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voltage regulators protection circuit

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Member level 2
Oct 7, 2017
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Hi, guys I want to know what is the purpose of the transistor Q2 in this circuit.
I think it acts as a bridge between Q1 and the voltage regulator and balances the current between them. If the droop across RSC is higher then Q2 starts conducting and thus cuts the input current to the transistor Q1 and supplies current to the voltage regulator.

Q1 bypasses some of the current through the regulator. The base to emitter voltage dropped across R1 decides how much it conducts compared to the IC current. Q2 conducts when the voltage dropped Rsc is high enough and as Q2 conducts it 'shorts out' Q1's bias voltage. It makes a crude but simple current limiter, only allowing a little more than the regulator and R1 will allow if overloaded.

The formula for Rsc is misleading because it only takes into account the current through Q1. The Isc it refers to is really only Q1 current, some additional current will still flow through the regulator.


The problem:
As soon as Q1 current is limited ... now U1 tries to maintain output voltage by increasing it´s current ... until internal limit.
But I think, in that case, the whole short circuit current flows through the voltage regulators.

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Previously I thought it was short circuit protection for the voltage regulators :rolleyes:
so, what is the point of calling it short circuit protection? Where to save the additional protection device we have to sacrifice the main element(voltage regulator) or wait for the regulator's overcurrent mechanism to kick in.

Most regulators of the kind used in this configuration (780x series) have built in protection from over-current and over temperature but before shutting down (and hence removing Q1 bias as well) there would be a risk that Q1 itself overheated. There is no rule about the proportion of current that would flow through the regulator and Q1, a good starting point for calculating R1 would be to assume Q1 started to conduct at about 75% of its maximum rating, that would ensure it's thermal shutdown would still operate at a reasonable current. A good practice is to mount Q1 and the regulator on the same heatsink so either one of the overheating would shut it down.


this doesn’t make a lot of sense to me.All you’re doing is limiting the current through Q1 and counting on the regulator to limit anything beyond that. So, what are you accomplishing? Just get rid of all the external stuff and you’ve got the same result.


But I think, in that case, the whole short circuit current flows through the voltage regulators.
As I already explained in your previous thread.
Nothing changed since then.
It still is a overcurrent protection.

Maybe you could be more clear of what you expect. .. How is your idea of an overcurrent protection?


I tried SPICE sim, two different models, cannot seem to get it to function
as a short circuit limiter, keeping in mind the reg has Isc internal as well.

The defining equations :


Should denom be (Ireq max(B + 1) - Io max), eg, brackets around those terms ?
Otherwise its unit inconsistent.

Regards, Dana./

The fundamental point is that the current flowing in to the regulator is (almost) the same as the current leaving it. If the regulator shuts down, so does the current flowing through its 'IN' pin so the bias is removed from Q1 as well.


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