BC517 to relais voltage to low

[case-sensitive]

Hello,

I try to run a 5 V miniature relais on a BC517 to the schematic as shown
**broken link removed**
I thought it could run on a 6V transformer, which would get me 8.4 V. Works well in Multisim, but when connected to a power supply I need to crank it up to 12.5 V until the relais sees 3V and swithches on. What is going wrong?

Cheers,

Case

 

[JimB]

As drawn, the transistor is connected to the relay contacts, the circle with a "K" is the coil.

The relay coil has a resistance of 167 Ohm and you have 100 Ohm resistor in series.
This means that the relay coil (when connected correctly) will ony see 167/(100+167) = 0.625 of the supply voltage at best. And that is neglecting the Vce(sat) of the transistor.

So, make sure the relay is connected correctly and remove R12.

JimB

 

[Boncuk]

With a power supply (battery) of 6V the relay voltage should be 5.08V using a normal NPN bipolar transistor like a BC547 with a base resistor of 2K7.

There is no reason to use a darlington transistor at all.

Boncuk

 

[case-sensitive]

the schematic asks for a BC517, which is not in Multisim. Found a thread which said I could use the BC372 instead.
Without the R12 simulation says 7 V across the relais, nowhere near the real life, needed about 9V to get 3 V across the relais. weird. Simulation also gets 7.7V for the BC547

 

[Boncuk]

I suggest you revise the schematic and label where you measure.

 

[case-sensitive]

**broken link removed**

maybe it did not contact poperly, the transistor, just built it on breadboard. Should have soldered it onto prototype board. Only measured 3 V.

 

[Boncuk]

Breadboards get bad connections once a part with thick wires has been inserted.

You might take it apart and reshape the contactors or get a new one.

Boncuk

 

[case-sensitive]

its rather weird, need about 10V for the relay to open. Then its 3 V across the relay and 7 V across the transistor (C to E). The relay has about 150 ohm, so the transistor has 350 (150 *7/3)?
R11 6k or 4k does not really make a difference, B sees about 10V....
measured each connection, no faulty contacts.

 

[ronv]

Try 1k for R11.

 

[case-sensitive]

I did it acc to this sheet
http://www.datasheetcatalog.org/datasheet/siemens/BC517.pdf
**broken link removed**
but its meant to be this
http://pdf1.alldatasheet.com/datasheet-pdf/view/69074/KEC/BC517.html
**broken link removed**

so torning it around works, now I have to twiddle around with R11 and R12 until I get about 5 V across the relay. It switches on at about 3 V, so even 4 V will do. Has to stay on for a while. Does it matter if the coil sees DC for a while, a few hrs, at one time?

 

[audioguru]

The pins shown on the Siemens datasheet are WRONG. The American datasheets have much clearer photos.
So you connected the pins backwards and the darlington transistor did not work properly.

Why did you use a darlington transistor for the low current relay coil? The darlington has about 0.7V voltage loss. An ordinary transistor has only a 0.1V or 0.2V voltage loss.

A relay coil that has its rated voltage can remain turned on forever with no damage.

 

[Boncuk]

The pins shown on the Siemens datasheet are WRONG.

They are shown CORRECTLY!

It's the bottom view is usually published in German data sheets.

Boncuk

 

[audioguru]

They are shown CORRECTLY!

No. Look again.
Transistor pins should be shown looking at the flat side with the pins downward.
Siemens shows them as EBC which is for American 2Nxxxx transistors. European BCxxx transistors are CBE.

 

[case-sensitive]

So you connected the pins backwards and the darlington transistor did not work properly.

yes. Now it stacks up to the simulation

Why did you use a darlington transistor for the low current relay coil? The darlington has about 0.7V voltage loss. An ordinary transistor has only a 0.1V or 0.2V voltage loss.

A relay coil that has its rated voltage can remain turned on forever with no damage.

its going to be 3 of these https://www.electro-tech-online.com/custompdfs/2011/10/07-01-3105.pdf Takamisawa SY-5-K and a LED, seems to be less than 100mA acc to multisim. Fair enough, 20m per relay and 10 for the LED. Could have got away with a smaller transistor. But, must operating voltage is 3.5V, transformer 6 which should get 8.4V DC. So all I need is about 4 V.
**broken link removed**
K8 and 9 are to get more relais in parallel, getting their current from this shematic.

 

[audioguru]

Your transformer is not 6V, it is 12V center-tapped. The peak voltage is 8.4V but the rectifiers reduce it to 7.6VDC.
Without R12 and with a darlington transistor the relay coils get about 6.9VDC.

 

[Boncuk]

No. Look again.
Transistor pins should be shown looking at the flat side with the pins downward.
Siemens shows them as EBC which is for American 2Nxxxx transistors. European BCxxx transistors are CBE.

It's the BOTTOM VIEW!

 

[audioguru]

It's the BOTTOM VIEW!

Didn't you see my attachment of three datasheets? They all show the bottom view.
The Siemens one has the C and E pins shown backwards because its flat side is downwards and the other datasheets show the flat side upwards.

 

[case-sensitive]

They all show the bottom view.

still did not get to the bottom of that? So there are actually 2 versions, 1 Europe, 1 others? Or the Siemens is plain wrong? Since EAGLE has it the other way, too. Anyway,
**broken link removed**
thats what I am planning to do, single tap transformer, SL1 will connect to another PCB which hosts the 3 reilais in parallel. Plus there is an LED so I see its actually on.

 

[JimB]

As drawn, that circuit will not work.
There is no supply to the transistor collector and relay.

Another thing, why are you using a transistor anyway?
What is the point?
You have a low voltage power supply, you want to turn on a relay when the supply is present, just connect the relay to the supply.
You don't appear to need the transistor.

Or am I missing some other point here?

JimB

 

[case-sensitive]

As drawn, that circuit will not work.
Another thing, why are you using a transistor anyway?
JimB

suppose it was in the initial schematic because its not exactly DC what comes out after the two diodes. Thought I get better DC going, its for some audio project, AC into the relay may create humm. Suppose I can well do without it.