Relay turn on resistance increases

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1. I have a digitally controlled resistance series network.
2. What I have done is placing a resistor at output terminals of each relay. I have 6 relays.
Whenever i need that resistor I turn off the relay, whenever I need to bypass the resistor I turn on the relay so effectively giving short.
3. Using normal SPST 12V sugar cube relay.
4. Problem is when i turn on relay with 12V coil, the relay turn on resistance at output terminals starts increasing gradually & then finally settles to around 2 ohms.
5. This 2 ohms resistor effects by circuit.
6. How do remove that or I have to use some other solution?

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Read the relay specs carefully. If it is a power relay, you need a minimum current to clean the contacts. You probably want a signal relay. What current is involved?

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What is the approximate current in the resistors ?

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Mike - bazinga! We posted at the same time.

Last Edited: Sat. Feb 28, 2015 - 07:38 AM
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http://www.electronicoscaldas.co...

 

this is relay I am using.

 

Relay have following written on it:

HL

JQC-3FC(T73)DC12V

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It's a power relay. You didn't tell use the critical information - how much current are you passing through the relay?

 

You thought relays were simple devices? Have a read:

https://www.components.omron.com...$file/M20Z54E11.pdf

 

The other seeming 'simple' component is the fuse. There is much to learn about these.

 

Last Edited: Sat. Feb 28, 2015 - 09:03 AM
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I am measuring resistance using 6 & half DMM from fluke.

So current passed through output terminals of relay is very low.

I haven't measured current, but I guess it will be less than few milliamps

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That explains the problem then. Pass a larger current through the contacts and measure the voltage drop across them. I dare say the problem will disappear.

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Attached is the circuit. This is how I add multiple relays in series.
Whenever I have take resistance value I turn off relay so path completes through that resistance.
In case I do not need resistance I turn on the relay , effectively giving 0 ohms in that circuit.
This is how I make multiple resistor value.

I am using multimeter to measure resistance value. maximum current will be 3A through these resistor.

Problem was relay resistance keeps on increasing, if I turn it on(upto 2ohms). Thus adding large resistor errors in final computational resistance.

I had checked some signal relays where contact/weeting current is low. But still I had found that its 10ma minimum. Again multimeter pass much less current.

So what alternative should I use instead of relays?

 

 

Attachment(s): 

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Did you measure the voltage drop as i suggested? I think the method you're using to measure the resistance is flawed. If a relay is passing 10A with a contact resistance of 2ohms it would melt. Something is clearly wrong.

Last Edited: Wed. Mar 4, 2015 - 07:18 AM
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Burnish the contacts to remove the oxidation. THere are tools for this. Like a fine grit fingernail file.

 

Some relays have gold plated contacts. Tin plated of course oxidize faster.

 

Big relays are intended to self-clean the contacts via many-AMP currents flowing and the sparking from make/break tend to clean the contacts, in lieu of a quick filing now and then.

 

Solid state circuitry would be better.

 

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As you don't appear to want to measure the actual voltage drop and are fixated on measuring resistance what happens if you set the DMM to your resistance range and then put the probe tips together and hold them together?  By any chance does that give you the same result as when measuring the relay contacts?

 

David