# Transistor base resistor calculation

Hello. I am new here.
I need question and help about transistor base resistor calculation?

Power supply on base is 24 vdc (Base signal voltage). Relay work on 60v dc. Can i use bc517 for powering relay about 300 ma consumption.

What is transistor base resistor and how calculate it?

Base resistance =

(Base Signal voltage - voltage drop aka Vbe(sat) / (Collector current / Beta aka Hfe)

This is a formula but can someone calculate my question?

If the relay supply is 60volt, you can't use a BC517 (Vce max is 30v).
Leo..

Other post deleted.

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bbs1975:
Base resistance =
(Base Signal voltage - voltage drop aka Vbe(sat) / (Collector current / Beta aka Hfe)

This formula is for lineair mode, and somewhat useless for switching.
For switching, you want to saturate (reverse bias) the transistor.
Base current = 1/20 of collector current for average small transistors.

Is this a relay or solenoid? Must be a big one. 60volt/300mA = 18watt..
Look for a medium power transistor or darlington (e.g. TIP20).
Leo..

As has been so frequently pointed out, if you want the transistor to saturate - to achieve the lowest possible voltage drop and power dissipation when turned on - you do not use the small-signal Hfe (β) but a value of 10 to 20.

The Hfe changes with current, so use one that is appropriate to the collector current which will be found in the data sheet.
More than the minimum base current is fine but it also slows down the turn off time of the transistor so don't go silly with the excess.

I find tip122 transistor. Vce is 100v.

For Ic = 3a i need 3 mA drive at gain = 1000. If we assume a 24v supply then R1 is such as to allow 3 mA to flow when Vbe = 2V
R1 = V/I = (24-2) /0.003 A = 7333 ohms.

Is it correct?

Transisot work as a switch

R1 = V/I = (24-2) /0.003 A = 7333 ohms.

Yes but in practice you can not get resistors of that value and you want to give it a bit more so I would use a 4K7 resistor.

bbs1975:
I find tip122 transistor. Vce is 100v.

Well, for 300 mA, a TIP122 is OK, but don't get into the habit of using them for low voltage switching or significant current - more than an Amp or so.

For serious switching purposes, look for logic-level FETs.

Wawa:
This formula is for lineair mode, and somewhat useless for switching.
For switching, you want to saturate (reverse bias) the transistor.

Technical point: saturation has both junctions forward biased, unlike linear mode where the
base-collector junction is reverse biased. Linear transistor action relies on most of the carriers
emitted from the emitter going stright through the base and being accelerated by the reverse bias
field of the base-collector junction.

In saturation this mechanism is absent, so the gain drops a lot. When the collector-emitter voltage
drops below about 1--2V the gain starts to reduce dramatically.

MarkT:
Technical point: saturation has both junctions forward biased, unlike linear mode where the
base-collector junction is reverse biased. Linear transistor action relies on most of the carriers
emitted from the emitter going stright through the base and being accelerated by the reverse bias
field of the base-collector junction.

In saturation this mechanism is absent, so the gain drops a lot. When the collector-emitter voltage
drops below about 1--2V the gain starts to reduce dramatically.

Excellent explanation.

Moral: Use FETs!

MarkT:
In saturation this mechanism is absent, so the gain drops a lot. When the collector-emitter voltage
drops below about 1--2V the gain starts to reduce dramatically.

Yes, so DON'T use hfe to calculate the base current. Look at the data sheet to find the appropriate current to give a suitable Vce(sat).

Russell.