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2QD controller
Assembly instructions

The 2QD is a simple 2 quadrant (drive and regen braking) controller. This page describes how to build it and points to other resources that are likely to be helpful.

The page is, initially, released in a partially completed form. so ask (via the list please) if you want more info.

Parts List

*130-21PCB, 2QD iss 9
*439-48MOSFET, BUZ100S
*142-092QD heatsink
*133-9AAL cap 25mm len x 22mm dia 50v 2200µC12
728-FAM6.3 PCB vert
441-40TO220 insulator
441-41TO220 bush (2.5)
129-313F IDC
129-393M PCB Vert
131-46CR25 Resistor 33R
231-60CR25 Resistor 1K0
131-66CR25 Resistor 3K3
231-68CR25 Resistor 4K7
1131-70CR25 Resistor 10K
331-78CR25 Resistor 47K
531-80CR25 Resistor 100K
131-83CR25 Resistor 180K
231-84CR25 Resistor 220K
131-90CR25 Resistor 1M0
131-F9Preset Hor mtg 100K
132-40CR16 Resistor 10R
132-40CR16 Resistor 10R
133-10CE cap 10pC3
333-22CE cap 150pC1, C10, C11
133-28CE cap 470pC6
133-40PE cap 10nC13
333-50PE cap 100v 100nC7, C9, C14
233-60AL cap 100v 1µ0C4, C8
233-78AL cap 16v 47µC2, C5
839-01Diode 1N4148D2, D3, D4, D6, D10, D11
239-32PNP Tr BC327, 45v 500mATr4, Tr7
239-33NPN Tr BC337, 45v 500mATr3, Tr6
439-34NPN Tr BC546CTr5, Tr8, Tr9, Tr10
439-3DPNP Tr BC556/7CTr1, Tr2, Tr11, Tr12,
139-84Quad Comp LM339
239-DE500mW Zener 9v1D5, D9
139-EF500mW Zener 47vD1
440-23M3 x 10 Brass Slot Pan
440-38M3 Full nut, Brass
440-3CM3 Washer, brass


Critical components

Vey few components need special mention - almost all are very standard, with no cautions needed. But take note of the following points.

The LM339 has a nasty habit of being susceptible negative spikes on its inputs: that is the function of the diode between pin 10 and earth (that's standard practise on LM339s, where such spikes are anticipated). However, experience has shown that certain makes of LM339 are far more sensitive than others and with some makes, the diode does not do the job. We have found problems with GS339 (GoldStar) and MC339 (Motorola) so avoid these manufacturers.
We have never had such problems with ST or National devices.
Drivers, BC337 and BC 327, Tr3, Tr4, Tr6 and Tr7
When they are switching, MOSFET gates can take a high current, so we use driver type transistors. In fact the effect of using standard ones is to slightly increase the switching times: this is not injurious and its effect on performance is barely noticeable.
Other Transistors
These are not critical. PNP types are BC557C and NPN types are BC546C. These have base in the centre. Beware of using 2N types: these are backwards compared with BC types, having the flat on the other side of the package.
The parts list specifies BUZ100S. However the current limit values restrict the voltage across the MOSFET to around 0.7v. Multiply the Rds(on) by the continuous current rating of the MOSFET and you will find the voltage you get is below 0.7v for most MOSFETs. So you can really use any type of MOSFETs!
10R gate resistors
These, ideally should be of the subminiature CR16 size. The reason for this is that when MOSFETs fail, they often put the supply voltage back onto the gate. Under this fault, the drivers will probably fail short circuit. A standard (CR25 size) 10R is likely to get hot - hot enough to burn a hole in the board. Although CR16s are not guaranteed 'fusible' experience shows that they almost always simply fail open circuit, without causing a fire or a charred hole in the board! 2200
Main capacitor, 2200µ, C12
This is a fairly critical component: it must be the plug-in type, with formed leads - the type shown in the drawing.
You may get away with other sorts, but this type of application pushes the capacitors hard and these plug-in capacitors have thicker leads than the smaller, wire ended types. In a motor controller, a substantial proportion of the capacitor heating is actually in the leads which consequently need to be as thick and as short as possible. It is a noteworthy fact that capacitor leads are all nowadays made of steel, copper coated. But steel is a very bad conductor, both of heat and electricity.
Current limit adjustment preset.
This is optional: the 'standard' is to use a 47K fixed resistor. Use a 47K preset in place of the 47K fixed resistor only if you wish to be able to reduce the current limit. The current limit is necessary to protect the MOSFETs so it is clearly not safe to increase it above the standard value, unless you know enough to work out safe new values. However, be warned that if you use MOSFETS of too low an Rds(on), you increase the work the main capacitor has to do. The main capacitor is the limiting factor on the current this type of layout will handle and the work the main capacitor has to do is dependant on a lot of factors (battery lead length and inductance, battery type, motor etc). A more powerful controller (e.g 4QD's Scoota controller) needs two main capacitors and there is also a lot more copper around the capacitor terminals to act as heatsinking.
The board as designed is made for a Z shaped heatsink, as the drawing. However, it is easy enough to arrange other shapes of heatsink. The better the heatsink, the longer the controller will be able to give sustained high current. But, of course, the more heat you remove from the MOSFETs, the more likely it is that some other component will get overheated!


This circuit and construction is relatively simple: if you overload the controller for too long, components will heat up until something gets hot enough to fail. However - you may be surprised at just how hot things can get! If you deliberately cook the board to death - you will probably melt the solder before anything actually fails!

Solder can be melted

  • around the main capacitor terminals.
  • around the source and drain lead of the MOSFETs, usually the drive MOSFET.
  • around the power and motor terminals.

There is no thermal shutdown included in the standard circuitry but it's not exactly difficult to fit. The obvious place to monitor temperature would seem to be the MOSFETs, but there is no room on the heatsink to easily mount a thermistor. And, as we have just seen, there are several places which may get hot! No point in monitoring the heatsink, if your system places most strain on the capacitor! Exactly which 'link' is the weakest depends on your installation - the layout, lead lengths and the mechanical loading on the motor, and the way you use the machine it's in. It's easy enough of course to reduce performance, so that all components are safe. But that will significantly reduce the performance!

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Page's author: Richard Torrens