need help about DC switching circuit> darligton pair

I need help about a simple circuit. I need to operate a relay(12v) with a 1.4v DC. So I am trying to use transistor as a switch, where 1.4v DC will be input. so that i can get 12 volt at output for relay.
I did this with, theoritical circuit but failed. after googling I found Darlington pair. I have Made circuit diagram. but during operation second transistor is heating very high. So please give me proper Idea of component rating so that it will work correct.

uploaded circuit **broken link removed**

hi and Welcome,
You have not said what the relay coil resistance is.??

The 2nd transistor will get hot , because its not being saturated, the Base current is too low.

What is the 1.4V source

Conventionally the two collectors are connected together

If the relay you are using really is an EMR011A12 (SP reed relay) then its coil should only draw ~ 10mA. That would not be enough to make either transistor hot. So it looks as though there is a short-circuit or wiring error somewhere.

Hi,

If you want to drive it that way you need a collector resistor in Q2 collector to limit the current through Q2.
That's not a darlington arrangement, but they way you have it you can get lower sat voltage for Q1.

I agree with MrAl. Without the resistor, when Q2 is switched on it passes a lot of current, which all goes through Q1 base-emitter junction and causes the heating. Q2 itself is saturated (or nearly so) so doesn't heat up significantly.

Why are TWO transistors used when the relay coil needs only 10mA and the input is only 1.4V?
A single transistor will work fine with a series 680 ohm resistor.

Why is a relay used? The LED and its series current-limiting resistor can be driven directly by the single transistor.

ericgibbs said:

hi and Welcome,
You have not said what the relay coil resistance is.??

The 2nd transistor will get hot , because its not being saturated, the Base current is too low.

What is the 1.4V source

Conventionally the two collectors are connected together

Click to expand...

resistance of relay coil is 200ohm, & 1.4V source is a normal DC supply from a 1.4 volt battery. Thank U for your reply?

alec_t said:

If the relay you are using really is an EMR011A12 (SP reed relay) then its coil should only draw ~ 10mA. That would not be enough to make either transistor hot. So it looks as though there is a short-circuit or wiring error somewhere.

Click to expand...

Sir, I was confused that I was wrong or circuit was wrong. It was working in multisim but not in practical.

and thank u very much for your reply.

MrAl said:

Hi,

If you want to drive it that way you need a collector resistor in Q2 collector to limit the current through Q2.
That's not a darlington arrangement, but they way you have it you can get lower sat voltage for Q1.

Click to expand...

Sir I don't know much about darlington arrangement, I googled and found it. After checking it on multisim I was trying to apply it on multisim.

alec_t said:

I agree with MrAl. Without the resistor, when Q2 is switched on it passes a lot of current, which all goes through Q1 base-emitter junction and causes the heating. Q2 itself is saturated (or nearly so) so doesn't heat up significantly.

Click to expand...

Thank U both sir, now I got that why the transistor getting heated I will again try it as you suggested.

audioguru said:

Why are TWO transistors used when the relay coil needs only 10mA and the input is only 1.4V?
A single transistor will work fine with a series 680 ohm resistor.

Why is a relay used? The LED and its series current-limiting resistor can be driven directly by the single transistor.

Click to expand...

Sir U are very right, now I have used only transistor BD139. I have not connected any resistance but working fine. Will it burn the circuit latter?

At last Thanks a lot for a brief and supporting reply from all of U seniors. This forum is very good.

One more thing that I need is to set a 555 IC for 4 minute time delay, how to do that, can anybody provide me information or any link?