Re: [CAD_CAM_EDM_DRO] 80v power supply

Ron Ginger wrote:

> As I noted in another note, Im getting into servo motors! My first
> problem is that the motors Ive found are 80v, 4Amp motors. I have not
> been able to find an 80 v supply.
>
> So, it seems to me that since the power line is 120 v I shuld be able
> to
> do a simple power supply. Suppose I took a light dimmer- the common
> triac variety, and hooked its output to a bridge rectifier, then to a
> filtetr cap. I should be able to adjust the dimer to the right value
> to
> hold the output voltage to 80v.

No, you can't do this. A dimmer pulses the power on and off. The peak
voltage
stays the same down to 50% dimming. Triac dimmers will not perform well
at
all running a transformer, anyway. You would need a special
transformer-type
dimmer, and then things will buzz ominously. Transformers don't like
big pulses and
non-linear waveforms in a big way.

Not using a transformer, as you actually proposed, is a MAJOR safety
hazard!
It means that ALL the servo amp and motor wiring will be carrying the
line
voltage on them, and the danger of shorts and contact with the wiring is

just too much. Also, you would need some kind of isolation between the
servo amp and the computer. Very few servo amps offer this isolation,
preferring to use a transformer at the input. (Some servo amps have
this
built in, or use a switching supply that provides the isolation
internally.)

>
> I understand the issues of not haivng an isolation transformer here,
> but
> I can work with that, Will the basic triac work to give me a lower
> output voltage, or will it be to badly regulated?

It would be badly regulated, if it would work at all. Most likely it
will NOT work, as
the bridge rectifier/capacitor filter doesn't look (electrically) like a
light bulb
at all! Try a standard light dimmer and light, and turn on and off a
few appliances
while watching the light. You will most likely notice the light getting
brighter and
dimmer, in much greater amounts, than without the dimmer. What is
happening
is the dimmer is actually AMPLIFYING voltage fluctuations.

> Any suggestion for a simple regulator that will work here?

Yes. Go to a few scrap yards and look for 'dry transformers', 'step
down
transformers', etc. These are small, point of use devices used to step
down 240 V to 120 V, for instance, to power lights, fans, electronic
devices, etc. in machine tools, inductrial air conditioning units, and
other manufacturing gear. They usually have 4 windings, so you can set
them up for 440 -> 220, 220 -> 220 or 220 -> 110 volt applications,
isolating the secondary in each case. They are typically rated in KVA.
If you set it up for the 220 -> 110 V setting (primary in parallel,
secondary
in parallel) and then supply 110 V to the primary, you will get 55 V AC
out. Full wave rectifying this output (with capacitor filtering) will
give roughly
78 V DC! How convenient! Now, using the transformer at half voltage
cuts the KVA rating in half, so if you need 1 KVA, you should get a
2 KVA transformer. That 1 KVA at 55 VAC represents something like
18 Amps! You could use such a transformer for 18 Amps peak, but
probably should derate it some more due to the low power factor
of the rectifier load.

One other thing you should do, is to actually put the secondary windings

in series, and use the 2-diode, center-tapped full wave rectifier
circuit. This may actually allow the transformer to run cooler, and you

get approximately 1 V higher output, due to one less diode drop.

You should be able to find such transformers for less than $1 / Lb,
without working too hard.

Jon