# What is the purpose of diodes in flip-dot displays?

In most flip-dot displays i've seen, the electromagnets have a set of two diodes connected to them. This is also mentioned by David, who asked a question about the displays before.

However, in the answers no one seemed to be sure why the diodes are put into the circuit.

simulate this circuit – Schematic created using CircuitLab

As shown in the schematic, each row has two diodes connected, one row has all anodes in common (Row nL) and the other row all the cathodes (Row nH).

Does anyone know why these diodes are put in the circuit as shown?

Possibly it has something to do with the "freewheeling" path, when the column or row voltage is switched off. Another idea is that by the addition of the diodes, the display can be changed per row/column instead of flipping each pixel individually. I can think of a circuit to flip a complete row at once, but usually the displays switch in a horizontal manner (the pixels flip starting from the left and moving to the right), and I don't see a way of making a simple circuit to switch the display per complete column.

• I didn't even know about these. But the Wiki does provide a nice physical picture and I can see where the magnet is slightly canted over so that one pole is a little closer than the other. Which makes me think that the reason is being able to reverse the voltage polarity across each underlying electromagnet coil in order to repel the permanent magnet in the flip 'thing' and force it over the other way without having to have a full H-bridge to do it.
– jonk
Commented Oct 24, 2016 at 20:22
• somewhat related: electronics.stackexchange.com/questions/125544/… Commented Oct 24, 2016 at 22:11

If we redraw your circuit without the diodes, we have this:

simulate this circuit – Schematic created using CircuitLab

Now imagine you drive Column-1 High and Row-1 Low (and Column-2 and Row-2 are Hi-Z) because you're trying to activate L1 in one direction.
You'll find that you'll also have current flowing through L2, L4 & L3 since the 3 of them in series are connected in parallel with L1.

The same circuit, drawn a little differently, but keeping all of the connections the same could look like this:

simulate this circuit

and now its much easier to see how this would work.

So, the diodes are there to isolate the coils form each other to prevent this situation and allow each coil to be driven independently.

• Thank you! That is a clear explanation. Interesting how redrawing the circuit can give so much insight. I think this is the answer, but I'll see what other people want to say (for now). You get my upvote already! Great explanation! Commented Oct 24, 2016 at 20:35
• If one drove active rows at full VDD, inactive rows at 1/3 VDD, active columns at 0V, and inactive columns at 2/3 VDD, then selected elements would receive full VDD while inactive elements would receive 1/3 VDD. I don't know how easy or hard it would be to adjust VDD so that active elements would consistently switch, and inactive elements would consistently not switch, but I would think it would likely be doable. Commented Oct 25, 2016 at 1:11