Relay interfacing suggestions needed....

JAM2014 · Joined: 24 Apr 2014 Posts: 138

Hi All,

I'm designing a PIC-based multiplexer that will be part of an automated test station. I need to drive 32 relays, only one of which will be on at a time. In the interest of saving I/O pins, I'm planning to use a 74HCT154 multiplexer to generate the 32 drive signals for my relays. The max. source/sink for each I/O pin on the '154 is 25mA. The relays I've selected have 5V, 10mA coils with internal diodes. My question is whether it's advisable to drive the relays directly from the '154, or should I use a driver between the two? On the surface, it looks like the driver might be unnecessary? On the other hand, I don't want to design in a reliability problem....

74HCT154: http://www.ti.com/lit/ds/symlink/cd74hc154.pdf
Relay: http://www.digikey.com/product-detail/en/coto-technology/9007-05-01/306-1063-ND/301697

Thanks,

Jack

gjs_rsdi · Joined: 06 Feb 2006 Posts: 468 Location: Bali

I don't think you will have any problem if you drive one relay at a time.
I drive 10 ma relay with 74HC00 in the past, but usually I am using a BC337
Did you check the stock of the relay?
It is written:

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

But that page also says:

Ttelmah · Joined: 11 Mar 2010 Posts: 19338

Beware....

The output of the 74HCT154, is being quoted into TTL & CMOS loads. Not into the current needed by the relay. The relay needs 3.75v to turn on. It'll probably work, but may well give problems.

Consider something using less wires, and with a higher drive capability.

The MCP23018, would only need two lines from your PIC (I2C). You can set two chips to different addresses. Use the pins as _output low_ to drive the relays. The low drivers will pull down to only just on 0.7v at 10mA.
The pins wake as inputs. Program the output latches to 0, and then just set the one pin required at a time as an output.

Also look for the CAT9552. This is easier to drive, single command to switch an output off/on.

<http://www.futureelectronics.com/en/Search.aspx?dsNav=Ntk:PartNumberSearch|cat9552|1|,Ny:True,Nea:True>

JAM2014 · Joined: 24 Apr 2014 Posts: 138

Hi All,

Thanks for the replies!

It looks like the MCP23018 has the advantage as (1) it's in-stock at Digi-Key, and (2) the CAT9552 appears to be EOL, and not recommended for new designs......

Thanks much!

Jack

Ttelmah · Joined: 11 Mar 2010 Posts: 19338

It very much depended on what this is actually 'for'. For a limited number 'off', rather than production, the 9552, is really easy to drive. However there is a driver for the I/O port expander, so the extra is minor.

Ttelmah · Joined: 11 Mar 2010 Posts: 19338

As a further comment, have you considered something needing less power to drive it. Depends on what voltage you actually want to switch, but I'm using a lot of these:

<http://www.mouser.co.uk/ProductDetail/Panasonic-Industrial-Devices/AQY282S/?qs=sEN%2fkO1EG6bFR4N8wKfhAA%3d%3d>

Great for switching things like 24v signals, require only 3mA drive. Single and dual versions available (284) in SMD or DIL (dual is 8pin DIL for example). Controls AC and DC (twin FET's in opposite directions). They do versions rated up to 400v (but current supported drops). The 500mA version (282), can handle 1.5A peak!....

temtronic · Posted: Wed Jun 15, 2016 5:48 am

these ..
MOSFET Transistor N-Channel 30V/4A AO3402
.. might work as well, or something similar...lots of FETs out these days.
Local supplier has them for 25 cents each...'normal' 3 pin TO-92.
In the good old days I'd use 2 resistors and a 2N3904......

Another possible option might be to use a bigger PIC ? IF an I/O pin can operate the relay OK there wouldn't be a power dissipation problem either (1 on at a time).

Jay

dyeatman · Joined: 06 Sep 2003 Posts: 1923 Location: Norman, OK

For a good FET relay driver ic to drive multiple outputs with a single package
I use the ULN2803 lot since it even includes the snubbers and is only 86 cents each.
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Ttelmah · Joined: 11 Mar 2010 Posts: 19338

The point was though that he didn't want a driver. He wants to interface a lot of small relays, using a latch or similar output directly, to give a minimum chip count, and minimum PIC lines.

He has a tiny relay that draws just 10mA, and using pull down, this is comfortably drivable from the I/O port expander directly. However for even less current he could use the little SSR, especially since the prices of these now are getting as cheap or cheaper than the mechanical relay, and would use less board space. Smile

temtronic · Posted: Wed Jun 15, 2016 8:17 am

Though he doesn't say what the signal source is (DC, AC, RF), he has chosen a reed relay, either for low current or maybe high isolation or ?? Reed relays, especially the gold plated ones have super reliable contacts (glass sealed !). It could be that SSRs are not an option.
I'd still try driving them direct from a PIC as the 500r coil spec is OK, though the 'must operate' spec (3.75) might be 'tight'. Still worth 5 minutes to breadboard and test. In the past I did operate a Form C version of a reed relay driven from 4000 chips with zero problems. Heck if I get time I'll grab a PIC, relay and cut some 'toggle_relay' code...

Jay

Ttelmah · Joined: 11 Mar 2010 Posts: 19338

Except he wants to use minimum PIC pins.

I've already posted that he can drive this fine off the MCP23018, using this as a pull-down driver. Two of these on just two PIC pins.

The SSR, was just an alternative to use slightly less power. Supports DC & AC, and would use less board space. Might well be suitable.