# Solving algebraic loop while using RTBox **URL:** \ **Category:** RT Box\ **Tags:** plecs, rt-box\ **Created:** [February 15, 2025, 7:42am UTC](https://forum.plexim.com/t/solving-algebraic-loop-while-using-rtbox/1786 "2025-02-15T07:42:08Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![Camberlam](https://avatars.discourse-cdn.com/v4/letter/c/f07891/32.png) [@Camberlam](https://forum.plexim.com/u/Camberlam)\ **Post date:** [February 15, 2025, 7:42am UTC](https://forum.plexim.com/t/solving-algebraic-loop-while-using-rtbox/1786/1 "2025-02-15T07:42:08Z")
Hello: I met some problems when using RTBox with simulation composed by power modules. it seems okay offline but when I tried to use it on external mode, it shows the error with algebraic loops, is there any possible solutions to fix it? thank you. ![image](https://europe1.discourse-cdn.com/flex006/uploads/plecs/original/2X/9/950ece7dd81b9993bbba0e2105430b54493a187e.jpeg) ![image](https://europe1.discourse-cdn.com/flex006/uploads/plecs/original/2X/e/e82b0d14baa89b408c1c8e577c4f7e577128691a.png) [NPCDABrtbox2.plecs](https://forum.plexim.com/uploads/short-url/i52YW8okeJ7DkPBtyjdFrhEF1mq.plecs) (82.1 KB) ---
**Author:** ![Bryan\_Lieblick](https://dub1.discourse-cdn.com/flex006/user_avatar/forum.plexim.com/bryan_lieblick/32/10_2.png) [@Bryan\_Lieblick](https://forum.plexim.com/u/Bryan_Lieblick)\ **Post date:** [February 18, 2025, 4:59pm UTC](https://forum.plexim.com/t/solving-algebraic-loop-while-using-rtbox/1786/2 "2025-02-18T16:59:09Z")
From the component help of the “3-Level Half Bridge (NPC)” power module: > The module as a whole is modeled with controlled voltage and current sources. The DC side of the inverter bridge has current source behavior and must be connected to positively biased capacitors or voltage sources You need to remove the ESR’s in series with the capacitor banks. You will need to place a resistance in series with your voltage source to prevent a “state source dependency” error.