# Transistor Limit Current

I am using a LM395T transistor for an inductive load (a motor). The voltage limit is well within the 36V threshold. But current isn't limited. The specs of the transistor say that 1.5 A is maximum. Unfortunately the motor is big can drive up to 20 A of current.

Does the transistor have inherent current limiting ability? That will limit the input. Or will it get fried? Basically, do I need to change the circuit to limit current flowing through the transistor?

A transistor does nothing on its own. However, that is not strictly a transistor. It's an IC with a power transistor and other stuff in it.

Also motors do not drive current, they draw current. But I would say that no matter what you do 1.5A is far too undersized for a motor that can draw up to 20A. And such a motor would not be able to do very much running off 1A anyways even if the transistor doesn't fry.

There are two fundamental kinds of "limiting" as far as current is concerned. The first is to completely interrupt current when it gets too high.

The second is to still allow current to flow but limit the amount of voltage to the load so it doesn't draw too much current. But the extra voltage from the source has to go somewhere and it will appear across the component which will cook it at motor level power currents. This is very similar to how a linear regulator heats up when the input voltage is too high for the output current. Your IC looks like it uses this method which is no good for high power stuff.

This is no different than choosing a power amp to drive a very familiar linear motor called a speaker. The biggest difference is #if you intend change directions at full power briefly, then the Back EMF and forward drive doubles your differential voltage and with inertia , well you might as well increase the power requirements x4.

Load : if 24V @ 20A = 480 W (if# then *4)

Consider a good linear amp has a Damping Ratio of 100, which means the inverse impedance ratio of driver to load is 1/100.

If 20A is just the rated load and say Vcc=24V then you can bet the DCR impedance is 10% of that Vcc/20A=1.2 Ohms.

BTW: (Speakers are 25% ~ 50% DC and the rest is the mechanical acoustic impedance)

This TO3 is OK but still not much better than your average power transistor with 1.5 Ohms Ri. In fact, it is only 0.5 Ohms at 1.5A if you compute the slope 0.5.

For a "good" DC design my #criteria is RdsOn total=1% of Motor Rating V/I or 10% of V/Isc

Isc (short cct)=10x Irated is typical for an efficient DC motor. Therefore the DCR of the motor = Vrated /Isc

You want your driver to be 90% efficient say during a step acceleration transient? right? then 99% the rest of the time in light loads.

Now you see how why I use 1% driver impedance to Rated load impedance. Zr=Vr/Ir or 2% in a full bridge.

In case you didn't get it, a BJT is just an ideal transistor with saturation hFE Vce< 2V down to 10% hFE and it has an ESR or Ri equal to the V/I slope at rated current.

We call this Rce , yet they don't teach this in school., but Diodes Inc uses it on all daasheets in their "SuperBeta" power transistors (hFE>500).

It's basically the same as RdsOn in FETs.

The specs for efficiency are yours and then you deal with the cooling design and torque differences.

For another example see here. DC motor control ULN2803