I'm updating an old design whitch uses a manual rotary switch to a new one with relay. A small contact resistance is important for me. The rotary switch version has a contact resistance about 3mohm. But I never seen a mechanical relay specified with such small values. Usually, they only give a maximum value such as '< 30mohm', '< 50mohm', '< 100mohm', etc. The working frequency is 50Hz. The current may as low as 1 micro-amp. So, can I safely replace the rotary switch with a relay? Or, there are other better choices?
Asked
Active
Viewed 5,108 times
0
-
1*why* does it need low resistance? Maybe you can use a better circuit. – Spehro Pefhany Feb 20 '16 at 10:35
-
1Current, voltage of signal, frequency, acceptable volt drop? – Andy aka Feb 20 '16 at 10:42
-
Why don't you post the schematics, to see really where the problem is? – Marko Buršič Feb 20 '16 at 11:31
-
1The problem with a relay at such low current is it may not break through the surface oxidation on the contacts. You might need a 'wetting' circuit. A solid state solution may be better _if_ it can be applied to your circuit - show us the schematic! – Bruce Abbott Feb 20 '16 at 14:05
-
1Among other things, 1 uA at 10 mohms will produce a voltage of 10 nV. Is this really important to you? If so, you need to watch out for a whole bunch of other problems before you worry about something like contact resistance. Thermal coefficient of wire resistance and thermocouple effects spring to mind, not to mention amplifier offset drifts. So why not explain your problem in more detail? – WhatRoughBeast Feb 20 '16 at 14:45
2 Answers
2
You may be able to use a MOSFET for your design. Depending on your design, you may be able to use a MOSFET with a very low RDSon (resistor when the FET is on)
There are some MOSFETS that have lower ON resistance than 10mohm.
I could have searched for a MOSFET for you but I don't have your schematics or specifications.
You should be able to find a good match at http://www.vishay.com/ (There are more MOSFET manufacturers)
Chetan Bhargava
- 4,612
- 5
- 27
- 40
0
I have no idea why you need such a low resistance with low currents.
Anyhow, a relay designed to carry large currents usually has low contact resistance.
Omron G9EA-1 (option -B-CA) has a 10 mΩ max resistance and 0.3 mΩ typical. You don't mention any specifications on coil voltage and this relay has a rated voltage of 60 VDC to 100 VDC depending on the model.
The -B model (without -CA) has 30 mΩ max resistance and 0.6 mΩ typical and is available for 12 and 24 VDC coil voltage.
Dejvid_no1
- 3,568
- 17
- 24
-
:) I think the '3 mΩ typical' should be '0.3 mΩ typical', the resistance is small enough, but, it's so........big!!! Anyway, it's a choice. – diverger Feb 20 '16 at 12:49
-
There are tiny MOSFET relays that fit your specs, but the ones I found required several mA to trigger. – Dejvid_no1 Feb 20 '16 at 13:00
-
-
1
-
Yes, I've checked that too. It states in connection method C, the resistance can go small to 5 or 8 mohm (typical value). But if I'm not wrong, the connection C is applied to DC load, only connection A support AC. I need to switch AC :(. But for connection A, the maximum values go beyond 50 mohm. – diverger Feb 22 '16 at 08:13
-
What do you think use g3vm in a "dry" circuit. Such as in the front of ADC to switch channels? It seems it's something like a opto. – diverger Feb 24 '16 at 08:36
-
There is probably and optocoupler inside, it's an SSR: https://www.ia.omron.com/support/faq/answer/18/faq02243/ – Dejvid_no1 Feb 24 '16 at 08:52
-
Actually, I'm confused by the "Release LED forward current", it states $$I_{FC}$$ minimum 0.1mA, measured when $$I_{OFF} = 10uA$$. I think $$I_{OFF}$$ is not only determined by $$I_{FC}$$, it should have some relations with $$V_{OFF}$$, but it doesn't give it. Am I right? – diverger Feb 24 '16 at 13:23
-