Utilizing basic 3-leg adjustable regulators in shunt mode

When elements have been round as lengthy, and in such widespread use, because the venerable LM317 and LM337 (and their siblings), you are inclined to assume each trick that may be performed with them is already within the (information)ebook. So, when the query arose whether or not these extraordinarily properly understood elements, which can be at all times described of their respective information sheets as solely sequence regulators, would possibly in some way be related as much as obtain a shunt topology, it appeared fairly far-fetched.  Consequently, the opportunity of the circuit in Determine 1 got here as a shock. The chip inner schematic is useful in understanding how this monstrosity works.

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Determine 1 Three terminal regulator (e.g. LM317) within the (optimistic) shunt configuration.

When related as proven, the usual “three leg” will start to conduct as INPUT present is introduced from zero and Vout approaches the programmed setpoint voltage Vset, inflicting Vadj to method -1.25V. This causes the inner Zener cathode to subsequently rise to 0V, and the inner regulator management amplifier to activate the inner Darlington move system. These design equations will then apply:

Let Vref << -1.25, e.g., -5
Vset = -(1.25 + Vref)(R1 / R2) – 1.25
R1/R2 = (Vset + 1.25) / -(Vref + 1.25)
R1 = R2(Vset + 1.25) / -(Vref + 1.25)

When Vset is attained, U1 transitions to a really low-impedance (single-digit milliohms) super-Zener-like mode, as proven in Determine 2 and clamps Vout to the programmed setpoint.  Be aware absolutely the worth marks that present the circuit can accommodate each I/O voltage polarities with acceptable selections for U1 and Vref.

Determine 2 Shunt mode I/V curve; when the programmed setpoint voltage Vset is attained, U1 transitions to a really low-impedance and clamps Vout to the programmed setpoint.

For a selected design instance, let:

Vset = 12V
U1 = LM317
Vref = -5V
R2 = 1k


R1 = R2(Vset + 1.25) / -(Vref + 1.25) = 1k(13.25) / -(-3.75) = 3.5k

 Unfavourable polarity regulation could be accommodated by doing the next:

  1. Exchange the LM317 with a LM337
  2. Exchange the detrimental Vref with a optimistic Vref

Be aware the max/min absolute worth of 40V > Vset > 3V and 1.5A > Iinput > 10mA per 317/337 regulator datasheet specs.

C1 could also be required for dynamic stability. See datasheet suggestions.

Stephen Woodward’s relationship with EDN’s DI column goes again fairly a great distance. Practically 100 submissions have been accepted since his first contribution again in 1974.

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