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Author Topic: Spindle On Off Setup No 5VDC at relay.  (Read 2425 times)

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Spindle On Off Setup No 5VDC at relay.
« on: April 27, 2011, 12:15:07 PM »
I am trying to set up Spindle on/off.  I have Setup output #1 to pin
16 and Spindle on/off to output #1.  Checked voltage on Pin 16 and GND
and I get 5VDC.  But when I hook up my relay to to it, the voltage
drops down to 0.24VDC. I am using a MDFLY breakout board (generic board) and the relay
has a nominal coil current of 89.3mA. and coil resistance of 56ohms. I
tested relay on computer 5V power supply and it works fine.  Do I need
a different relay?  Do  you know what the problem might be?
Re: Spindle On Off Setup No 5VDC at relay.
« Reply #1 on: April 27, 2011, 01:01:18 PM »
Could be you need a smaller relay, any idea what the BOB's rated output current is?

Try some other loads on it like an LED/resistor, filament bulb, etc and see if it fires them up ok. also, does the relay work with a plain 5v supply to it?

I used some darlington pair optoisolators on my machine, these provide 5kv isolation and will drive a small relay very well. Worth considering. A phone charger makes a lovely packaged 5v supply.

Edit: is it one of these? http://www.mdfly.com/index.php?main_page=product_info&products_id=87

If so you will have only the power available from the parallel board to drive your relay, and it probably won't be enough. most parallel boards only really supply logic level currents.

http://campbelldesigns.net/Combo-board.php - This sort of thing may be more to the point since it is designed to drive relays. Plenty of options avalable, and most of them feature optoisolators which will stop your computer going to glory in the case of your wiring having a fry-up.
« Last Edit: April 27, 2011, 01:13:08 PM by BluePinnacle »

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Re: Spindle On Off Setup No 5VDC at relay.
« Reply #2 on: April 27, 2011, 01:20:39 PM »
You need a driver circuit of some kind to drive the relay. The 5 volt that you are measuring is the voltage from the computer LPT port and that only sources a couple of milliamps and is not enough to drive a relay on it's own.

You could use an opto isolator, the output side would need a separate 5 volt supply to power the relay.
You could use a TTL gate, again a 5 volt supply would be necessary.
You could use a different BoB which already has the relay and drive circuit.

There are many options.

Tweakie. 
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: Spindle On Off Setup No 5VDC at relay.
« Reply #3 on: April 27, 2011, 01:39:48 PM »
you could use an npn transistor with a limiting resistor to sink the relay to ground.
Re: Spindle On Off Setup No 5VDC at relay.
« Reply #4 on: April 27, 2011, 02:09:48 PM »
Many thanks to both of you.  I will try one of the solutions and let you know.