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I'm in over my head with a project and need some help. I'm trying to use an RCX output port to control a 5VDC SPDT Relay from Radio Shack (part #275-240A). I need to limit the voltage of the port to 5 volts. How can I accomplish this? Is a voltage (...) (25 years ago, 27-Sep-99, to lugnet.robotics)
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| | RE: 5VDC Voltage Regulation
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(...) The RCX puts out a variety of waveforms depending on the power level. If you have acecss to an oscilloscope, you can see that the higher power levels give you more "on time" that lower levels. The DC level at full on is about a diode drop (...) (25 years ago, 27-Sep-99, to lugnet.robotics)
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| | RE: 5VDC Voltage Regulation
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You need to limit the current through the relay. Just add a resistor in series with the relay. You can experiment, using a voltmeter, or you can calculate the proper value from the relay specs. Relays specs usually list the resistance of the coil. (...) (25 years ago, 27-Sep-99, to lugnet.robotics)
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| | Re: 5VDC Voltage Regulation
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Ralph Hempel <rhempel@bmts.com> wrote in message news:000001bf0901$f5...@pro150... (...) I would just put a 33Ohm 1/2W resistor in series with the relay coil. According to the spec for the relay, the coil wants 5V at 90ma. The RCX outputs about 8V (...) (25 years ago, 27-Sep-99, to lugnet.robotics)
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| | RE: 5VDC Voltage Regulation
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(...) Leave it to Michael to actually go through the math and put the right answer up for the group to see - nice work. Cheers, Ralph Hempel - P.Eng ---...--- Outside of a dog, a book is a man's best friend. Indside of a dog, it's too dark to (...) (25 years ago, 27-Sep-99, to lugnet.robotics)
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| | Re: 5VDC Voltage Regulation
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(...) This doesn't seem to work. I am trying this first since it is the simplest solution so far and I have a 330hm resistor at hand. Without the resistor the voltage across the coil with he port set to level 7 is 7.17. When I add the resistor in (...) (25 years ago, 27-Sep-99, to lugnet.robotics)
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| | Re: 5VDC Voltage Regulation
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Michael Gasperi wrote: =>> =>> I would just put a 33Ohm 1/2W resistor in series with the relay coil. ^^^^^ Wes Matchett <wes@augustschell.com> wrote: =>... =>This doesn't seem to work. I am trying this first since it is the simplest =>solution so (...) (25 years ago, 27-Sep-99, to lugnet.robotics)
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| | Re: 5VDC Voltage Regulation
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Bingo - thanks, I guess I read that a little too fast. -Wes (...) (25 years ago, 27-Sep-99, to lugnet.robotics)
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| | RE: 5VDC Voltage Regulation
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(...) Ummmm, I think Michael meant to say 33 Ohm resistor. That's what his calculation says and like soooo many others, I just looked at the answer without checking the math... Cheers, Ralph Hempel - P.Eng ---...--- Outside of a dog, a book is a (...) (25 years ago, 27-Sep-99, to lugnet.robotics)
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| | Re: 5VDC Voltage Regulation
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This indeed does work fine, with he proper resistor. :) Voltage across the coil is now 4.74VDC. Thanks to all for the input - when I get a working model I'll put it on my web site. -Wes (...) (25 years ago, 27-Sep-99, to lugnet.robotics)
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| | Re: 5VDC Voltage Regulation
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Don't forget to apply a reverse biased diode across the relay. If you forget this the coil will generate large voltage spikes when the power is removed. This may hurt your RCX. (25 years ago, 27-Sep-99, to lugnet.robotics)
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| | Re: 5VDC Voltage Regulation
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(...) Rubbish. The same happens for motors, and the RCX handles this. (25 years ago, 27-Sep-99, to lugnet.robotics)
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| | RE: 5VDC Voltage Regulation
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Man, don't sugar coat it .... Say what you really mean. (...) (25 years ago, 27-Sep-99, to lugnet.robotics)
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| | Re: 5VDC Voltage Regulation
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(...) OK. What I was thinking of was that if a current flows through a coil, that be a relay, transformer or motor, the current can't change at an instant. If you break the circuit current will keep running through the coil for a short time. This (...) (25 years ago, 28-Sep-99, to lugnet.robotics)
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| | Re: 5VDC Voltage Regulation
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Jacob Schultz <dungeonmaster@get2net.dk> wrote in message news:FIrMLo.H8K@lugnet.com... (...) A "freewheeling" diode is a good idea generally, but the RCX must already have diodes built into the driver IC. It has to turn on and off the motor (...) (25 years ago, 28-Sep-99, to lugnet.robotics)
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| | Re: 5VDC Voltage Regulation
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(...) You can't have the diodes on the motor because it goes in both directions. If you put a diode out there, you would have a dead short in one of the directions. Such things need to be built into the H-bridge, not the motor leads. So, I seriously (...) (25 years ago, 28-Sep-99, to lugnet.robotics)
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