I currently have a matrix orbital LK202-25 USB display I need to drive a relay off the general purpose outputs on this device. They output 5v 20ohms according to the manual. http://www.matrixorbital.ca/manuals/LK_series/LK202-25-USB/LK202-25-USB.pdf But by removing the resistor you can use higher resistance components I have a 5v 500ohm relay which in turn will control my 12v relay (due to this switching 240v!) Now the output comes out at 4.8v but the relay isnt switching. I really need some input on this, im know next to nothing about electronics but can solder pretty well and put things together. Any ideas on how I can get this to power a relay from the source? Im trying to drive this relay http://www.maplin.co.uk/Module.aspx?ModuleNo=2605 JH12N http://www.matrixorbital.ca/manuals/LK_series/LK202-25-USB/LK202-25-USB.pdf
Looking at the Mo datasheet the outputs aren't 20Ohm they're 20mA, 240ohm which is why your relay isn't switching, there isn't enough current. Use a MOSFET to driven by the MO to switch the relay. Take the Mos output to the gate of the mosfet, the drain of the mosfet to one side of the relays coil to 12v. The Drain of the mosfet goes to ground, you will probably also need to put a high value resistor (somewhere between 1k and 10k) between the MO output and ground. Try to find a mosfet with a low Vth (threshold voltage) or a logic optimised mosfet, mosfets won't turn on fully unless the gate voltage is above the threshold voltage. Moriquendi
Sorry if this sounds stupid but I don't really know what I should be looking for here. http://www.maplin.co.uk/Module.aspx?ModuleNo=33843&C=SO&U=strat15 Am I along the right lines?
If the digital output is 5v, surely the Vth won't need to be anything special? A 2N7000 should be fine, I would have thought. Also, don't forget a diode in reverse parallel with the relay's coil to prevent the back-EMF of switching damaging anything.
http://forums.bit-tech.net/showthread.php?t=171774 just seen this thread that I posted last year?? not that I remember this at all! Seems fairly straight forward? What resistor would I need before the transistor then? Also could how could I implment this into 5v relay, but I only have a 12v power supply. Is there any simple/cheap way to downconvert 12v to 5v with just enough 'juice' to power two of these relays? Thanks for all your help
As capnPedro says the 2N7000 should be fine. That is a logic optimised or at least low Vth mosfet I think, some higher power mosfets that I have seen have a Vth of more like 4-5v and the closer your gate voltage is to the Vth the higher the Rds so more energy gets dissipated as heat => fried 'fets. That shouldn't be a problem in this case though. Also as Pedro says, the anti-parallel diode is important, I shouldn't have left it out. Moriquendi Edit: replace the BJT (Bipolar junction transistor) in that diagram with a Mosfet and you don't need a resistor at all. Mosfets are much simpler than BJTs to use in situations like this.
What mosfet would I need? Also will this still work if my matrix orbital is being powered from a seperate power source to that of the power source to the relay?
The 2N7000 should work fine as Pedro suggested, it's shown on the maplin page you linked to. It will still work provided the grounds of the two supplies are connected. Moriquendi
