Re: [CAD_CAM_EDM_DRO] Re: Power supply

Peter,

I stand corrected. I misread the specs on that particular transformer
as 0.500 kW, not 0.050 kW, as it actually says.

However, it seems to me that a transformer that is delivering 50 watts
at 12 volts would be doing so at approximately 4.17 amps. The buck
transformer doesn't really 'care' what you do with those 4 amps. In
buck operation, the new transformer is not actually delivering power to
the load, but instead, reducing the apparent line voltage seen by the
original power transformer, by about 10%. You could still pass 4.17
amps through the secondary of this buck transformer, and into the
primary of the original transformer without overheating the buck
transformer.

In turn, the primary of the original power transformer could handle 108
volts at 4.17 amps max before overstressing the buck transformer. I
have used this arrangement in many situations without burning anything up.

My analysis is that the original transformer puts out 65 VAC. Reduce it
by 10% (120 VAC Line - 12 VAC buck voltage, and you will have 58.5 volts
before the rectifier bridge. Subtract two diode drops in the bridge
(2*0.6 volts), (= 57.3 Volts) and multiply by sqrt(2) and you will see
81.2 volts on the filter cap, with no load on the supply. As soon as
you put any load on the supply, the output voltage would drop by an
amount determined by the size of the filter caps and the size of the
load. Sounds close enough for government work ;o). If you really
wanted to be sure, you could use the 24 volt strap option, drop the
primary voltage down to 96 volts, and the filter cap would never see
anything more than 71.8 volts, no load.

I believe that you could pull (108 primary volts * 4.17 amps) or 450
watts (VA, to be picky) out of the supply before the buck transformer
would overheat. That would correspond to 5.5 amps at 81.2 volts, though
we know that the 81.2 volts would drop immediately toward 57 volts as
soon as you apply any load to the filter cap.

The user never did spec how many watts he (or she) needed for the lathe,
so we are kinda shooting in the dark, anyway.

Dave


Peter Reilley wrote:

> A .05 KVA transformer that is driving a perfectly
> resistive load is capable of supplying 50 Watts.
> There is no getting around that 50 Watt limit.
> Volt-Amps (VA) is the same as Watts for a
> resistive load.
>
> This transformer can supply about 4 Amps at
> 12 volts or about 2 Amps at 24 volts. It will
> draw about .4 Amps if the primary is wired
> for 120 Volts or about .2 Amps at 240 Volts.
>
> >From this it can be deduced that the wire used
> in each of the two secondaries is capable of carrying
> about 2 Amps. The wire in each of the two
> primary windings is capable of carrying .2 Amps.
> You cannot exceed these current limits without burning
> up the transformer.
>
> Pete.
>
>
>
>
> ----- Original Message -----
> From: "David Speck" <Dave@...>
> To: <CAD_CAM_EDM_DRO@yahoogroups.com>
> Sent: Saturday, June 24, 2006 11:49 AM
> Subject: Re: [CAD_CAM_EDM_DRO] Re: Power supply
>
>
>
>> Peter,
>>
>> It's true that the buck transformer is rated at 4 amps, but if applied
>> to the primary side of the power circuit as is usually practiced, then
>> that means you would be drawing 4 amps through the 120 volt primary, or
>> delivering about 480 watts to the secondary of the original
>> transformer. The original enquirer didn't spec the size of the original
>> transformer, but 480 watts sounds like a reasonable initial guess.
>>
>> Dave
>>
>> Peter Reilley wrote:
>>
>>> That transformer is not adequate.
>>> It is spec'ed as .05KVA. That means that it is good
>>> for about 4 Amps when wired for 12 Volts.
>>>
>>> Pete.
>>>
>>> ----- Original Message -----
>>> From: "David Speck" <Dave@...>
>>> To: <CAD_CAM_EDM_DRO@yahoogroups.com>
>>> Sent: Saturday, June 24, 2006 10:39 AM
>>> Subject: Re: [CAD_CAM_EDM_DRO] Re: Power supply
>>>
>>>
>>>
>>>
>>>> This buck / boost transformer would do the job for $29.99 w/ $10.00
>>>> shipping on eBay's "Buy it Now"
>>>>
>>>> Dave
>>>>
>>>> GE BUCK BOOST TRANSFORMER CATALOG# 9T51B0102 Item number: 3853854254
>>>>
>>>> ====+====+====+====+====+====+====+====+====+====+====+====
>>>>
>