need help about relay!!

[msovais]

in this circuit FWD/RVS relay is shown with 2 separate contact pins and 2 separate NC and NO pins...

i wanna ask whether they are using 2 relays with one transistor or configuration of relay is just like that internally..?

 

[Nigel Goodwin]

It's two relays, and clearly shown as such. One is used for ON/OFF and the other for reversing.

 

[msovais]

i m not talking abt above power relay..
the 2nd FWD/RVS relay...???

 

[Nigel Goodwin]

It's a DPDT relay, also called a 2 pole changeover, again it's clearly marked as such, by the dotted line from the coil to the contacts. It's a standard relay!.

 

[hamm3rst0rm]

yeah.. nigel is right

 

[msovais]

hmm...in this case if i dont use that relay...can i use two simple relays with one transistor?

 

[Nigel Goodwin]

hmm...in this case if i dont use that relay...can i use two simple relays with one transistor?

You could use two single pole changeover relays - assuming you already have them? - if not you may as well get the correct relay, like I said they are VERY common.

 

[msovais]

im not been able to switch two single pole relays with one transistor(2n2222).

 

[ericgibbs]

im not been able to switch two single pole relays with one transistor(2n2222).

hi,
What are your base drive voltage inputs, if they are +5V, its most likely that with a 10K base resistor you are not driving the 2N2222 into saturation, when you have two relay coils in parallel.

Whats the supply voltage to the relays and what is the coil resistance?

Eric

 

[Hero999]

Do you have the mandatory free-wheel diode connected in reverse parallel?

 

[audioguru]

The relay might be a honking huge one for switching on a winch. The poor little 2N2222 will curl up and die.

 

[Nigel Goodwin]

The relay might be a honking huge one for switching on a winch. The poor little 2N2222 will curl up and die.

Well we are lacking almost all information!

 

[things]

Well we are lacking almost all information!

pretty much, he has thrown a circuit at us, and is asking how to avoid using a relay, because he doesn't have one. wouldn't you think if the circuit COULD use transistors, they would, since transistors are much faster, and sometimes more reliable???
just buy or find a relay somewhere, they would only be a couple of bucks, and it would save you all this trouble

 

[TrevorP]

Those diodes are protection from the relay right?

 

[audioguru]

Connect a 9V relay to a 9V battery. Then hold the relay's wires in one hand and disconnect the battery. You will feel a big shock (don't use both hands or it might kill you).

The voltage when the relay is disconnected might be hundreds of volts that could destroy a driver transistor. Adding a reversed diode across rhe coil clamps the voltage spike to 0.7V more than the positive supply voltage.

 

[Hero999]

Doesn't it clamp it to 0.7V below the supply not 0.7V above the supply?

Some of the biggest shocks I've had have been from playing around with transformers and large contactors. The moment you disconnect the battery from the relay the same current will cary on flowing, for an instant, and if you're in that current path it isn't nice. Currents above 5mA are considered harmful and can potentially kill you, however in practice when you interrupt the 20mA goint through a relay it only passed through you for a split second so it isn't too bad. but a big contactor with a current of 2A is a different story.

 

[ericgibbs]

Doesn't it clamp it to 0.7V below the supply not 0.7V above the supply?

Some of the biggest shocks I've had have been from playing around with transformers and large contactors. The moment you disconnect the battery from the relay the same current will cary on flowing, for an instant, and if you're in that current path it isn't nice. Currents above 5mA are considered harmful and can potentially kill you, however in practice when you interrupt the 20mA goint through a relay it only passed through you for a split second so it isn't too bad. but a big contactor with a current of 2A is a different story.

hi hero,
The 'spike' at the collector is postive going, wrt Vcc line, so it forward biases the clamp diode into conduction, thats why its clamped to Vcc + the Vdiode fwd drop.

Eriv

 

[Hero999]

Yes, I agree, the collector is lifted above the supply voltage by 0.7V, I was incorrect.