# Need help working out watts & ohms

Discussion in 'The Projects Forum' started by madazz, Aug 12, 2009.

Aug 12, 2009
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hi

i need some help in working out what the ohms & watt output of my 12v appliance is. i need 160 ohms/13 watts to drive a relay and am not sure if my unit will put out enough power. can anyone help please.?

thanks aaron

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Last edited by a moderator: Aug 12, 2009
2. ### jj_alukkas Distinguished Member

Jan 8, 2009
751
5

What is your device? Is it an inverter or a logic circuit with transistor driver? Where did you get a relay that draws 13 watts at 12v?? The heaviest of relays draw a maximum of less than 1 watt.

You can calculate the wattage of your relay at 12 v using p=V^2/R. So a 12v 160ohms relay will pull 0.9W. What is the driving stage of your device? a circuit diagram of that helps decide whether it is capable.

If you need, these are basic power formulas

P=VxI
P=V^2/R
P=I^2xR

Aug 12, 2009
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The unit i have is a ECM, it is powered by 240VAC and has a transformer in it reducing to lower voltage, It is used for environmental control.(Co2, Fans, Lights Etc..)

The Ecm has a circuit board inside it, which is a bit to complex for me to draw a circuit diagram. When the ecm is programmed to, the 12vdc output will get power, from this power i need to run a relay to switch a 240vac appliance on/off.

This relay was what i was told i needed to use, but was then told i'd need 160ohms to drive it. ?

i have included a photo of the relay, 12V DC Relay has a coil resistance/impedence of approx. 160 OHMS

can i find out what the ohms output is witha cheapo multi meter ? i have one but don't know how to use very well.

i had orginally thought about using a solid state relay input 3-32VDC - output 40-480VAC 40A. -SOLID STATE RELAY
SSR-40 DA 40A/480V

would this work better ?

many thanks aaron

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• ###### Solid State Relay idea.jpg
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4. ### jj_alukkas Distinguished Member

Jan 8, 2009
751
5
What output is your device intended to drive at 12v?

By sketching circuit diagram, I meant only the last part out of which the 12v output from the device is coming. It will mostly be a transistor or IC. Just note to which pin the 12v output is connected and the part no.

You cannot measure a device's output impedance and if you do, it is of no use at it will be in kiloohm range. However you can measure your relay impedance by setting multimeter in the 200 or 2000ohms range and connecting it between the coils of the relay. If your mechanical relay is not working, you can use a solid state relay.

If you are told that a 160 ohm relay is needed, then it will work. But it is safer to find out the device which is driving it first than to unknowingly damage it.

Aug 12, 2009
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i'm not sure i understand your 1st question ? the device i'm wanting to run from the 12v output is a dehumidifier - 240VAC. my ECM only puts out 12V on the output for the relay box for the dehumidifier to plug into, therefore the reason i need a relay. 12v to 240v so it can turn the dehumidifier on/off when the ECM tells it to. it does have a ic in it also are a few transistors (from memory). best thing i can do is take a photo to show u, i cannot draw a diagram as i wouldn't know where to start.

now as far as being told this is the relay i need, i was asking which one i needed and the guy (who was selling them) said that he had one but asked if my ECM put out enough Ohms to drive the relay. 12V 160 OHMS. this is what i need to know .

what happens if the OHMS are to low ?

what happens if the OHMS are higher ?

could i just use the solid state relay, like the diagram i drew ? or will i have the same problem with the ohms? If the worst thing that can happen is the relay won't work or could be damaged then thats ok. i DONT want to damage my ECM!

ill get a pic as soon as i can and post shortly, thanks for all your help!!!!!

aaron

Last edited: Aug 12, 2009
6. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
The relay coil is 160 ohms. By Ohm's law, the current it takes to pull in the relay is 12 volts/160 ohms = 75 milliamps. That is going to be the entire load of the relay on the ECM.

The relay contacts are what will switch the 240VAC to the dehumidifier. If you take the dehumidifier's rated power in watts and divide by 240, the result is the current that those relay contacts have to be able to handle. Relays are always rated for contact voltage and current.

It would be best to mount the relay in a box so the terminals are covered. Feed the 240VAC in with a line cord, and mount a socket in one side to plug the dehumidifier into. The line goes to the common contact, the socket goes to the N/O (normally open) contact.

Aug 12, 2009
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The enclosure idea is what i'd planned on doing, i've included a pic of the RELAY unit (made by company that makes ECM) i'm basically building the same thing but saving myself \$300+

my question wasn't weather the relay would run my dehumidifier but if my ECM would drive the relays coil. (needs to be 160 OHMS) thats what i'm trying to find out, weather my ECM puts out enough Ohms to drive relay coil.

aaron

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8. ### rjenkins AAC Fanatic!

Nov 6, 2005
1,015
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160 Ohms would be the minimum resistance to go for, a 12V relay with a higher coil resistance would work fine.

The IC or transistor on your controller will only be rated for a particular amount of current. You know it's OK with 160 Ohm load.

As beenthere says, a 160 Ohm relay on 12 Volts draws 75mA or 0.075A; this is the actual 'known safe' output current from your controller.

A higher resistance relay will draw less current from the controller.

A lower resistance relay will draw more current, and could possibly damage it, if it needs more than the controller can safely give.

The SSR40-DA only needs 7.5mA at 12V input. It should work fine.

9. ### jj_alukkas Distinguished Member

Jan 8, 2009
751
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Your question has my answer. SO your device is designed to drive relays. Then no probs, get a 12v relay win impedance >= 160ohms. Lesser input impedance means that you are shorting the device. So play it safe.

At 160 ohms, P= 0.9W
When OHMS are low, Power used will be greater than what the device can handle and either relay wont latch or burn the driving device out. Think of it as shorting your ECM's output.

When ohms are TOO high, power draw will be lower and the relay wont latch. But this doesn't happen untill you drive a relay which is more than 200ohms above rated.

SO FINALLY, NEVER USE A RELAY LESS THAN 160 OHMS. You can use a 200ohms one without any problems.

Aug 12, 2009
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Thanks you all So So much, really appreciate all your help. u guys are the best. ill see how i go then.

azz

Aug 12, 2009
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hey joseph

hey mate got another problem may be u could help. u know how i thought my output was 12v i had assumed that i tested it last night its a 5vdc signal. so i cannot use the 12v relay as it will not work on 5volt.
the output is designed o drive a relay but the manafacture will not let me know anything except i can buy 1 for \$350. will the SSR relay be ok to use ? or any ideas what i might do. ? also can u tell me that SSR relay is 480v 40a so is that 240v 20a ? (when running on 240V)

12. ### jj_alukkas Distinguished Member

Jan 8, 2009
751
5
SSR's are mostly rated for inputs 3-32VDC. If you have an SSR in mind, check its datasheet for operating voltage. SSR's are almost universal. So would be no problem. However, if you use an ordinary relay, select one for 5 or 6vdc with 200ohms impedance.

It would be better if you confirm the voltage to your SSR's datsheet.

When a relay is rated for maximum 480v 40A, it will allow a maximum increased current draw of 20 - 40 % at half the voltage. You cant use the power formula here and say it will hold 80A. But I can say with confidence that it will support a load of 50A at 240V. So no probs on the output.