Circuit Design Help

[Townie]

Hi
Im trying to work out a way to have the battery meter read the voltage when the charger is plugged in, regardless of power switch.
The Dc socket is wired so it disconnects the V- when the charger is plugged in to prevent the amplifier drawing current from the charger atm.
I have a 6 pin DIP 150mA opto solid state relay and a High/Low trigger relay board.
I thought of a pull down resistor on the Dc socket switch terminal and trigger the relay from that but that would just power on the amp, if the power switch is on.

Here is the circuit.
https://drive.google.com/file/d/1Q7YPJXcwTm54s2l_BgacQlmE-DTVCRoj/view?usp=sharing

[Lesolee]

I assume that putting the battery meter in the position shown is unacceptable because it is on all the time ?

[Townie]

Correct

[Lesolee]

Just to clarify:
(1) the battery is connected to V+ and V-  ?
(2) the ‘charger’ does not charge the battery  ?
(3) the redrawn circuit below correctly represents the current design (apart from the meter positions)  ?

This being the case we can’t put the meter in the M1 position because it is ON all the time.

Is it acceptable to use low drop Schottky diodes in series with the battery or the charger? This may give more freedom to move things around.

[Lesolee]

The first try uses two meters to show the concept:


then the second uses an extra changeover pole to switch the meter.


(1) The charger socket is not "live" to the battery so you can stick wire into it and not blow up the battery
(2) The relay is not powered from the battery so there is no additional current drain from the battery

[Townie]

Thinking like this.
The dc socket is the charger for the battery.
V+ and V- are connected to the battery
Would this work?

[Townie]

I think I need to remove R3

[Townie]

I need to change the gate resiator values too.
Datasheet says typical trigger voltage 0.5V 50uA current. Battery full charge voltage is 25.2V.
What resistors values would I need ?

[Townie]

Actually I definitely need to change those gate resistor to the scr's too. I'm trying to work them out but there is a trigger current and a holding current which has me confused. The transistor also needs to be saturated as well I think as its functioning as a switch correct?

Here is the datasheet for the scr https://www.mouser.com/datasheet/2/302/BT169D-527397.pdf

Im planning just using a BJ547 for the transistor.

[Lesolee]

It is difficult for us to look at the circuit as drawn. The wiring is done as a wiring diagram, rather than a circuit diagram (like mine), so currents run around in circles rather than top to bottom.

Anyway, as a general consideration, an SCR is not a resistive device and will not act as a zero-volt-drop switch (ever). A bipolar transistor is in the same category. The volt drop across it is always non-zero so you won't measure the battery voltage accurately.

A FET on the other hand is resistive so the volt drop can be negligibly small at low currents. I used relay contacts because they are zero-volt-drop and require no power to keep the contacts closed.

So, if I have this right the spec is:
(1)   The meter measures the voltage when the ON/OFF switch is on
(2)   The meter also measures the charger voltage when the charger is plugged in
(3)   The charger needs to be directly connected across the battery (without a diode in the way).
(4)   The amplifier must not be allowed to take power when the charger is plugged in.

It would be nice if the DC-DC converter could have an enable control to shut if off, rather than having to put a switch in series with it.

We have to use the charger switch, somehow, since it is the only way to know if the charger is plugged in. Of course this doesn't mean that it is actually charging because somebody could plug in the charger socket but not plug the charger into the mains!


Talking about basics, it seems to me that your circuits do not charge the battery.

[Lesolee]

This one should be closer to your needs.