relay minimum permissible load 1

[dau]

Hi Forum,
I was going to replace the push button switch in my input voltage divider by a bistable relay. Now I discovered that the relay has a minimum permissible load (10µA @ 10mVDC). Can I replace the switch by the relay and ignore the minimum load? Why doesn't the switch have such restraint?

Regards

 

[Noway2]

The relay contacts have a minimum load rating to ensure that the signal will break through any contact corrosion. In your case, 10uA @ 10mVdc is a very lite rating and you should be able to work with this. It is inadvisable to ignore the ratings as you won't be able to guarentee that the circuit will work.

If you don't already understand it, you will need to learn about Ohms Law, which will let you calculate the voltage, current, and resistance relationships. You will then need to evaluate your circuit, ie what is the supply voltage, what is the load resistance, etc, to calculate and make sure you are above the ratings of the relay.

 

[LionelHutz]

I would only ignore it if you eventually want the circuit to fail. You need to meet the min relay requirements or the contacts will mechanically close without making electrical contact.

 

[dau]

@ all
I'm certainly aware of the relay specs but can anyone tell me the difference between a relay contact and a switch contact? That's the question that puzzles me.

 

[waross]

In small sizes there is a lot of similarity between many types of relay contacts and switch contacts. (The actual contact points, not the mechanism.)
Switch contacts may be snap acting or operate at the speed that the button is pushed. Relay contacts are generally fast acting.
Better quality switches and relays will have a contact wiping action to clear any contact film.
A few generations ago, when solid state switching was being introduced to industrial automation, GE recommended about 100 volts to pierce contact film.
Your ratings sound like a solid state relay. Is it a solid state relay?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dau]

@ waross
no, it's a usual TQ-relay by Panasonic (5V). When using solid state relays I suspect leakage currents and capacitances spoiling my signal. I'm just reading something about thermal emf of relay contacts, but shouldn't the same apply to switch contacts? There must be a significant difference.Don't all DMMs use switches to select the range?

 

[OperaHouse]

Many small signal switches are gold flashed or have a mechanical wiping action. I have seen a number of inappropriate switches used low voltage/current applications fail after about a dozen or more years. I have to replace a lot of switches in old test equipment. Just because the switch doesn't have a published spec does not mean it doesn't exist.

 

[itsmoked]

dau; many switches have gold flash as OperaHouse states. Very, very, few relays do. Can't say I've ever seen one that did. (I'm sure some do.) Of course gold doesn't corrode so there is no contact issue. (Your switch) The relay is likely silver or some other metal that does have contact oxidation hence the minimum rating.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[dau]

@ operahouse & itsmoked,
that's a very valuable information. I've checked my TQ-Relay and it has Gold-clad silver contacts. Now it gets complicated, I fear.

 

[VEBill]

"Gold-clad silver"

I've read somewhere that such contacts can be used for low level signals, provided that their not ever used for high current switching. Once they're used for high current switching, then the gold cladding may be arced away and then they become silver contacts good for only high current switching from then on.

Stand-by for others to confirm.

 

[sreid]

Switching at low currents and voltage is known as "Dry Switching" (no arc). Gold contacts are required for dry switching. I've always assumed this also applies to zero current/zero voltage.

A simple experiment might resolve this issue for you. Measure the contact resistance. Switch the relay once per second for 24 hours (86,000 cycles). With the contacts closed, measure the resistance and see if it has gone up.

 

[dau]

@ all
thank you for all your contributions. Probably I must face the truth that it is difficult to replace an ordinary push switch assembly like the ITT Cannon TJ type we've used so far. The problem is that hardly any distributor in Germany has such assemblies available. I was looking only for this reason for a substitute. Perhaps anyone can provide me with a link to a 'switchless' (professional) voltage divider with 2 ranges 1:1 and 10:1 with a voltage protection up to approx. 300Vac and an input resistance of 1 MOhm. Frequency is from dc to 10MHz.

Regards

 

[itsmoked]

VE1BLL; I'll confirm that.

Let me also point out that the real key is the voltage. The voltage is what gets thru any contact resistance or oxides.

dau; If you're going to have more than a measly 10mV across this relay quit worrying about it and just use it.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[BobbyNewmark]

What you want is a reed relay. The contacts are two metal reeds hermetically sealed inside a glass capsule. When the coil is energized, they attract each other and complete the circuit. Models with gold-plated contacts are available. Vacuum-filled versions are available for kilovolt work. There are normally closed versions that have an internal permanent magnet; magnetism from the coil cancels it to open the contacts.

Keep in mind that it does not take much magnetic field to close the circuit, so you have to keep them away from stray electromagnets and idiots with permanent magnets. (Mount in center of enclosure, and make the enclosure at least a few inches in size.)

Mercury-wetted relays can also be bought. They literally avoid the dry contact problem for small signals, yet the mercury surface is not damaged by high currents or reactive loads. (I bet the CE bureaucrats hate these.)

 

[OperaHouse]

This is what they make sockets for! Even a really bad choice in a relay will probably last ten years. I wouldn't worry unless failure has some consequences in the application.

 

[dau]

Thank you all for your comments. They've helped me a lot.
Reed relays are inappropriate, I think, because the device will be used in a power electronics environment. For Mercury-wetted relays I'll check the www, haven't heard of them before. But in one respect I agree with many of you: I'll buy such a gold-clad relay and simlply test it. The only difficulty probably is to convince my boss to give it a try.

 

[waross]

Someone has already mentioned that just because a device does not have a published spec does not mean that the spec does not exist. (The spec, meaning the electro mechanical attributes and capabilities of the device may not have been determined but they exist.)
The gold plated silver contact relays that you are considering will probably out perform the push button you are replacing.
What are the specs of the switch?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dau]

@ waross,
I've tried to find a URL for our used switch but couldn't find one. I'm completely fed up with corporations that aren't able to find their own parts like Philips....and it seems also ITT Cannon. But perhaps they don't manufacture it anymore. I'll try to upload our old pdf. Our future strategy isn't decided yet so don't bother too much over this issue. But it's rather interesting to follow this conversation.

Regards