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Led working but relay not working with BC547

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vikas1450

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I want to drive 5v single channel relay by BC547, I am using 1k resistor at base but relay is not working. I replaced relay with a led and it works fine. So can anyone tell me why led is working but relay is not working.Also I tested voltage by replacing relay at the terminal and it was 5v but still relay not working. Please help.
 

I want to drive 5v single channel relay by BC547, I am using 1k resistor at base but relay is not working. I replaced relay with a led and it works fine. So can anyone tell me why led is working but relay is not working.Also I tested voltage by replacing relay at the terminal and it was 5v but still relay not working. Please help.
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The datasheet of BC547 shows a DC current gain in the order of 110 - 800. So biasing base with 1K resistor should be sufficient to drive the relay. What is the resistance of the relay coil. You might have replaced the relay by LED with current limiting resistor. What is the value of the resistor.
 

swapan said:
The datasheet of BC547 shows a DC current gain in the order of 110 - 800. So biasing base with 1K resistor should be sufficient to drive the relay. What is the resistance of the relay coil. You might have replaced the relay by LED with current limiting resistor. What is the value of the resistor.
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Resistance of relay coil is 70ohm, and in led I am using 1k resistor as current limiting resistor. I am not adding any external resistance to the relay coil.
 

Hi,

So did you consider what's the difference between LED and relay?
* different DC current --> calculate the base resistor according the countless descriptions in the internet
* relay is an inductive load --> use a freewheeling diode

Klaus
 

I am not adding any external resistance to the relay coil
Click to expand...

Maybe an unnecessary question, but did you check if the relay is really rated for 5v? if so, the only remaining options are, either burned parts or wrong circuit assembly; by the way, we are assuming you are connecting base resistor either to 0v or to 5v on these experiments.
 

Hello vikas1450,
Provided the relay you're using is rated at 5 volts, as suggested by andre_teprom, then
the circuit should work.
See pictures below:

foASAsV.jpg

The above showing the relay is active.

RDGqK59.jpg

The above showing the relay de-activated.

I used a 555 Timer to simulate the relay switching as an example.
I don't know what you'll be driving your relay with.
I hope the above helps.
Regards,
Relayer
 

The other possibility not yet mentioned is that BC547 comes with different pin outs, some are EBC some are ECB.
If wired incorrectly you could be sinking LED current through one of the transistor junctions directly to/from the source with the transistor not amplifying at all. The much lower current requirement of the LED would still let it work but there wouldn't be enough current to operate the relay.

Brian.
 

hFE does not apply as a saturated switch. You should use Ic/Ib=10~20 Vce(sat)=0.6V max

So for 4.4V/70Ohm=63mA Ib=3.2mA Min @Vbe=0.8typ and with Rb=1k , Ib=(5-0.8)/1k= 4.2mA Should be enough if you can verify the voltages.
 

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