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Electronics => Beginners => Topic started by: hot_wire on July 01, 2013, 01:24:17 am

Title: Trimmer pot in dummy load design
Post by: hot_wire on July 01, 2013, 01:24:17 am
Hello

I have been looking into dummy loads recently and I came across the following design which has been on display on the EEVBlog:

http://www.arachnidlabs.com/blog/2013/02/05/introducing-re-load/ (http://www.arachnidlabs.com/blog/2013/02/05/introducing-re-load/)

This appears to be the schematic corresponding to the load:

https://github.com/arachnidlabs/reload/blob/v1/reload-schematic.png (https://github.com/arachnidlabs/reload/blob/v1/reload-schematic.png)

I am trying to understand this design and most of it is rather straight forward, but I have one question. I noted that the SENSE voltage is fed into a second OpAmp stage (IC1$G2) which has a gain of 2. My guess is that this second stage serves the following purposes:

1. 100 mV measurement output voltage corresponds to 1 A which is much nicer to read than 50 mV ~ 1 A.
2. It is not possible to mess around directly with the SENSE node. (I.e. applying voltages by mistake, attaching a low-impedance meter or shorting it out, etc.)
3. The trimmer can be used for calibration adjustment.

I am however wondering whether it is relevant that the trimmer is part of the second stage or whether it could be designed into the feedback loop of the first stage just as well.

Thank you
Max
Title: Re: Trimmer pot in dummy load design
Post by: Odysseus on July 01, 2013, 05:52:18 pm
It certainly can be added to the first stage by moving the entire resistive feedback network from the second opamp to the first, but just connect R4 to the sense node instead of the output of the opamp.

However, this doesn't really accomplish quite the same function.  Now, the current range of the dummy load will be increased by factor of 2, and measuring the real current draw must be done by measuring across the sense resistor itself.