# HELP me design a discrete current sinking positive V regulator

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#### CharlieLaub

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I would like to design an adjustable hard limiter, adjustable in term of the voltage that it limits at. This is for a DC application, so I only need a single ended type.

I've designed the following circuit (see attached schematic):
• output from previous active stage passes through a 100 ohm series resistor
• next is a shunt diode, connected to the regulated voltage
• finally, the circuit connects to the load, for which the resistance is known (34k Ohm)

The regulated voltage from the LM317 is set to around 4V. When the input from the previous active stage is less than about 4.5V there is no current flowing through the diode, however, when 4.5V is exceeded the diode starts to conduct. As a result, the voltage into Rload is limited to about 4.60-4.65V. The diode shunts current away from the load, and acts like a voltage divider with the 100 ohm series resistance.

I'm currently using an LM317 adjustable reg in the circuit. This allows me to set the voltage at which the diode starts conducting and limiting the output voltage. There is a small issue - the LM317 can not SINK current, only source it. As a result of circuit sims, I know that I will need up to about 10mA - 20mA flowing through the diode for this limiter to work correctly. So my workaround is to set the "quiescent" current flow of the LM317 to about 20mA. Instead of sinking current, the LM317 can source LESS current, and this is how it can be made to act like a current sink.

But while it works, this seems like a bit of a Rube Goldberg solution to the problem. The other approach that I thought of using was just to tweak the rail voltage of the last active gain stage (it's non inverting) so that the op-amp itself limits the output voltage. That might work, too.

I am wondering if there is a simpler solution - a discrete regulator or other circuit that can inherently sink current that I can replace the LM317 with?

-Charlie

#### Attachments

• DC_clipper_using_LM317.GIF
14.8 KB · Views: 83

#### abraxalito

You could try a TL431 shunt reg - that'll be happy sinking current. Also try a transistor (as emitter follower) in place of the diode?

Last edited:
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