle of the bridge therefore sees the polarity reverse. I'll draw a diagram when I get back tomorrow.

Brian.

Hi,

Any DPDT switch with low enough resistance can do the job (OK, it should be "break-before-make")

There are lots of switches. Look at a distributor: mouser, farnell, digi-key, RS....


Klaus

betwixt said:

I think Klaus based the design on the assumtion that the current would be much lower, it was before you told us the resistance. You can try the MAX device but I suspect it won't work properly, it is more for routing signals, audio, video etc rather than controlling a small motor mechanism.

Click to expand...

ok, so I will opt for another solution then.

I'm about to head out to a meeting so I can't produce a schematic for you right now but what I visualize is two PNP and two NPN transistors in a bridge configuration and one additional NPN transistor working as an inverter. Basically, the mechanism sits across the middle of the bridge and in one mode the top left and bottom right transistors conduct, in the othe rmode, the top right and bottom left transistor conduct. The middle of the bridge therefore sees the polarity reverse. I'll draw a diagram when I get back tomorrow.[/QUOTE]

ok, thank you!

KlausST said:

Any DPDT switch with low enough resistance can do the job (OK, it should be "break-before-make")

Click to expand...

So would this one work?

Hi,

So would this one work?

Click to expand...

It's a DPDT...so it should work.

Klaus

Sorry, meeting plus travel ran to 27 hours and my head is still spinning!

The relay will work provided you have 12V control signal and cross wire the two poles correctly. It certainly wont work on 3V although it may be possible to find lower voltage relays.

Brian.

No problem! My control signal would be around 6V or 1.5V, so this is not feasable for me. Could you point me to one at farnell which is able to switch either 5V or 12V and which allows to be controlled at either 6V or 1.5V (on) and ~0V (off) with one control signal. I am at a loss here.
Alternatively, I could use your transistor version - but using a single component would be easier of course.

Follow this lllloooonnnggggg link. You might have to copy and paste it in to your browser, it's all one line but Edaboard might split it.

https://uk.farnell.com/webapp/wcs/s...2,111783587,111783594,112181938,112184204&vw=

It lists all the suitable ones but really, all you want is one of the smaller box ones with pins (like code 1629053). You control it by switching the voltage across the coil pins. All those relays are DPDT types (double pole, double throw) which means the contain two identical change-over switches. What you have to do is connect your mechanism between the two 'common' pins (that's the side of the switch that gets moved from Normally Open to Normally Closed) then link NO one one switch to NC on the other. You apply the supply to the NO and NC of one of the relays. Basically, the two switches operate simulateously when you apply power to the coil but they are wired so they reverse the voltage because their contacts are cross-wired.

There is one other point: I strongly suggest you wire a diode, I suggest 1N4001 through 1N4007, any will work, with it's cathode end (the end with the band around it) to whichever end of the coil you apply the positive voltage to. The coil itself will work either way around, it's the diode and applied voltage that are important. The diode will help by eliminating voltage spikes as the coil de-energises when you switch it off.

Brian.