multi chan relay driver

[Christopher]

all

Might seem like a dumb question, but i have designed in a uln2803 on a test board to control 10 off 12v 250ma relays spread between two devices.

Initially the idea sounded great, less board space and less for me to solder!

Thinking about it in greater detail 2v ce drop is a lot which may not turn the relay on and will heat up the package


Any simple solutions? Yeah I could quite easily what on 10 sot223 fets and 10 kickback diodes etc but that is a lot of routing and soldering for a quick project. This will be controlled from a 3v mcu


Maybe I'm in dreamland again

[sleemanj]

Any simple solutions?

Use relays with a lower coil voltage and current.  12v 250mA seems quite high.

[Christopher]

These high current ac contractors are already specced into my design. There is a chance all 10 will be on so there is a lot of current I am trying to switch.

Maybe individual mosfet is the way to go

[David_AVD]

I would be going with separate FETs.

[TonyStewart]

ULN2803 Vce(sat)@250mA  = 1.2 typ, 1.4 max @25C  Pd=IcVce=0.3W typ 0.35W max. X8=2.4Wtyp

Except Vce(sat) drops 150mV as T rises from 25 to 85C and reduces Pd by 0.15Vx0.25A x8=0.3W

DIP spec=0.76W max @85C

However if you only use 4 inputs with 4 outputs paired with 2 IC with built in clamp diode.

A better solution would use lower current coils at higher voltage eg 48V


But discrete SMD FETs with RdsOn <1 ohm are cheap with 250mA clamp diode on coil, is preferred.

[SeanB]

Split the load between 2 devices, and drive 5 from each device. Keep the 3 per package as spares, or use them to drive status LED's on the board.

[Christopher]

Redesigned the board with 10xIRLL011 logic level fets.

[Rudolfo]

Special relay driver can support up to 1A and reduce power be 50%, like the iC-GE (http://www.ichaus.de/product/iC-GE and http://www.ichaus.de/wp1_mcu_interface ).

[ludzinc]

These high current ac contractors are already specced into my design. There is a chance all 10 will be on so there is a lot of current I am trying to switch.

Maybe individual mosfet is the way to go

<Humour Mode>
How much an hour do you pay your contractors? 
</Humour Mode>

Contactors are power hungry beasts.  You need to switch the contacts in a contactor *hard* (so they don't bounce, burn out , especially when switching inductive loads).

Typically you control an external relay, that then switches the contactor. 

Is this an option for you?

[Jeroen3]

Although not the same price, what about BTS5210G or similar high side power switches?