[sdiy] Current sink schematic/calculations help.

[Justin Owen]

-----Original Message-----
From: Veronica Merryfield [veronica at merryfield.ca]
Received: 16.12.2011 18:15:37
To: synth DIY
Cc: Justin Owen
Subject: Re: [sdiy] Current sink schematic/calculations help.

>> When measuring Iabc between the collector of Q1 and the INV input of IC1B I'm not getting anything that matches these calculations - they're not really making sense at all and changing R values isn't giving me anything I can get my bearings from. I'm getting values from a few hundred uA to a few hundred mA.

>How are you measuring Iabc exactly?

Measuring current between the Q1 Collector and the INV input of the I-V converter.

> Have you tried with just a resistor load for Q1 and Iabc?

As in removing the I-V converter, connecting a resistor from the Q1 Collector to GND and then measuring Iabc between the Q1 Collector and the resistor to GND? Yes, tried that. Also tried connecting that resistor back to the I-V converter - same 'switching' behaviour or barely any change in Iabc or max and min values of Iabc that I couldn't match to any calculations.

Any clues?

Thanks - appreciated.