Question about relays

[simonbr]

I have a project I am working on that I am using a relay, but I'm not sure if its the best way.

I have a 5vdc point that goes to 0vdc at some point in time. When its at 0v, I have to ground a trigger wire. I found a small relay in my spare parts that is working but I'm not sure if I'm using the relay properly because its kept energized most of the time. Is keeping a relay energized a bad idea, can it burn out or cause problems?

The relay in question is hjr1-2c l-05v (data sheet)

 

[betwixt]

Relays are quite durable and will last virtually forever if run at their rated voltage so you don't have to worry about it being activated for long periods.

I'm afraid that without more information about what the trigger wire connects to, it isn't possible to suggest alternatives but it is quite likely that a better method exists.

Brian.

 

[simonbr]

Thanks for the reply.

I am driving a 12vdc chime that does a "ding dong" sound continually when the trigger is on ground. The circuit is 3 wires, 1 - 12v, 1 - ground, 1 - trigger. This is what is connected to the relay.

to control the relay, I'm using a 5vdc source that is almost always present. When the 5vdc goes to 0v, this is when I need to remove the trigger from ground (float it) and it stops the chime.

Is this enough information for other ideas?

Thanks again

 

[betwixt]

That explains a lot, forgive the questions but the 'clients' on Edaboard range from school students to power station engineers so it was necessary to put a scale on the size of problem.

The relay solution will work fine but there are alternatives which are more efficient. As the trigger current is likely to be in the microamps region, a relay is really overkill.

If you need electrical isolation between the chime circuit and your control circuit, an opto-coupler will suffice and be much smaller. You connect it the same way as a relay but it has no moving parts, the only thing you have to be careful of is getting the polarity right as unlike a relay they are not tolerant of reverse connection and you have to limit the current to the input side. Typically they are a 6-pin or 8-pin plastic package, much smaller than a relay and run cold, taking only a fraction of the power used by the coil in a conventional relay.

If the chime and your control circuit do not need isolation and share a common ground connection (usually the negative side of the supply), you can still use the opto-coupler method but there is an even easier way, use a single transistor. What you are essentially trying to achieve is a voltage level conversion from the 5V to the trigger wire voltage which is quite simple to do.

Personally, I would use the opto-coupler as it works in all circumstances and is inexpensive and more versatile.

Brian.

 

[D.A.(Tony)Stewart]

Electrical switches can be isolated like relay coils or common mode like an grounded emitter depending on requirements for high voltage, current and speed.

Relays can sustain power always, but some power solenoids cannot and are rated for duty cycle.

On a broad electronic application scope;

• BJT transistors are current mode saturated with a current gain of only 10 to 50 depending on Vce rating with equivalent Rce
• MOSFET transistors are voltage controlled, fast and come in a wide range of ON resistance 1mΩ to 100Ω such that you choose one 1% of load resistance for high efficiency or low load regulation
• REED Relays have a gain of 50 suitable for low rep. speed <500mA range in a small package
• <2A signal Relays , most have gold plated contacts for uA current use without oxidation
• >2A power Relays require 10% of rated current to "wet" the contacts of oxide with arcs and can have contact rating to coil current gains as high as 2000 ( much higher than BJT's )
• opto-isolated switches provide high V isolation with very low current gain as low as 30% or others with active gain

polarity is user control by design.

• We say positive logic when +V closes contacts , but
• we say negative logic if high input produces contact closure to Gnd.
• Single transistors are Inverting switches thus high input for low output or reverse.
• switches used on low side can be ON with short to Gnd
• switches used on high side are On shorted to Vcc
• thus in automotive use on 12V they also have protected smart High side and Low Side drivers protected from overtemp/overcurrent & shorts/reverse spikes
• ( but not for drivers high on smoke haha)