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Building servo driver


Have any of you built the servo driver illustrated at the ELM web
site? Here is a link to the schematic I
am intending to build this circuit for a miniature (watchmaker’s
size) cnc lathe but I am not certain that the schematic is correct
and would value the opinion of someone more versed than I in matters
electronic. The thing which is worrying me is the connection from
the motor power supply to the logic power supply. The circuit shows
the motor supply of +6 - 15v (I would actually like to use 24v and am
using higher voltage FETs) connected directly to the output of the
7805 regulator and the motor 0v connected to ground. I don’t see how
that could work and wonder whether maybe the 0v line should be
floating? Any advice would be appreciated.

Best wishes,
Ian W. Wright

Ian, my own weakness is electronics, meaning I’m no good at it
either. Here’s some links about homemade CNC that might be helpful,

There’s more, but there’s always more. Quite the cottage industry
there…Good luck!

Ian, sorry for the long reply.

I have not built this circuit.

You have misread the diagram, the input of the 7805 is connected to
the power supply (pin 1), the output (pin 3) goes to the logic and
interface circuitry.

Do not use 24 vdc on this circuit, you will spend much time
repairing. Reason; First, the input zeiner, does not show a spec.
Could be over-voltaged at 24vdc and it is only rated at 3 amps. 7805
regulator is a limited power handling capable device as a TO-220
package, unless you get it in a TO-5 configuration, at 24 vdc you
will have a potential of 19 watts developed during regulation, the
device is rated at 15 watts max and you better have extremely
effective cooling getting near 15.

I/O is rated as RS 232 levels, if you float ground, plan on buying a
new computer, you will burn out the RS 232 port and whatever is
connected to it, simultaneously with the logic circuit in the
diagram. Using a beefier FET will buy you nothing as everything else
will be dead. Additionally Q5,7,9 and 11 would have to be re-rated
for the installation, they will more than likely expire at the first
kick of the servo under load.

Use the circuit as is, 15 VDC at the input with a Max of 8 Amps will
supply 120 watts and spin this device with considerable force, you do
not need to go to 24 volts to do so would require a major redesign of
an otherwise good circuit.


The circuit shows the motor supply of +6 - 15v (I would actually
like to use 24v and am using higher voltage FETs) connected directly
to the output of the 7805 regulator and the motor 0v connected to

Look again, the 6-15 V supply is connected to the input pin of the
7805 not the output pin. Also if you use 24V as a supply you will
want to make sure you have the 7805 well heat sinked as it will
dissipate more of its power as heat at higher input voltages.

James Binnion
James Binnion Metal Arts


I have built several machines using driver boards from xylotex.

Never mind that I hold an engineering degree, and various electrical
engineering patents, purchasing that driver and matched motors was
cheaper, and quicker, than building my own circuit. hystreresis, eddy
currents, fourier inductive ringing, hexfet voltage spike induced
breakdown, just keep all of those as fancy sounding phrases and buy a
pre built board. you will save alot of effort, and get better

the boards are like $125-$150.

also, use stepper motors, unless you are building a large (weighs
more than 1000 KG) machine.

good luck

Thanks for the replies. Terry, The only reason that I was thinking of
a 24volt supply is that the motor I have which I was intending to
start with is rated at 24volts. I already have a small 4-axis cnc
milling machine which I built from scratch but which is driven by
stepper motors. For most things is is OK if a bit slow but I now
want to be able to use it to cut threads and do rigid tapping and the
existing rotary axis is just too slow to get a good finish. So I am
intending to mount a watchmakers lathe on the machine table and
drive the lathe spindle with a servo motor to get smoother and faster
rotation. Ultimately I’ll probably replace all the motors with
servos but I have a severe limitation on funds for the project -
hence the surplus motor and ‘build-it-yourself’ electronics. Looking
at the spec for the 7805 it seems to indicate that it is good for up
to a 35volt input - with decent heat sinking. Now I realise that I
hadn’t looked carefully enough at the schematic and the 7805 is
working ‘the wrong way round’ - i.e. not as I had assumed left in,
right out, it all makes sense. I think I’ll have to take my brain out
and polish it!!! The zener you mention is a 27volt one and all the
FETs (1N530 and 1N9531) are 60-100volt (none of the ones specified in
the schematic were available over here so I have had to look for
equivalents), perhaps I could change the zener for a 35 volt one
without upsetting things too much as it is only for transient
suppression. However, I will try 15 volts as the supply to start with
and see if the servo has enough power for my needs.

Best wishes,
Ian W. Wright
Sheffield UK


The 35v zeiner should work, at 35 it is still in spec for the
2SC2712 and there is limited opportunity for an over current.

Yes, the input range for the 7805 is 35VDC, but it is really limited
as to how much current is available at that voltage differential.

Good luck on your project, if you need help or want to discuss, drop
me a line.