4QD controllers Service section VTX and NCC series
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Introduction

This page described the changes on issues 18 through 24 controllers. Earlier boards used a different circuit, main change being a different current limit circuit. The expansion connector was also altered to easily accept a tacho feedback board.

This history should be used in conjunction with issue 24

• circuit
• circuit description
• layout

Issues

• 18
  First release of revised circuit.
• 19
  1. C 23 added. This is a ceramic capacitor between PWM output and earth, its function being to help suppress noise transients that may occur from motor brush gear.
• 20 and 21
  1. 100K added, pin C of expansion connector to earth. When the expansion connector is fitted, the link at TR14's emitter is scratched through and, if no device is fitted in the expansion connector, pins B and C should be joined by a header (supplied). If the resistor is not fitted then, if the header is not present, then output will drift up to full speed. The resistor is to prevent this.
  2. Earth track to Tr3 etc. rerouted. Previously, this ran from Tr19 to the right of IC. It now jumps through a link to the left of IC1. There was some indication with certain external wiring layouts interfering with the operation of the hiside under certain (never traced) conditions.
  3. Fuse track lengthened.
• 22 and 23
  1. Transistor numbers added to the PCB artwork. These numbers cross relate to those used on the Pro series for consistency.
  2. 10K resistor added across battery line. This is to discharge the main capacitor if the battery is disconnected. With the ignition off, the controller draws essentially zero current and the main capacitor can take (literally) hours to self-discharge. While it is still charged, the circuit is susceptible to handling damage if not treated as live. Even with the 10K, the discharge can take up to a minute.
  3. During issue 22, the base-emitter resistor of Tr17 was changed from 10K to 22K. If the battery connections are very bad, or the supply is of exceptionally poor quality, a high current transient can in theory cause the battery supply to drop excessively. If it does, so too can the internal supply 9v line. If this drops to around 6.5v (with the original 10K) then Tr17 turns off, turning on the hiside. It could therefore happen that both hiside and loside MOSFETs could conceivably conduct simultaneously resulting in high dissipation and failure.

     It is not quite clear under what wiring faults such a condition could occur, but it is recommended that this resistor be changed during service of issues 18 through 23 as a precaution.
• 24
  1. 10R gate resistors to hiside MOSFETs. This has little effect on the circuit's operation but should help reduce chain reaction effect of MOSFET failure.
• 25
  1. C11 on the circuit diagram is a 470pF which defines switching timing. With issue 25 it was split with 100pF in the existing C11 position and C11a of 330pF between the bases of the drivers and 0v. It slightly improves timing.

Other relevant pages

• Circuit Diagram NCC Issue 24
• How PWM works
• Pulse width modulators
• MOSFET current sensing
• Testing drivers and output

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