Dear colleagues,

I've a design issue I'm sure you can clarify me.

The input power of my circuit is between 15V and 12V. The point is that I have around 12 relays that work with 12V, so I need a stable source of 12V from the (12V, 15V) power supply. I'm not sure if a low drop-out voltage regulator with a 12V output is a good option since maybe sometimes I'll have 12V in Vin...

Anyone can help me with a solution from my design?

Thanks in advance.

Best wishes!

 

If all you are driving are 12V relays then you don't need to regulate 12-15V input.

 

Are you using a transformer with a secondary output of 15vac, or a dc supply between 12-15v,? whichever you're using it needs to be able to give out the full current with all relays activated.

 

Hello,

Thanks for your answers. I'm using a DC power supply comming from a solar panel.

The thing is that I wanna use the relay as a swich to allow the voltage comming from the solar panel to pass or not. The point now is that I need to stabilise this voltage, wich I need to fix near 12V, but the problem is voltage input from solar panel power source varies from 12V to 15V...

 

I wanna use the relay as a swich to allow the voltage comming from the solar panel to pass or not.

What is controlling the relay? If there is no circuit driving the relay, and you are relying on the relay coil to act as the discriminator between passing and not passing the panel power, note that the actual pull-in voltage of a "12 V relay" can be as low a 8 V. And once the relay has pulled in, the release voltage could be 6 V or less. Of course since we have absolutely zero information about your circuit or the components, all of this is useless guesswork.

To paraphrase Rear Admiral Joshua Painter in "The Hunt For Red October":

Engineers, son, don't take a dump without a schematic.

ak

 

And why, exactly, do you need to stabilize the voltage?
As noted, you don't need to do that for the relays.

 

Hello,

Thanks for your answers. I'm using a DC power supply comming from a solar panel.

The thing is that I wanna use the relay as a swich to allow the voltage comming from the solar panel to pass or not. The point now is that I need to stabilise this voltage, wich I need to fix near 12V, but the problem is voltage input from solar panel power source varies from 12V to 15V...

Where is the solar panel output power going ans why do you need full physical isolation of them form the circuit they are providing power to?

 

Dear colleagues, sorry for the lack of information, AnalogKid is right about that.

In the pic attached you can see (in a raw way) my first idea to control a water pump that needs 12V to operate. In a first approach, I decided to use these two transistors to control 12V. Here I have two issues:

- 12V comes from a solar panel that is very irregular, producing an output from 12V to 15V. This is the voltage I want to stabilize, since the pump does not allow more than 12.5V.

- The other thing is that a colleague has told me better to use a relay as a switch better than the PMOS you can see in the SCH... I think a relay is better use to drive AC, but I'm not sure which option is better.

 

With only 3v difference to play with to create 12v, its going to be a low drop out regulator, would your pump run on a constant current circuit?

 

What is the pump current? If it is not large, and there is some room for a heatsink, then the P-channel MOSFET can be both switch and LDO pass element. After all, a P-channel or PNP is the pass element in most IC LDOs anyway. The advantage to growing an LDO from scratch is a much better L. That is, a much lower low drop out voltage. For example, if the pump draws 2 A and the panel is making 15 V, that's 6 W in the pass transistor, something a moderate heatsink can handle without a fan. With a 10 A or 20 A MOSFET, the voltage drop across it at 2 A can be only 40 *millivolts*. That's way better than any purchased LDO IC.

1 - opamp, preferably with common mode input voltage range down to its negative rail
1 - voltage reference, something like 2.5 v
1 - P-channel power MOSFET, with Rdson below 50 milliohms
Rs and Cs

The opamp+FET is configured as a non-inverting DC amp with a fixed gain of 4.8. Since the MOSFET is operating common-source (inverting), this effectively swaps the opamp's inverting and non-inverting inputs.

The uC output is configured as open-drain, and shorts the reference voltage to GND to turn off the output.

ak

 

I think you may be concerning yourself over nothing being I doubt that pump motor is that sensitive to voltage as to even need any kind of over voltage protection at 15 volts.

Your typical vehicle's electrical system is rated at 12 volts and all components in it are rated for 12 volts yet in reality they operate at 14 -- 15 volts all the time.

 

Relays have pull in and drop out voltages as specs. They are not that sensitive to overvoltages.

Time to make and release time may figure in the calculations too.

 

I'm missing a lot of information to give a reasonable answer, for example:

As others have asked, so I am amazed that a 12 volt pump maximum must be supplied with 12.5 Volt.

What is the minimum pump voltage? and what is the consumption of the pump, and preferably also a graph of the relationship between supply voltage and current consumption?

And I also missing data on the the solar panel? - so you can compare with the pump data for whether there actually is a problem.

 

Thank you for the rich information you offer.

The pump current is constant and is about 2A (for transitory peaks), never more. So, according to your words, PMOS should deal with it without any problem. After @AnalogKid suggestion, I've gone through attached file, your advice is very interesting, thank you so much.

I've phone the pumps's owner and he's gonna send me the whole information about voltage tolerances.

And just to clarify concepts, in which case you will use a relay as a switch instead of a FET? I guess in my case, it makes no sense to drive 12V @ 1-2A using a relay, right?

 

Attached file here, sorry...

 

Use DC-DC. For example TRACO company. There are other firms. Is the input voltage 9-18V. And the output voltage of 12V.