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5V REED relay question — Parallax Forums

5V REED relay question

Drud0616Drud0616 Posts: 26
edited 2004-12-01 13:42 in BASIC Stamp
Is it safe to connect a 5V Reed Relay to the BS2 pins? Just wondering because that what I am planning to use in order to power water pumps at 9V when the pin is high. Thank you in advance

Comments

  • Jon WilliamsJon Williams Posts: 6,491
    edited 2004-11-19 02:26
    Probably not -- use a transistor buffer.

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    Jon Williams
    Applications Engineer, Parallax
    Dallas Office
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2004-11-19 15:27
    I'd have to go with Jon on that...Although Reed Relays are built a little different than standard relays, they still have·a coil...A transistor is such a small and inexpensive part to add (With a resistor) to ensure you're not going to harm the BS2 or draw too much current.



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    Chris Savage

    Knight Designs
    324 West Main Street
    P.O. Box 97
    Montour Falls, NY 14865
    (607) 535-6777

    Business Page:·· http://www.knightdesigns.com
    Personal Page:··· http://www.lightlink.com/dream/chris
    Designs Page:··· http://www.lightlink.com/dream/designs
    ·
  • nick bernardnick bernard Posts: 329
    edited 2004-11-20 03:02
    HAIL
    try a tpic6a596
    its a high current (open drain) shift register by ti

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    League Bowling.... it's not a sport, it's a way of life
  • agentileagentile Posts: 101
    edited 2004-11-20 09:51
    The reed relay you are using has an activation current.· Many of the ones I have used are suitable for use with TTL chips.· If a TTL chip can drive one, then the BS2 can probably also do so. ·If you have a data sheet on the relay, check to see what the activation current is.· Then check to see what the output pins of the BS2 can supply.·



    agentile
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2004-11-20 18:34
    Actually there are TTL Reed Relays available...Usually the contact rating is only 250mA or 500mA, although I have seen some between 2A-5A...The 5V ones usually require between 360mW - 510mW of power and have a nominal 100 ohm coil.· I used them to switch small signals on a controller board once.· Many even have internal clamping diodes for the coil.· I find though with most standard Reed Relays require around 20mA, which would mean the Stamp would have little to supply to anything else.· 2N3904 Transistors are around .04 cents each in qty 50, and resistors are cheap as well...

    I would still just put a transistor/resistor combo, and don't forget your clamping diode.



    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Chris Savage

    Knight Designs
    324 West Main Street
    P.O. Box 97
    Montour Falls, NY 14865
    (607) 535-6777

    Business Page:·· http://www.knightdesigns.com
    Personal Page:··· http://www.lightlink.com/dream/chris
    Designs Page:··· http://www.lightlink.com/dream/designs
    ·
  • Drud0616Drud0616 Posts: 26
    edited 2004-11-20 20:28
    I've been using the schematic with a relay and a transistor schematic as a reference, but I can't get the circuit to work

    www.geocities.com/SiliconValley/Orchard/6633/vbonoff.html

    What is the easiest way to set up a transistor buffer. Calculations? ect

    Thank you
  • DaveGDaveG Posts: 84
    edited 2004-11-20 22:38
    Question: You mentioned that you are turning on a water pump with this relay. What are the voltage and current requirements
    of the pump?

    You have to be careful when switching inductive loads with a tiny reed relay, the switching arc can weld the
    contacts shut, unless you incorporate some type of suppression.

    In my opinion, a conventional relay is a better choice for turning on a motor.

    Dave G
  • Drud0616Drud0616 Posts: 26
    edited 2004-11-21 02:52
    well here is the problem...I ordered the pumps from all-electronics.com, unfortunately they cannot provide me with any current requirements which does make it hard to design any circuit around, but they operate at 5-12VDC.· Really the only reason I purchased them was because of their price.

    HERES THE EXACT PUMP
    http://www.allelectronics.com/cgi-bin/category.cgi?category=search&item=PMP-2&type=store

    Dave, the explanation you gave about switching inductive loads may apply to what happend to me. I noticed that when I applied 5V across the coil the pump turns on using the 9V battery, but when I removed the 5V, the pump was still on for a short period of time(3-5 seconds). Could that have something to do with the contacts being welded together? If so what do you mean by a conventional relay?

    One more thing, I noticed this problem after I connected a BJT before the relay in order to safely hook the pumps to the BS2 pins.

    Post Edited (Drud0616) : 11/21/2004 3:24:40 AM GMT
  • DaveGDaveG Posts: 84
    edited 2004-11-21 05:25
    I looked at your pump on AllElectronics and I would estimate that it requires from 1/2 to 2 amps.
    The best way would be to measure it with a ammeter if you can.

    A conventional relay has heftier contacts than a reed relay and its action is more mechanical. I would suggest that you
    use either of the following relays, available at Radio Shack stores.
    1. #275-005, coil: 7-9VDC @ 18ma, contacts rated at 2amps
    2. #275-248, coil: 12VDC @ 30ma, contacts rated at 10amps

    You can use a 2N2222A or 2N3904 transistor to drive either of these relays. Be sure to put the 1N4001 diode across
    the relay coil, as shown in the diagram you referenced in your 12:28pm post. That diagram is correct and should work.

    By the way, what type of 9volt battery are you using? It needs to be hefty enough to drive the motor. The type of
    9v battery used in smoke detectors is much too small for your application. I would suggest using a Radio Shack
    #273-1776 AC Adapter, 12 volts @ 1 amp.

    Dave G
  • Drud0616Drud0616 Posts: 26
    edited 2004-11-22 04:24
    I was using a regular 9V duracel battery which seems to drive the motor pretty well, only thing is I notice it dies farely quickly and it gets expensive to buy batteries over and over again.
    As for the relays that you spoke of...How would I·go about appying power to the transistor, relay, and pumps? I hope I don't need a seperate battery to all parts. Can I just hook up the coils, transisor collector, and pumps to the 12 VDC AC adapter?

    Post Edited (Drud0616) : 11/22/2004 4:26:38 AM GMT
  • DaveGDaveG Posts: 84
    edited 2004-11-23 02:48
    Attached is a schematic of a relay driver for your pump.
    There are many other ways to solve your problem, but this solution
    uses components available from most Radio Shack stores.

    Dave G
    RelayDriverCircuit.jpg
    1024 x 572 - 126K
  • Drud0616Drud0616 Posts: 26
    edited 2004-11-23 06:12
    very cool...thank you Dave, but unfortunately now I have more questions regarding this schematic smile.gif

    Keep in mind that I'm doing all of this on a bread board right now

    1) I am planning to buy the 9VDC adapter, cut the end of the wire and stick the lead directly into the bread-board. Now I was under the impression that Vin of the BS2 had to take a value of 5VDC, why else is the a 5V regulator on the board of education (which I have been using as well).

    2)According to the schematic the BJT collector is connected to one end of the coil and 9V is connected to the other. Is that correct?

    3)What is the device that is connected in series to the (+) lead of the pump?


    Basically how is the relay connected to the BJT, 9VDC, and pump? I really appreciate all the help everyone has given me so far.
  • trendtrend Posts: 112
    edited 2004-11-23 13:52
    If you know the voltage, you can find the amps.. given you have a voltameter.

    Just measure the resistance of the coil.

    Then use the resistance in Ohm's law (V = I x R)

    here is a page on ohm's law

    http://64.233.179.104/search?q=cache:Q8cFlBfTL7gJ:www.grc.nasa.gov/Other_Groups/K-12/Sample_Projects/Ohms_Law/ohmslaw.html+ohms+law&hl=en

    I guess this would of been more helpfull earlier :/
  • DaveGDaveG Posts: 84
    edited 2004-11-23 19:03
    My previous schematic contained an error which is corrected in the new attached schematic.
    Please take note of it.

    I would suggest that you download the schematics of the BS2 and BOE Board from Parallax so you
    can see how they have the on-board regulators connected.

    Essentially, the BS2 has its own tiny reguator, on the Stamp module. The input to the regulator is called Vin
    and the output is Vdd(5V). Vdd powers all the chips on the module.

    The BOE Board has another, larger on-board regulator which also regulates Vin down to Vdd(5V). However, this
    regulator does not supply the Basic Stamp module. When you hook +9v to Vin on the BOE Board, it connects to
    both regulators, separately.

    The collector of the 2N3904 is connected to one lead of the relay coil and the other end of the coil is connected
    to +9V, as shown in the schematic.

    The device(s) connected to the + lead of the pump are the contacts of the relay, which are actuated by the relay coil.

    I would suggest that you look into some basic electronics books to familiarize yourself with the various schematic symbols.
    Although the symbols for relays do tend to vary quite a bit.

    Dave G
    RelayDriverCircuit1.jpg
    1024 x 572 - 155K
  • Drud0616Drud0616 Posts: 26
    edited 2004-11-24 04:31
    Very helpful...Thank you Dave I'll let all of you know how it goes when I hook up all of the pumps. Have a great Thanksgiving
  • Drud0616Drud0616 Posts: 26
    edited 2004-12-01 00:54
    Is the BJT connected as a switch in the diagram? When I apply 5V to the base I should be getting 5V out of the collector right? I don't seem to be getting that

    The 5V is supposed to bring the coil voltage down to 4V in order to open the contact between the pump and the 9V adapter. Am I reading this right?

    Post Edited (Drud0616) : 12/1/2004 1:09:48 AM GMT
  • DaveGDaveG Posts: 84
    edited 2004-12-01 03:17
    The 2N3904 transistor acts as a switch in this circuit. When 5V is applied to the base, through the 4.7K resistor, the
    transistor "turns on", causing current to flow through the relay coil. In turn, the relay contacts close and connect
    the 9V power supply to the pump. When the transistor is "turned-on", the voltage at the collector should be about
    0.15 volts, not 5V.

    By the way, do not connect 5V directly to the base, without the 4.7K resistor, because it will fry the transistor.

    To answer your question, 5V does not come out of the collector. The action of the transistor is more complicated
    then that. I would suggest that you find a beginning electronics book and learn how transistors work.

    A good reference is "Practical Electronics For Inventors" by Paul Scherz, available from Amazom.

    Dave G
  • Drud0616Drud0616 Posts: 26
    edited 2004-12-01 07:37
    Is the 2200 nfd capacitor necessary?· I didn't put it in and the circuit seems to still work fine.
  • DaveGDaveG Posts: 84
    edited 2004-12-01 13:42
    The 2200 microfarad(not nfd) capacitor is NOT necessary if the Stamp is working fine.

    In some circuits, an inductive load, like your pump, can cause the voltage from the 9V power
    supply to dip down several volts each time the pump comes on. This dip in voltage can
    cause the Stamp to reset each time. One way to prevent these transient dips is to install a
    large capacitor near the pump. The capacitor briefly supplies the energy to fill in the dip.

    Dave G
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