Can 2 voltages be used on the same PCB?

Using Arduino Project Guidance
#1

I am using Arduino Uno to receive the pulses from a GPS receiver, both connected to 12V. After every 60 pulses (seconds) a 12V pulse is sent to one of two relays with contacts connected to 24Volt. These then pulse a 24V coil in a mechanical memory. Relay 2 has the 24V + Ground inverted to give me polarized pulsing.

If I disconnect the 24V leads to the 24V coil then all runs smoothly. The 12V relay coils click normally. If I connect the 24V leads then the GPS receiver turns off for a minute or two after pulsing, I think it is after pulsing as the pulse is only 200ms and not easy to judge the time lapse.

My question at this stage is: Is it back EMF or is it not possible to power the board with two voltages and a common ground. I have diodes in what I judge to be the correct places.

#2

Two voltage with common ground is totally fine.

Post the schematic.

#3

A schematic of how you have things wired would be very helpful. Yes, anything with coils in them tends to produce strange pulses, but you may also be running into an issue with the power supply not being able to provide the power for everything you have hooked up. Hard to tell without seeing a decent schematic and some info on what the components and power supply are.

#4

Common grounds can have voltage differentials due to high currents passing through them. That is why a "star ground" configuration is preferred in such cases (all reference grounds connect to the same physical location).

#5

Relays K1 and K2 clicks alternatively every 60 seconds to give me a polarized pulse on T1.
Turning the circuit on and off kills the GPS receiver for a minute or more.
Where do I put Diode D5 and D6 to prevent back EMF?
Is that the only reason why the GPS dies for a period?
If I disconnect T1 (no coil connected) then the circuit runs perfectly.

I attach the schematic as requested.

I really need your help with this one.

DipTrace Schematic - Parow Polarized Schematic.pdf (21.7 KB)

#6

That schematic is hard to read, but it looks to me as though you have the transistors connected in an emitter follower configuration.

The voltage on the emitter of the transistor (to which you have the relay coil connected) is going to be around 0.7V below the voltage on the base, you also have a diode in series with the base resistor, so the voltage on the relay coil is likely to be below 4V when the Arduino output is high.

Do your relays really operate at 4V?.

#7

That schematic is hard to read, but it looks to me as though you have the transistors connected in an emitter follower configuration.

The voltage on the emitter of the transistor (to which you have the relay coil connected) is going to be around 0.7V below the voltage on the base, you also have a diode in series with the base resistor, so the voltage on the relay coil is likely to be below 4V when the Arduino output is high.

Do your relays really operate at 4V?.

Thanks for your reply JohnLincoln.

What can I do to improve readability of the Schematic? I would like to improve.

I browsed “emitter follower configuration”, but do not fully understand it. However, the two transistors get their pulse from Arduino Pin8 and 9, respectively. Each transistor then energizes one of two relays to give me a polarized pulse to the 24V coil of a mechanical memory of a public clock some 3 meters away. These pulses are 24V, although the coils of the relays are 12V. The incoming power of the circuit has a 1amp slow blow fuse.

The relay coils get 12V from a voltage regulator (not shown in my schematic sent earlier) to reduce the 24V (2 X 12V lead acid batteries in series) to 12V and the transistors connect the relay coils to Ground for the duration of the pulse supplied by Arduino Pin 8 and 9.

The relays have 12V coils, but I do not know what voltage they actually get since the pulse is too short for the digital volt meter. I assumed it is close to 12V and they have a real healthy click.

My concern is that I do not know where to connect diodes D5 and D6 which is shown in the schematic, but not fitted on the board. Are they really needed seeing the 24 volt circuit is totally isolated from the 12V circuit that has the delicate components? Only the contacts of the relays are connected to 24V and when they turn off, the Memory coil is totally isolated from all other components, unless there could be spikes created on ground (which is also disconnected), but I would not know how and where and how to handle that.

The puzzling part is that with the wires to the Memory coil on T1 disconnected the system runs for hours without any fault. The moment I connect even the coil of a 12V relay to T1 then the GPS receiver dies/stalls for a minute or more and then turns on again. It must be a safety built into the GPS receiver.

Any ideas why it does not like the 24V coil to be connected to T1?

I again attach the schematic.

DipTrace Schematic - Parow Polarized Schematic.pdf (21.7 KB)

#8

That schematic can't switch the relays at all. If you are actually getting the relays to click then you have not wired it according to the schematic.

The reason I think this is because the transistors are on the positive side of the relay coils. VBatt is 12V right? So when the transistor is 'on' and conducting, the main body of the transistor will be about 12V. The 5V Arduino can't control a transistor that is at 12V.

#9

MorganS:
That schematic can’t switch the relays at all. If you are actually getting the relays to click then you have not wired it according to the schematic.

The reason I think this is because the transistors are on the positive side of the relay coils. VBatt is 12V right? So when the transistor is ‘on’ and conducting, the main body of the transistor will be about 12V. The 5V Arduino can’t control a transistor that is at 12V.

That’s not how I read it. It looks to me like VIN (+Batt) goes to the transistor collectors. They are emitter followers, the emitter driving the relay coils. Pins 8 and 9 drive the transistors through the strange combination of series resistors R1, R2 and D2, D3. Thus the transistors will amplify the 5V from the output of the Arduino pins, less the junction drops of D2, D3 and the transistors. So the relays will have about 3.6V across the coils when they are energized.

It almost goes without saying that this is bad. It shows that the designer doesn’t understand transistors at all. Unless the coils are 3.6V coils, which I doubt. The transistors have to dissipate a lot of power, they will probably get warm or maybe even burn up.

#10