Relay turn on resistance increases

[Vindhyachal.takniki]

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?

[HAL-42b]

This should help understand the problem.



Cool guy, just came across his channel yesterday.

[Vindhyachal.takniki]

http://www.electronicoscaldas.com/datasheet/JQC-3FC%28T73%29DC12V_JIHJIK.pdf
 
this is relay I am using.
 
Relay have following written on it:
HL
JQC-3FC(T73)DC12V

[wraper]

First of all this is some crap Chinese relay. Secondly relay contacts do have minimum switching current, probably the current is too low to clean them. http://en.wikipedia.org/wiki/Wetting_current

[Seekonk]

This is fairly common.  Likely this relay is rated for 120V AC.   If you connect these up to a 100W light bulb and cycle them about twenty times to clean the contacts first they will likely wok in your project for many years before they act up again.

[DmitryL]

This is fairly common.  Likely this relay is rated for 120V AC.   If you connect these up to a 100W light bulb and cycle them about twenty times to clean the contacts first they will likely wok in your project for many years before they act up again.

Brilliant! All these inventors of low-signal relays with reed, gold-plated, mercurium-wetted and other weird things contacts just didn't know that they can "burn-in" cheap power relays with 100W bulb to get rid of "_minimal_ switching current (and voltage)" effects.

Think about patenting your idea!

BTW, there is a very good (as usual) article on relays here:
http://sound.westhost.com/articles/relays.htm

[sacherjj]

I was in a presentation at the APEX show this week with some interesting information about relays vs solid state devices for test fixtures.  They currently have solid state relays with low enough contact resistance to beat relays.  However, the don't all have good values for all of the following:

Ron, Roff, Capacitance.  With a low Roff, you get some leakage.  With a high capacitance, you cannot use for high frequency signals without issues.

Best case had good values for two out of the three.  For uses that require all three, there still isn't something better than the standard reed relay for test fixtures and similar low current uses.

[Vindhyachal.takniki]

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?

[macboy]

You could use MOSFETs instead of relays. You are using low value resistors, and a good MOSFET will have a lower on-resistance than most relays will. An important difference is that you no longer get isolation between the load and the circuit driving the MOSFETs, as you had with the relays.

[Seekonk]

If speed isn't an issue use a VOM1271 to drive the FET.  I have also used opto isolators driving 300K gate resistors from a 9V battery.  These have run for years without needing to have the battery changed.