TL431 based Relay

Hi,

biggest problem I see with your implementation is the collector resistor with 1k. This resistor will limit the coil current heavily, and also with respect to the rated coil resistance, the applied coil voltage will not be sufficient to energize/close the relay.

E.g. the 12 VDC version of this relay has a rated coil resistance of 360 Ohm, which leads to an applied coil voltage of only 3.17 V.

Get rid of your collector resistor. The maximum collector current determined by the relay is about about 40 mA, the base current is about 6.5 mA. Assuming a maximum collector-emitter saturation voltage of the 2N2907 with 1.6 V, would lead to a power dissipation of 64 mW. An assumed base-emitter saturation voltage of 1.3 V will lead to additional ~8.5 mW. This total worst-case power dissipation of 72.5 mW, is not a problem for the transistor in the TO-18 case.

BR

you cannot use TL431 like this...it draws 1mA bias current and make the Q2 come on regardless...use the ZR431 instead....or maybe the TLV431, but that is only 1.24v ref

Hi,

cupoftea said:

you cannot use TL431 like this...it draws 1mA bias current and make the Q2 come on regardless...use the ZR431 instead....or maybe the TLV431, but that is only 1.24v ref

Click to expand...

I do not get where you come up with 1 mA. The TL431 can sink 1 mA up to 100 mA, but its off-current is in the low µA range.



BR

Thanks thats interesting...i dont know what "off state cathode current" is...but TL431 will always draw 400uA to 1mA of "bias current" when it has more than 2.5V on its cathode.

Hi,

cupoftea said:

but TL431 will always draw 400uA to 1mA of "bias current" when it has more than 2.5V on its cathode.

Click to expand...

I´m sure this is not true.

Just look at the "Test conditions" for I_OFF. It clearly says:
* V_KA = 36V !!!
* Anode = 0V
* V_ref = V_REF_Anode = 0V
(test circuit 8-3 @ TI datasheet)

Klaus

cupoftea said:

i dont know what "off state cathode current" is.

Click to expand...

You should know that it's a well defined datasheet property. It's measured for Vref = 0, thus irrelevant for discussed operation mode. On the other side Vka versus Ik is specified for Vk = Vref only. But as long as no other specification exists, I'd expect that Vref versus Ik looks similar, can't guarantee Vref of 2.5V for Ik < 1 mA max. respectively 0.4 mA typ.

Thanks, you are right, if the Ref pin of TL431 is connected to 0V then it does appear that the cathode current sinks down to 1uA (100nA to 1uA).
This is a surprise, since the cathode is actually also the Vcc pin of TL431, so its surprising that it suddenly stops drawing the 0.4-1mA of bias current when its ref pin is taken to 0V.

Hi,

cupoftea said:

so its surprising that it suddenly stops drawing the 0.4-1mA

Click to expand...

from the datasheet it seems it NEEDs a minimum cathode current of 0.4...1.0mA for proper regulation.

But it is not 100% clear.

Klaus

FvM said:

You should know that it's a well defined datasheet property. It's measured for Vref = 0, thus irrelevant for discussed operation mode. On the other side Vka versus Ik is specified for Vk = Vref only. But as long as no other specification exists, I'd expect that Vref versus Ik looks similar, can't guarantee Vref of 2.5V for Ik < 1 mA max. respectively 0.4 mA typ.

View attachment 191174

Click to expand...

This snippet is showing that the TL431 is in cutoff at (0V ≤ Vref < about 1V). At this region, the cathode current is about 0uA. It is in the linear region at (about 1V ≤ Vref < about 2.5V). At this region , the cathode current increases, although not linearly, from about 0uA to about 400uA. It is in the saturation at Vref = about 2.5V. So in saturation, the cathode current increases rapidly above 400uA (limited by the resistance in the path) without any further increase in Vref. The saturation region is where it needs to be operated in this application. This simply tells us that for your relay circuit, you should design for IR5 (= Ib + IR3) to be greater than 400uA. Whatever value IR5 is to be designed for, Ib has to be sufficient to cause transistor (Q2) to operate in the saturation region and Ic has to be sufficient to turn on the relay, considering the relay coil resistance.

Notice that VR3 + VR3 = 12V - 2.5V because Vka = Vref.

Cheers!

stenzer said:

Hi,



I do not get where you come up with 1 mA. The TL431 can sink 1 mA up to 100 mA, but its off-current is in the low µA range.

View attachment 191169

BR

Click to expand...

So just like FvM mentioned regulation is not guaranteed at Ik < 1mA. So IR5 should be designed to be greater an 1mA. Just being greater than 400uA does not guarantee regulation.

Hi,

ok as pointed out by the graph attached by @FvM the PNP transistor will get conductive and will turn on the realy. This can be overcome/delayed by increasing the time constant of the RC network connected to the Vref pin (e.g. increasing C1). But of course, than the circuitry is not that well defined as by depending on Vref only.

BR

Akanimo said:

This snippet is showing that the TL431 is in cutoff at (0V ≤ Vref < about 1V). At this region, the cathode current is about 0uA. It is in the linear region at (about 1V ≤ Vref < about 2.5V). At this region , the cathode current increases, although not linearly, from about 0uA to about 400uA. It is in the saturation at Vref = about 2.5V. So in saturation, the cathode current increases rapidly above 400uA (limited by the resistance in the path) without any further increase in Vref. The saturation region is where it needs to be operated in this application.

Click to expand...

The TL431 is more of a zener diode than a transistor. So here I should have been saying breakdown rather than saturation. So the snippet posted by @FvM shows that it breaks down at Vref of about 2.5V (2.495V typical, with min and max values also specified) with an Ik current draw of 400uA typical with some drawing anywhere up to 1mA. So limiting IK to below 1mA in your application may starve the TL431 and so it may not regulate.

Just had to correct the impression about cutoff and saturation.

Akanimo said:

The TL431 is more of a zener diode than a transistor.

Click to expand...

I don´t agree. Your statement is only true for the datasheet test setup when REF and K are connected, makig it a 2 pin device.

In the user´s schematic it is a 3 pin device. REF is independent of K.
Then the circuit more works like a transistor with a V_BE_th of 2.5V. But even then - if you go into detail - you see it does not work this way.

Let´s focus on the OP´s circuit.

Klaus

You can use the transistor level equivalent circuit to evaluate the behaviour for operation points not specified in the datasheet. Transistor designators according to the transistor level model given by Helmut Sennewald. You can show that minimal operation current is mainly defined by the parameters of band gap reference Q1/Q2.



You see in simulation that minimal operation current for correct Vref is almost independent of Vka, thus above shown diagram "Cathode current versus cathode voltage" can be also read as "Cathode current versus reference voltage" for Vk > Vref.