Page Contents

• Introduction
• 4QD 150 and 4QD-200
• 4QD-300
• The circuits
• The modification

Introduction

Any motor controller must exit gracefully when the battery voltage fails: if battery failure is not properly handled, then the MOSFETs will live up to their other name, i.e. Smoke and Fire Emitting Transistor.

The 4QD series controllers automatically switch themselves off if the internal supply fails: this internal supply, being derived from the battery, is a measure of the battery - if it falls too low.

However there has been some evidence that, at high currents, certain batteries can have a cell fail in such a way that this protection is inadequate! What appears to happen is that the dud cell (which, if dud, will be conducting at reverse voltage, rather than providing power) can suddenly switch to an open-circuit. Since there is now no current, the cell starts to conduct again and this 'relaxation oscillator' seems to be too fast for the in-built switch-off to operate.

Normally, a good battery will drop its voltage steadily as it discharges and as the current increases. If the battery voltage gets too low - the controller simply stops working. However if the battery has a dud cell, it seems that the battery voltage may catastrophically fail so quickly that the normal controller switch-off is not fast enough. Undervoltage protection should prevent this.

In consequence, we will be supplying controllers with modified battery undervoltage protection. This undervoltage will be permanently engaged and will be owner adjustable between 14 and 40v. The controllers should work properlydown to around 14v, so keeping the undervoltage at this level should prevent the suspected failure mode.

4QD-150 and 4QD-200

To date, the problem only seems to occur at the high currents of the 4QD-300 (400 amps). So we are not retrospectively modifying the 150 and 200 models.

However Issue 13 baseboard are now in production and include the same modification. See the instruction manual.

4QD-300

This page details the modification as it can be retro-actively fitted to 4QD-300 controllers only of issues 01 through 03. Issue 04 will have the modification included.

The circuits

On the left is the old circuit, on the right, the new. Both use an 8v2 zener in series with the Vbe of the transistor as a source, with a resistive divider to detect battery voltage.
When the battery voltage is high, the 8v2 zener conducts and the transistor turns on. If the battery voltage falls sufficiently there is no longer enough voltage on the zener's positive to keep it conducting and the transistor base current fails. The 10K collector resistor now pulls up the collector and a voltage is fed to the 'ramp down' pin: this causes the demand speed to reduce, limiting the drain on the battery so the voltage cannot fall lower than the set level.

The new values give an adjustment range on the preset of 14v to 40v.

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The modification

Although a simple modification to the circuit board, fitting it involves complete disassembly of the base unit so these details are not intended as instructions for DIY modification. If you wish to fit the modification yourself, 4QD will accept no responsibility for the consequences. See the instructions for disassembling the unit.

The first diagram shows the component layout before modification with the component affected arrowed. This diagram should be entirely clear to anyone competent to do the modification! It shows both sides of the circuit board, viewed from the component side. Component side copper is shown in orange and solder-side copper is shown in green.

• Remove the 6 components shown with the red arrows (some of these will not be fitted).
• Drill out or otherwise remove the pad arrowed yellow (or cut the copper side track on one side of this pad).
• Drill out the three holes marked blue at accept the new 100K preset: 1.5mm is probably correct. You can use a 1mm drill and elongate the holes with this to better fit the pins of the preset.

The second diagram shows the board after modification: three components (arrowed in red) must be fitted:

• 22K resistor
• 100K preset
• 47K resistor

In theory this will give an adjustment range of 14 to 40 v. However the 40v maximum is with the entire track of the preset in circuit and preset resistors are inexact components. The maximum value will depend on the real value of this preset.

The table shows typical voltage against preset positions:

Position	U/V	For Battery
Fully clockwise	40
3 o'clock	37	48v
centre	27v	36v
9 o'clock	17v	24v
Fully anticlockwise	14v

The undervoltage must be engaged by linking the pads on the control board marked BDP. This is explained in the instructions, a copy of which is available via our instruction manual section.)

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Page Information

© 2003-2006 4QD
Page's Author: Richard Torrens
Document URI: www.4qd.co.uk /serv/4QDuvp.html
Page first published 29th March 2003.
Last modified: Monday, 28-Aug-2006 21:12:00 BST