help required to know transistor TCD100 details

[Briez]

I recently seen a circuit in which a TCD100 transistor is being driven by microcontroller to operate 12v relay. I searched on google about datasheet of TCD100 but I couldn't find. . Can any one tell me its technical details?..

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[D.A.(Tony)Stewart]

TCD100 is not a valid complete part number but learning to operate a transistor like a Relay is important.

Transistors have a linear current gain which is not the same as the saturated switch current gain. Due to typical silicon switch geometry, they only have a current gain of 10:1 in the VI curves which show Vce(Sat). Special types called Ultra Low Vce(Sat) switches are 50:1 and some in between.

The also have an effect Rce resistance when saturated which can be derived from the V/I curves or table values for Vce(sat)/Ic. This is similar to the MOSFET RdsOn.

Relays are much slower and bulkier but can have much higher current gains, we consider the same voltage for DC coil and contacts such as Automotive. The "best" Engineering designed and best quality Relays are made by OMRON and it is reflected in the quality of their component specs. Consider this 30A 12V relay

There are a few models with different coil resistance which affects the rated current, holding current and max current at max voltage, as these are designed to operate over the entire automotive range of 7.2 ~16V with 12V nom. The best coil is 225 Ω which draws 54 mA. The current gain for this coil is Isw_rate / Icoil = 30A /54 mA =555x

Remember this is 10x better than the best transistor. Some relays can be 2000x, and others 100x

So to drive a transistor switch on the low side of the coil requires 55mA so a conservative design can be driven from with mA base drive using OHm's Law for the Base resistor voltage drop. ( So 12V-0.7V)/6mA =1.9k ( or next lowest value) In some cases CMOS can drive this relay directly as CMOS on ARM chips has a 25 Ohm driver and others vary from 50 to 75 Ohm ( based on V/I drop).

In all cases the relay coil causes an inductive spike when open, that can be clamped with a diode that only needs to be rated for the coil current ( small signal diode in this case) and tied across coil in reverse polarity.

Always check coil resistance for determining the driver current from given worst case voltage range.

 

[wp100]

Hi,

You might also want to consider using a ULN2803A chip which can drive up to 8 relays; all extra components, as mentioned above, are built in to the chip so it connects directly to the Micro and Relay .

 

[Briez]

Yah that's right but I want to know about TCD100 transistor. ..

because I couldn't find its datasheet on google. ..

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[wp100]

Hi,

Likewise nothing showing under that code, without further circuit details it hard to say.

If the micro is connected directly to the transistor, ie no external base resistor, then likely its a 'Digital' transistor like these https://www.onsemi.com/pub_link/Collateral/AND9129-D.PDF