Controlling a lot of relays with shift registers

Using Arduino General Electronics
#1

Hy everyone.

I would like to ask you guys what do you thing about my circuit below? Would it works like I say there below?

To describe everything a little...

I have to make an application that controls 24 reed relays, more exactly those are 12V/10mA/500Ohm reed relays (DIP05-1A72-12L). I will use shift register TPIC6B595 to extend digital outputs with my Arduino Nano. The shematic of application is like at the picture below:

At the picture above only 4 relays are shown I have to control 24 reed relays. Also there should be some kickback diodes I know but there are not in the schematic above.

So Is there anything I should be afraid of by doing application above? Is there any advice for anybody of there is something missing or I didn't do correctly?

Tny very very much for answers!

#2

Andraz23:
Also there should be some kickback diodes I know but there are not in the schematic..

Then how are we supposed to give feedback when half the schematic is missing? To make the most out of your question, post a full schematic.

#3

AFAIK TPIC659 have inbuild open drain mosfet outputs, so no external mosfets needed.
A diode across the relay is not even needed, because they also have inbuild clamping circuits.
Check datasheets for voltages/currents of the different models.

I have just designed an 8-relay board with the TPIC6C596 for battery cell measurement. Chainable.
Parts/boards ordered. Maybe I’ll post pictures when it’s build.
Leo…

Flying Cap.png

#4

In fact, the circuit above won't work with open drain because the MOSFET inputs will float. They would need pull up resistors. But you wouldn't need that with a shift register with normal logic outputs.

#5

You don't need the MOSFETs, the TPIC6C595 or 6B595 will work fine by themselves.
For example, this board driving 8 relays directly:
http://www.crossroadsfencing.com/BobuinoRev17/

1077×655 107 KB

Video of 3 daisychained

And controlling strings of 120V LED lights:

Shield versions:

2174×1721 618 KB

1064×486 101 KB

(last one actually controlled by Mega's D22-D29)

#6

Thank very much for great freedback...

Wawa:
A diode across the relay is not even needed, because they also have inbuild clamping circuits.

Here is one problem that I see with that. Yes the TPIC6C595 has inbuld clamping circuits, but if you look at datasheet there you can se that the ''Drain-to-source breakdown voltage'' is only 50 V . More exactly there you can se like shown in the picutre bellow:

I measured the voltage them mosfets furns off and there was about V_DS = 250 V . So I think that external diode across the relay coil is needed. Am I wrong?

CrossRoads:
You don't need the MOSFETs, the TPIC6C595 or 6B595 will work fine by themselves.

Thanks, I see that now, so new shematic should look like this:

There are just 4 relays above, but there will be 24 of them.Will this works or maybe I don't se something? Something missing?

Tanks again for great support!!

#7

Andraz23:
Thank very much for great freedback...

Here is one problem that I see with that. Yes the TPIC6C595 has inbuld clamping circuits, but if you look at datasheet there you can se that the ''Drain-to-source breakdown voltage'' is only 50 V . More exactly there you can se like shown in the picutre bellow:

The breakdown is the clamp. You don't need the diodes.

#8

aarg:
The breakdown is the clamp. You don't need the diodes.

Even if the reverse voltage at the coil comes up to 250 V, so the 250 V ? Isn't that to much. I also measured V_DS voltage with DSO and get V_DS=250 V...

#9

If the coil voltage rises above 50volt (33volt for the TPIC6C596), the internal mosfets start to conduct, preventing a further voltage increase.
Look at the "output circuit" diagram in the datasheet.
There is a zener diode between drain and gate of the output mosfet.
A clamping circuit like this provides a slightly faster "relay off" than a diode across the relay.
Leo..

#10