Sounds like you have bought the perfect relay: when it is powered by a 24v source, it cuts in the 24v source. As the source is depleted to 20v, the relay cuts out to the 12v source.

Nothing else is needed.

Unless you insist on sticking to the 16v cut-out vs. 20v cut-out.

Hi I tried to use the relay as you suggested. At 12.6 volts the coil energizes. Could I put a resistor in series with the coil to increase when the coil energizes

 

Not to worry: Here is a circuit that will work with your new relay and your 20K trimpot:

View attachment 91838

Things you should note. With the values of R1, R2, and U1 shown, it's CutOut voltage is adjustable from ~30V to ~14V. Its CutIn voltage is about 1V higher. With the pot centered, the CutOut is ~20.3V, and the CutIn is ~21.3V, as shown on the green trace on the plot.

The 1V of hysteresis is very important. As the load discharges the battery bank, the voltage is dropping slowly (red trace). If it were not for the hysteresis, the relay would disconnect the load, the battery voltage would rise due the load going away, and the relay would pullin again immediately, and the relay would cycle on/off rapidly. With 1V of hysteresis (as controlled by R3), the battery voltage would have to rise by more than 1V for the relay to be pulled in.

You might want more hysteresis than the 1V I show here. If so drop R3 to ~1Meg.

Hi Mike

Asking for help again,

I have attached my circuit diagram based on the assistance you gave me.

The relay does not energise I have adjusted the Trim pot in both directions.

The Voltage on Pin 1 of U4 is between 0.5V and 0.7 Volts.

When I adjust RV1 on the one side of R10 I have 17.5 which is equal to the supply voltage and 6.21 Volts on the other side.

On resistor R9 on the junction of RV1 and R9 the voltage The voltage does not change its stays on 0.6Volts

 

Thank you for this.

If the coil resistance is not 320R can I adjust R4 so that the total resistance is 470R?

The resistor R1 is not available in Zimbabwe, Can I use a Preset pot or a multi turn trim pot instead and set it to the correct resistance?

The TL431 has a 100mA maximum rating and is also restricted by IxV=W, if neither of those limits were exceeded, I'd leave out R4.

The TL431 switches on when it see's 2.5V on its Vin pin. You could probably use anything from 2k2 to 10k for R2 and a 100k preset pot for R1 - then you can set the switching threshold just where you want it.

The TL431 has a pretty sharp knee curve, so that circuit should be OK.

 

Hi I tried to use the relay as you suggested. At 12.6 volts the coil energizes. Could I put a resistor in series with the coil to increase when the coil energizes

Using a zener to drop a fixed amount of voltage might make the switching point a little more precise.

But you have to watch the zener dissipation and make sure its rated accordingly.

 

...
I have attached my circuit diagram based on the assistance you gave me.

The relay does not energise I have adjusted the Trim pot in both directions.

The Voltage on Pin 1 of U4 is between 0.5V and 0.7 Volts.

That is wrong. Just as the relay is poised to pull-in, the voltage at the REF pin of the TL431 should be increasing through 2.495V!

I suspect you have the TL431 miswired. Download the actual data sheet for the part you actually bought, and verify the pin order yet again.

I labeled all of the relevant circuit nodes, and reran the sim at two fixed voltages, once at a battery voltage of 24V (when the relay is pulled-in), and again at 21V (where the relay is dropped-out).

Here are all of the node voltages for the two cases:



Compare them to the voltages in your circuit...

 

That is wrong. Just as the relay is poised to pull-in, the voltage at the REF pin of the TL431 should be increasing through 2.495V!

I suspect you have the TL431 miswired. Download the actual data sheet for the part you actually bought, and verify the pin order yet again.

...

There is an application example in some of the TL431 datasheets for an under/over voltage detector - it uses 2x TL431, but should be dead easy to adapt for this use.

The TL431 is available from a lot of sources, but most of them publish nearly identical datasheets - but not all the same ones. There's about 2 or 3 versions of the datasheet, and I'm not certain all of them show the app example I mentioned.

 

That is wrong. Just as the relay is poised to pull-in, the voltage at the REF pin of the TL431 should be increasing through 2.495V!

I suspect you have the TL431 miswired. Download the actual data sheet for the part you actually bought, and verify the pin order yet again.

I labeled all of the relevant circuit nodes, and reran the sim at two fixed voltages, once at a battery voltage of 24V (when the relay is pulled-in), and again at 21V (where the relay is dropped-out).

Here are all of the node voltages for the two cases:

View attachment 93969 View attachment 93970

Hi Mike

Thank you for the help.

I firstly had used a 50K Multi turn trimpot. I didn't have 20K as they were incorrectly supplied, so I used a 10K and changed R1 to 50K.

The other mistake I made on the PCB. I had miss labeled the trimpot and so the centre pin was not going to pin 1 of the TL431. it was actually going to ground.

Having rectified this the circuit works fine now. Thank you very much for your help

Best wishes

Rod



Compare them to the voltages in your circuit...