Question on running a relay

Hiya Guy's
I'm on making up a parrallel port based cicuit board for my CNC project and when it comes to relays well I admit I'm pretty green as I haven't used them in the past. Below is a schematic of a transistor emitter follower to power the relay from a parrallel port pin. Now the relays I managed to get very cheap are NAiS ST1-DC48V SPDT relays that in my opnion would run fine of a 12 volt dc source after looking at the datasheet. But my main question is those relay's have a resistance of 9K so would this effect the those resistors and by the way I'll use a BC548 as they are compatible with the C8050 transistor. As I'm not sure I'm asking as I don't want to go and make big stuff up's and like I said I'm totally green with relay's.

Cheers Bryan

i don't see and expect any problem in the ckt , but i doubt about the 9k resistance of the coil.

bryan1 said:

Below is a schematic of a transistor emitter follower

Click to expand...

Small point, but that's NOT an emitter follower! - it's a common emitter circuit, a completely different thing!.

To 'akg', a quick google soon find the datasheet, which gives the coil resistance as 9000 ohms, the 24V version is 2400 ohms.

My only concern is operating the relay at only 25% of it's rated value, the contacts might not have very good pressure on them!.

As Nigel stated, I would have concerns about using the relay at 12 volts. Don't confuse drop-out voltage with pick-up voltage. Most relays will "hold" the contacts in at a much lower voltage than required to overcome the mechanical inertia to pull them in. Once the armature is near the coil pole the required EMF is much lower ( by the law of squares I think ) than it is at a distance.

The drop out voltage is given so that in the case of a slower decaying coil voltage source, a time to operate can be calculated.

Bryan,
Your relays are designed for a 48V coil voltage. The datasheet spec's a minimum coil voltage of 38.4V to make the relay activate.

Your LED is shown backwards and won't be very bright with only 4.5mA or less through the 2.2k resistor.

Bryan,
When I was doing relay circuit design, we used a rule of thumb that the coil current has to be 1.67 times the operate current. {We used current rather than voltage since the magnetic field is proportional to current, so the design tables we used specified the operate and release currents}

So I deduce that the operate current of your relay is about 3.2 mA. (48/9/1.67)

At 12 Volt, the current will be about 1.33 mA and therefore the relay won't operate.

What voltage does the parallel port provide?

Virtually any transistor can be used in this circuit. All you need to do is to ensure there is enough base current to saturate it. The rule of thumb is that the base current should be about 1/10 th of the collector current.

Hiya Guys,
Thanks for the reply's and I'll have to lash out and buy some 12 volt relays. I sorta supected those 48 relays would be a problem and after reading the reply's I shouldn't be a scrooge and use what I have laying around. Eh Audioguru tht clip of the circuit I posted came straight off the Oatley parrallel port driver circuit board. The reason I'm making my own is firstly the cost and I thought I had every part laying around. Oh well time to visit the ripoff mobs again I reckon.

Thanks for all the input Guy's


Cheers Bryan

Bryan,
One point that we all missed in our replies is that the diode across the relay coil could be a diode such as a 1N4148, the 1N4004 is an over kill.

Hiya Len,
As I mentioned in my last post that schematic was copied straight off the schematic of the Oatley board. On talking with my friend this morning I'm redesigning the board to use opto isolators and the relays will be used on the driver boards not the parrallel board. This will mean I should be able to use a simple breakout box on the computer side and just run wires out to the machine.

Cheers Bryan

I'm doing roughly the same thing, my son wants to be able to control party lights from a computer.
I think a good first stage is to write a software midi instrument who's notes are lights controlled by solid state relays (less noise than mechanical) controlled by a parallel or USB I/O board.
For hardware, Jaycar have a couple of useful kits: One gives direct USB to analog and digital input and output; $AU70. The other is controlled by a parallel port and provides relays for the 8 outputs; $AU60. I don't know i I'd run 240Volts through them. I wanted to hack a powerboard to include solid-state-relays to control each socket separately. Since I'm Mac based I'll use a USB - Parallel adapter $AU39.