Voltage dependent switching relay

[SirSparks]

I have a photo voltaic system (12 volt 1000W max) with 120 volt shore supply and 120V inverter supply. Presently I have a manually switched DPDT relay which operates well but I would like to automate it to switch from battery bank (inverter) to shore supply below about 12.2 volts. As an electrician I am not brilliant at soldering electronic circuits so am thinking I could use an SSR with a series zener input to switch the DPDT relay automatically

Relay coil draw is less than one amp I am hoping that a series 12v Zener across the SSR input and fed with battery voltage with perhaps a voltage dividing pot also in series to make it all adjustable would work.??

I am not sure though that the 2 volts or so normally required to operate the SSR might already be present even when the voltage is below 12V

Any advice greatly appreciated

Thinking about this I am also concerned that the Zener once fired will simply cause the relay to latch or oscillate even if the voltage drops below 12.2V ?

 

[BrownOut]

I think your concerns are well founded. Looks like you're just gonna have to learn to solder.

 

[Grossel]

Hi. If subject is urgent, its better to just use the word "Urgent" rather than writing a copy of your last post. And you have to get used to the "Edit" button.
Well, enough about this.

You will find it very hard to get your relay to trigger at the right voltage level by using a zener to recuce voltage. It just isn't suitable for that. If you have a pot in parallell with the zener to compensate, you will probably be bugged by those things:

• The relay trip level will change as the relay's relatively angle to earth is changing (eg. flip relay upside-down)
• Even if you got the relay to trig exactly at 12.2 volts, you will not have control on when it releases at low voltage
• There is no such things as two relays that are completely identical. If you replace the relay you need to adjust it all over again to get it to flip at exact voltage level.
• When high current flows through the relay contacts, even lower voltage is required to allow the contacts to release.
• In the moment of reacing upper voltage level (just enough to the relay activates), if coil voltage fall, but not enough to the relay to release, you might have poor contact (ref. a 12V rating relay feeded with 9V won't keep the contactors very tight)

In your case, I would recomend a Schmitt-trigger. If properly designed, you will overcome all the problems I mentioned above. The relay coil will have only full voltage or no voltage.

Also, there is circuits (IC) that is designed to cut power on low voltage. Those are called "Low Dropout Regulators".

 

[SirSparks]

Thanks guys, you saved me a lot of heartache. I'll check in to your recommendations.

 

[Dragon Tamer]

You are on the right track, a zener would be a good way to go to generate a reference voltage but you will need a sensor circuit to check if the reference voltage is above or below the measured voltage. The easiest way to do this is with a comparator.

We will need to know if your voltage is AC or DC. If you are using AC then the circuit will be a little more complicated. If you are using DC, then it is fairly straight forwards.

 

[BrownOut]

Yo dude:

I have a photo voltaic system (12 volt 1000W max) with 120 volt shore supply and 120V inverter supply.... I would like to automate it to switch from battery bank (inverter) to shore supply below about 12.2 volts

 

[SirSparks]

DC input between 12v and 14.4V. DC output to energize existing 12V relay coil which switches between shore power (below 12.2V) and inverter power (above 12.2V). I will also need to incorporate a time delay or else battery voltage "pull down" will keep the circuit oscillating

If X/0 = Infinity does 0/X = Ford ?

 

[Dragon Tamer]

You could probably do this with nothing more than a few relays. All relays have a minimum drop out voltage that could be useful for switching sources. To prevent oscillations, you could have a second relay that is engaged by the first de-energizing, when the second relay is engaged, it prevents the first from resting.

And no 0/X = 0 this is proven

 

[BrownOut]

Your best bet is to use a simple comparator IC with hysterisis. Using hysterisis will prevent oscillations. We can show you a simple way to set this up, but then you have a few other issues to resolve: the compatator required DC voltage to operate. Where will that come from? If you power it from the photo-volatic, then it will slowly power down in the evening, and slowly power up in the dawn. You could use a simple wall transformer plugged into the boat's power. Typically, it would keep the circuit powered through switching.

 

[SirSparks]

Sounds good. I have 585 AH 12 volt (or nearly 6Kw) of batteries and in daylight a charge rate of up to 80 amps an hour (1Kw) from the PV's. Hence no problem with a few watts.

12.2V is about 50% DOD (depth of discharge) at that point any more discharge is detrimental to the battery bank in the long term. So I have three alternative charging methods; 1 amp float, 6 amp float and 75 amp 3 stage charging.

 

[BrownOut]

Hi, I haven't forgot about your question, just trying to find the time to put something together. Working two jobs doesn't leave my much time these days, but I'll try in the next few days.

 

[SirSparks]

Hi, I haven't forgot about your question, just trying to find the time to put something together. Working two jobs doesn't leave my much time these days, but I'll try in the next few days.

Thank you, I have been considering an old style car voltage regulator, or a PV controller I have which monitors battery voltage and will switch off battery loads when below 11.5V trouble is that 11.5V is too low I need 12.2V to protect my battery bank better. I thought maybe I could use some resistors across the battery input to the controller to bring it in line with what I need.

I have quite a few of these controllers if anyone is interested.

 

[Reloadron]

I may be off base with this suggestion but why not consider using a comparator like a LM311 circuit. Run a LM7805 off the battery side to get a stable 5 volts. Power the LM311 off that single supply 5 volts.

You want something to happen at 12.2 volts correct? So 12.2 volts / 3 = 4.066 volts so set the reference using a 10 K pot across the 5 volts for 4.07 volts. Take the battery voltage across a 3:1 voltage divider and use it as the signal in. When the battery voltage divided by 3 equals about 4.07 volts the actual battery voltage is 4.07 * 3 = 12.21 volts.

The circuit would draw literally tiny power as in mA (Don't know off hand) and the 7805 only needs 7 volts to operate. Incorporate some hysteresis in the comparator circuit as when the battery is unloaded (Battery to Shore Power) the battery voltage will creep up.

So at Battery of 12.2 volts shore power comes on, when you want to return (and how) to battery power? That scheme needs to be worked out. Again, I may be way off base with this line of thinking.

Ron

 

[BrownOut]

Here's essentially what you're looking at. The circuit requires a comparator and a few resistors. R6 will need to be a combination of a fixed resistor and a variable "trimmer" resistor to get the level set correctly. You'll need a steady source of power. You might use a wall plug in the 120V circuit, and with good bypass capacitor, around 10uF or so, it should work during switchover. To set the circuit up, you should use a variable voltage you can adjust to your desired trip point. To make one, just use a fixed DC voltage and a variable resistor connected across the voltage and connect the wiper to the circuit. Using a 10K variable resistor will ensure you won't have over current and blow it up. Oh BTW the actual comparator don't have to be the same. Use whatever will drive you actuator, I just grabbed one from the library. You'll probably need a transistor between the comparator and your relay, or whatever you're planning on using.

 

[Reloadron]

Attached is a variation of BrownOut's circuit that I made yesterday, It runs along the lines of what I mentioned in my post as to using a comparator. One big difference is I would likely power off a LM7805 regulator (not shown) to get a stable ref voltage. I also added some hysteresis. In my cartoon R1 and R2 would be replaced with a 10 K Ohm pot and adjusted for the output I mention above.

The output would eventually be used for switching between on board or shore based power. Everything would still need tweaked and peaked and designed around exactly what you want to happen and when.

Ron

 

[SirSparks]

Thanks guys, you have all given me good tips and plenty to think about.

 

[BrownOut]

I also added some hysteresis.
Ron

My circuit also has hysteresis, provided by R4, though much less than Ron's. Be careful about having too much. You're looking at a voltage that changes by around 5-8% or so from fully charged to half charged. Too much hysteresis will prevent the comparator from working properly.

 

[SirSparks]

I've ordered a DC milliammeter from Ebay to help. Not an electricians normal tool ! I do have another option I am going to try first as it is easier and cheaper hopefully only needing on pot and one resistor in addition to that PV controller I already have about 6 of.

The controller switches OFF a relay for loads (up to 15 amps) if voltage falls below 11.5V. I think I could add a simple voltage splitter to the battery input so that only 11.5 arrives from the battery when they are in fact 12.2V. Also they are free!

 

[BrownOut]

What you need is a resistor voltage divider. I'll try to post something from home later, unless someone else jumps in.

 

[SirSparks]

I'll need to know the input current (hence the mA meter) to size R and then I thought it would be simply one fixed R and one variable R in series with the slider going to the Battery input of the controller?

Remember I have a few of these if anyone is interested, just send a SAE (small jiffy bag);

Green Energies LLC 12V 15 Amp PV Controller.