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08-12-2009, 03:12 AM   #1
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## Working out ohms

hi

i need some help in working out what the ohms output of my 12v appliance is. i need 160 ohms to drive a relay and am not sure if my unit will put out enough power. can anyone help please.?

thanks aaron

s11.mitchel@yahoo.com.au

08-12-2009, 07:06 AM   #2
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Your output is probably rated in watts, so if I remember correctly, P=E2/R, in a resistive ckt(no AC). P-power in watts=voltage squared divided by the resistance.

08-12-2009, 11:47 AM   #3
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Quote:
 Originally Posted by madazz hi i need some help in working out what the ohms output of my 12v appliance is. i need 160 ohms to drive a relay and am not sure if my unit will put out enough power. can anyone help please.? thanks aaron s11.mitchel@yahoo.com.au
That doesn't make sense. "Need 160 ohms to drive a relay" is kind of a nonsensical statement. Relays require a voltage to drive them, and draw a certain amount of current. The relay coil itself will have a resistance measured in ohms, but it does not "require" a certain resistance to drive it. The output impedance of your appliance is probably close to zero.

Do you mean that the relay has a 12V, 160 ohm coil? If that's the case, then it will draw 12/160=0.075A, or 75mA, from the 12V source that operates it. Thus, you need to make sure the output of your 12V appliance can provide 75mA.

 08-12-2009, 07:40 PM #4 Member     Join Date: May 2009 Location: Easton MD Posts: 1,893 Rewards Points: 1,104 Is this an impedance matching problem for an electronic project regarding maximum transfer of power to limit signal attenuation?
08-13-2009, 08:29 AM   #5
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Quote:
 Originally Posted by Code05 Is this an impedance matching problem for an electronic project regarding maximum transfer of power to limit signal attenuation?

i think so if i understand what u mean. i have a control unit which has a 12v output, from that output i need to run a dehumidifier (240VAC) obviously it cant run on 12v & there fore need a relay. 12VDC to 240VAC

the relay i was looking at buying is 12v, 160 Ohms coil relay and was asked before i brought it if my unit puts out enough Ohms to drive it. ? i have no idea and no idea how i can find out. i know its 12v but as far as ohms ???

i was thinking about using a SSR-40 DA 40A/480V (solid state relay) but i guess ohm still comes into play. look at pic i drew to give u idea.

thanks aaron
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Last edited by madazz; 08-13-2009 at 08:33 AM.

 08-13-2009, 08:39 AM #6 the Musigician     Join Date: Jul 2008 Location: I'm right here! Posts: 10,404 Rewards Points: 2,000 I = E over R DM __________________ [SIGPIC][/SIGPIC]Click here to see some of my original magic tricks and trick boxes!
 08-13-2009, 08:59 AM #7 DIYer   Join Date: Oct 2008 Location: Virginia Posts: 910 Rewards Points: 500 This is not an impedance matching problem per se. You must know the current output capability of the 12 volt source. Once you know how much current in milliamps (ma) it can put out, then you can figure out if the relay coil will pull too much or not.
08-13-2009, 11:50 AM   #8
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Quote:
 Originally Posted by madazz i think so if i understand what u mean. i have a control unit which has a 12v output, from that output i need to run a dehumidifier (240VAC) obviously it cant run on 12v & there fore need a relay. 12VDC to 240VAC the relay i was looking at buying is 12v, 160 Ohms coil relay and was asked before i brought it if my unit puts out enough Ohms to drive it. ? i have no idea and no idea how i can find out. i know its 12v but as far as ohms ??? i was thinking about using a SSR-40 DA 40A/480V (solid state relay) but i guess ohm still comes into play. look at pic i drew to give u idea. thanks aaron
This is not impedance matching. That's the only context where you would refer to the input/output of something in ohms, and it's much more advanced and nuanced than anything you're doing here. Like I said in my previous post, you have a 160 ohm coil, so you need to provide 12/160=.075A of current to drive the relay. What you care about is whether your source can supply 75mA, not "what its ohms are". You're not asking the right question.

 08-13-2009, 01:06 PM #9 Member   Join Date: Nov 2007 Location: Nashua, NH, USA Posts: 7,917 Rewards Points: 1,440 Ohms (for resistance or impedance) in this case describes an input as opposed to an output. It is still common for relay inputs to be described using ohms which forces the user/customer to do additional calculations (see above) to figure out what kind of power supply (volts and amperes) is needed to operate the relay. Still, there is a minimum voltage and a maximum voltage that the relay coil can take. Relay "input" versus "output" can be confusing since the input current is what controls the relay and the input current or voltage does not show up at the output. So "output" is not really a good word to use. The relay outputs are really switch terminals and the switches are turned on or off depending on what the relay is intended to do. The switches have maximum voltage and current ratings but no minimums. "Normally open" and "normally closed" refer to the switch positions when no current is applied to the coil (input). Ohms for an output is applicable to electronics as opposed to what we are discussing here, electrical circuits. __________________ The good conscientious technician or serviceperson will carry extra oils and lubricants in case the new pump did not come with oil or the oil was accidentally spilled, so the service call can be completed without an extra visit. Last edited by AllanJ; 08-13-2009 at 01:36 PM.
08-15-2009, 05:45 PM   #10
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Quote:
 Originally Posted by mpoulton This is not impedance matching. That's the only context where you would refer to the input/output of something in ohms, and it's much more advanced and nuanced than anything you're doing here. Like I said in my previous post, you have a 160 ohm coil, so you need to provide 12/160=.075A of current to drive the relay. What you care about is whether your source can supply 75mA, not "what its ohms are". You're not asking the right question.

hey yeah i'm not a electrician so i don't really know. i obviously have been asking the wrong question then. i have a small knowledge for simple circuits but not in working stuff out. yes 75mA is what that 12v relay needs to drive it, i dont know if my unit will be capable of driving it.
on another note i actually tested voltage last night and was surprised to find out that its not 12volt like i thought its actually 5volt there fore the 12v relay wont work can i use the ssr that can run on 3-32vdc or how else can i run it so that 5v signal can switch my 240v appliance on.

08-15-2009, 05:53 PM   #11
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Quote:
 Originally Posted by madazz 75mA is what that 12v relay needs to drive it, i dont know if my unit will be capable of driving it. its actually 5volt ssr that can run on 3-32vdc so that 5v signal can switch my 240v appliance on.
Digi-key probably has a relay with a high-impedance coil that may do this.
Otherwise you can have a low power relay driving a relay that can switch 240vac.
How much current can your 5v deliver [get spec's or measure it]? How many amps does your 240v [AC?] appliance draw? Is it a motor load or a lamp load?
Your 5v source may need the relay coil transient suppressed to keep the source from being damaged.

08-15-2009, 07:54 PM   #12
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Quote:
 Originally Posted by Yoyizit Digi-key probably has a relay with a high-impedance coil that may do this. Otherwise you can have a low power relay driving a relay that can switch 240vac. How much current can your 5v deliver [get spec's or measure it]? How many amps does your 240v [AC?] appliance draw? Is it a motor load or a lamp load? Your 5v source may need the relay coil transient suppressed to keep the source from being damaged.

the manafacture wont give me the specs how do i measure the current ? its a motor load i guess ?? its a dehumidifier that i'm trying to run, so its not lights (high inductive load)

what do u mean transient suppressed ? wouldnt it be suppressed already, the outlet/5v output was designed to run a relay/relays that runs 240v appliances such as air conditioners and dehumidifiers.

i had thought about getting a low power relay and driving a larger one with the small one to switch my appliance on/off. i just dont know what one to use or where to start, i can buy the relay box from manafacture for \$350 and i bet all thats inside is a simple relay, i paid 1000 for my ecm unit and i reckon they built it in 1 hour its a pretty small circuit board that does everything. Anyway what do u suggest i should do, i'm lost now!

so cause the voltage is 5v (low) i want a relay with a high impendance rating ?? don't mind buying 2 relays at all if thats what i'd need to do. Jaycar sell a small SSR relay similar to the one i mentioned before. much smaller can only switch 3 amps 240v so i'd need another to run the dehumidifier. i have the specs for that relay what am i looking for ? the control current, input resistance, min load current.

i really dont want to damage my ECM.!

as far as dehumififiers Amps i dont have it hear at the moment ill find out and post back. thanks for all your help!!!!!

aaron

 08-15-2009, 08:17 PM #13 Member   Join Date: Jul 2008 Location: NW of D.C. Posts: 5,990 Rewards Points: 2,000 For sure, the contactor inside the dehumidifier can handle the motor load, so it could be one of the possibly two relays. Post a schematic of the dehumidifier. It's probably pasted on the inside somewhere. And, a schematic of the output stage of your ECM if you have it. If we OR your controller with the internal contactor then your controller can always turn off the dehumidifier but will enable it to be turned on. For an AND function both your controller and the internal contactor must be asking for humidity for the thing to turn on. Or your ECM totally controls the dehumifier. Last edited by Yoyizit; 08-15-2009 at 08:21 PM.
08-15-2009, 11:56 PM   #14
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Quote:
 Originally Posted by Yoyizit For sure, the contactor inside the dehumidifier can handle the motor load, so it could be one of the possibly two relays. Post a schematic of the dehumidifier. It's probably pasted on the inside somewhere. And, a schematic of the output stage of your ECM if you have it. If we OR your controller with the internal contactor then your controller can always turn off the dehumidifier but will enable it to be turned on. For an AND function both your controller and the internal contactor must be asking for humidity for the thing to turn on. Or your ECM totally controls the dehumifier.
ok sorry but i'm totally lost, i dont understand what your saying in the last paragraph????? ill post a pic of it soon waitig for batt to charge.
i dont have dehumidifier here at the moment so cant tell u much about that.

My ECM is a Environmental Control unit it monitors temp/humidity and controls a fan (240V) for cooling and a control output for a 240v co2 solenoid. now my unit moniters the temp and RH and depending on what i've set the min/max temp will control fans for cooling, it will also shut fans off when injecting co2. now it has a seperate output to plug a relay box in. (the makers of ECM sell them for \$350+) from the relay box it plugs into the power point 240v then another cord goes into the ecm (5v output) and i can then plug my dehumidifier into the output on the relay box. when the RH has gone above my set level the 5v output gets power, sending a 5v signal to the seperate relay box (which is what i'm trying to make) which turns the dehumidifier on. hope this helps, ill post pics shortly.

i just need to know what relay i should use or how i find out the current it can put out so i can get the right relay, i dont wanna fry the ECM unit!

aaron

Last edited by madazz; 08-16-2009 at 12:03 AM.

08-16-2009, 08:32 AM   #15
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You don't need the dehumidifier or other appliance (that plugs into the circuit controlled by the relay switch contacts). The relay coil behaves the same whether or not it is actually controlling anything.

You'll need to call the manufacturer of the control unit that is putting out the 5 volts that will supposedly feed your relay.

As far as transients go, when a coil (doorbell, relay, etc.) is turned off, a momentary "kick" voltage will go back through the circuit. This usually does not cause problems with mechanical switching such as a doorbell button or wall switch and with the 5 volts or 12 volts or 24 volts or whatever coming froma transformer. Electronic switching and electronic power supplies can be damaged by this transient. A capacitor can be placed across the relay coil terminals to soften this transient, unfortunately I don't know how to calculate the size needed.

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