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I'm using a BJT in emitter-follower configuration to control the current flowing through a high voltage load (170 V DC). The base is connected directly to an analog output of a BH2221FV DAC.

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During normal operation the current through the HV load is Ic = (Vb - 0.6) / R1 (Ic = 4.4 mA max - Vb = 5 V max) and the base current is a few microamps, but if the load fails and the collector is left unconnected the base current rises up to 2 mA, far beyond the maximum allowed by the DAC output (1 mA).

There is a way to limit the current in such a case without affecting the normal operation? Should i put a base resistor and, if yes, in which way must be calculated?

Thanks!

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    \$\begingroup\$ That's not an emiiter follower. What does the DAC do when 2 mA Iout flows? Maybe its output accuracy falls out of spec, but does it burst into flames? If it doesn't actually suffer damage, then is there any point in worrying about the output current in a load fault condition. \$\endgroup\$
    – Neil_UK
    Dec 10 '20 at 14:11
  • \$\begingroup\$ Marco, why don't you consider something like this? It's self-limiting and, I think, more manageable in behavior, as well. I'd feel better about your DAC, as well. Just a suggestion. \$\endgroup\$
    – jonk
    Dec 10 '20 at 17:24
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You may be worrying too much about exceeding a 1mA limit- that sounds more like a limit for accuracy than safety of the chip. Looking at the datasheet it's a recommendation, not an abs. max value. I doubt metal failure modes are going to manifest at a couple mA, and power dissipation is not significant, even 12 channels at 2mA add up to less than 60mW.

A base resistor high enough to reduce the DAC current to 1mA would be in the 3.3K range, so the effect on the collector current in normal operation (gain of 40 minimum) would be less than 10%.

You could also add a reverse-biased diode from each collector to a voltage source (eg. 5V) that would prevent the collector from going lower than 4.3V so the transistor would never saturate (though bad things might happen if the diodes were installed backwards or broke down unless that 5V supply was protected).

But I don't think any of that is necessary.

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