Hi all,



I replaced an arrow display unit of an elevator but it was common cathode arrow display and the old one was common anode display so i need to invert the control signal to be able to control the new display unit properly.

*i tested the arrow display and measure the current which was about 50mA.

*the system voltage 24v

*I searched about not gate IC can support 24v,only found cmos ic's can support 18v.

Dose simple circuit using transistor and resistors like in schematic below enough to do what i need ?




Regards

 

Common cathode would refer to an LED display with multiple independent LEDs. I don’t know what you mean here, since there is only one thing to light, what is common?

Bob

 

If all you want to do is invert a signal and low current then yes, your drawing is a transistor not gate. Just about any common NPN switching transistor should work like a 2N2222.

Ron

 

I am guessing that there are both up and down arrows and one common terminal it may not be possible to access the connections inside to separate the connections.

 

It looks as though the arrow display is lit by five(?) individual LEDs? Do you mean those LEDs in the new display have their cathodes commoned?

 

I am guessing that there are both up and down arrows and one common terminal it may not be possible to access the connections inside to separate the connections.

Exactly..

The following figure show the system.


I tried the following circuit to control up arrow but it didnt light .

 

Does the circuit with the transistor represent the original interface, the part in the block between the controller and the display? Or is that a proposed solution?
A simple fix, if the controller will support it, will be to have each 5 volt output operate a relay to switch the 24 volt power. At that point the polarity can be whatever is needed, and be easily changed for the next replacement arrows.

 

No, it will not.

we need to know how much current is drawn. Then we need to use a PNP transistor or P-MOSFET selected based on the current.

Bob

 

The attached circuit tested as proposed solution to invert the 24v signal comes from the control board to meet the new unit.

I measured the current of each arrow and it was about 50mA

 

If the original was common anode, then the outputs for UP and DOWN are connected to ground to light the arrows. You do not have a 24V signal, the 24V would be constant. Can you verify this?

Bob

 

A quick calculation of your 2N3906 and 1KΩ circuit - you're drawing over half a watt when the light is intended to be off. And I'm assuming it's off more than it's on.

(24V - 0.6Vf) ÷ 1000 = 0.0236A
23.6mA X 24V = 566mW (0.566 watts)

How are you going to deal with that much heat on the transistor?

 

The circuit in post #6 cannot work because it can't supply the positive voltage needed. (it places a 1k resistor in series with the load)

If you switched to a PNP the problem is the output from the 5 volt controller must be able to handle 24 volts...or poof.

You may have to consider using a relay.

Or do your inversion after whatever that small block marked 24/ & 5/ is instead of replacing that unit as it seems in your diagrams.

 

After thinking about it a little more that small level shifting block between the controller and the display is likely to already be a couple of NPN transistors needed to drive the common anode display, so you need to drive some PNPs with the output from there.

You should raise the base resistor value and can probably eliminate the collector resistor. (no logic levels needed here)

 

I assume when you tested for the current in the display you connected 24 volts directly to the display and the current control is already contained in the display.

If that is the case...try this.