general relay question

[anthony_c]

does anyone know offhand how many volts it takes to trigger one of the bosch style mini-relays?

[Patcon]

Do you really want to know Volts? not amps?

Is your car not running 12volts?

[SteveJ]

17 minutes ago, Patcon said:

Do you really want to know Volts? not amps?

Is your car not running 12volts?

Yes, there is a threshold voltage for generating enough of a field in the coil to pull in the contacts. There is a different value at which point the relay releases.

@anthony_c I think this link may have what you're looking for. http://bowery.com/maserati/home/files/bosch relays.pdf

 

[anthony_c]

Most of Bosch's relays show a pull-in voltage of less than or equal to 8v but they don't explicitly indicate a minimum.

Did they mean >= 8v, or is the stated drop out of 5v also the minimum pull-in.

[SteveJ]

"Pull in" means that the contacts are open until it gets to 8VDC. At that point, there is enough of a field that the coil pulls in the contacts, and the normally open circuit is now closed. So that is the minimum operating voltage.

"Drop out" means that after the contacts have closed, they will stay closed until the voltage on the coil drops below 5VDC. At that point, the contacts drop out of the circuit, and the circuit is open.

It makes sense that they are not equal because it takes more energy to change the state of the contacts from open to close than it does to maintain the contacts closed.

https://media.digikey.com/pdf/Other Related Documents/Panasonic Other Doc/Small Signal Relay Techincal Info.pdf

[Patcon]

I assumed voltage could play a role but couldn't think of a situation where voltage would be that low. Less than 8v's in a 12v system

[anthony_c]

4 minutes ago, Patcon said:

I assumed voltage could play a role but couldn't think of a situation where voltage would be that low. Less than 8v's in a 12v system

reference "interlock relay" and work backwards from there

Edited December 2, 2019 by anthony_c

[SteveJ]

1 minute ago, Patcon said:

I assumed voltage could play a role but couldn't think of a situation where voltage would be that low. Less than 8v's in a 12v system

The fuel pump relay on the 260Z has a low pick up because of the circuit utilized.

You want the pickup to be lower than the typical system voltage. Here are two examples that may give you an idea why:

1. When cranking the engine, the battery voltage (an therefore the system voltage) drops down to 10VDC or so. You want the fuel injection and fuel pump relays to pick up.
2. The alternator dies. You still want systems to operate properly until the voltage has decayed some.

[Captain Obvious]

3 hours ago, Patcon said:

Do you really want to know Volts? not amps?

It really is the current that matters, but to make things easier to think about (and apply in use) they spec it by voltage instead. The relay coil has resistance. You apply a voltage to that resistance, and current flows. The higher the voltage, the more current will flow. Apply a high enough voltage, then enough current will flow to create a strong enough magnetic field to pull the relay in.

But for most applications, you don't want "a relay that pulls in at 160 mA". You want "a relay that works on 12 Volts".

 

[anthony_c]

my initial guess was that the original relay had failed so i picked up a generic mini-cube type relay at napa. after some additional testing i found the voltage on yellow is hovering around 2v. if i crank the starter it rises to a max of 8v. 

probably not the relay.

[Captain Obvious]

2 hours ago, anthony_c said:

Did they mean >= 8v, or is the stated drop out of 5v also the minimum pull-in.

The pull-in voltage is the voltage at which the relay will be guaranteed to pull-in. So a relay with a pull in spec of 8v means the relay is guaranteed to actuate if you put 8 Volts across the coil. Less voltage might not pull in, but eight volts is guaranteed to pull in.

[Captain Obvious]

8 minutes ago, anthony_c said:

i found the voltage on yellow is hovering around 2v. if i crank the starter it rises to a max of 8v. 

probably not the relay.

I feel like we're coming in at the middle of a story... What's "the yellow". Is there a yellow wire going to your relay? What relay?

@SteveJ, You're already on top of this one?