Hi,

I am working on a project at home, a Hot Wire Foam Cutter! It calls for a power supply and a schematic is given from hobbycnc.com. The power supply is going to supply 24volts and about 4-8amps to a circuit card. This circuit card in turn will give electricity to a wire which will be used to cut foam.

http://www.hobbycnc.com/downloads/powersupply.zip
It's the BOW POWER CIRCUIT.pdf

I purchased:

one 24 V.C.T. @ 5 AMP POWER TRANSFORMER (Model TX-245 from
allelectronics.com).
one 35 AMP 600 PIV
BRIDGE RECTIFIER (Model FWB-356 from allelectronics.com)
one Capacitor 40V 10000uF (Mouser P/N 75-36DE103G040AM2A from Mouser.com).
Bleed Resistors set up as 3pcs 5.6K 1/2W in parallel.

I hooked it all up and nothing blew up, but when I checked the voltage (at TB7) and I get 37.5 Volts.

Can you explain to me what is happening and what is going on?

Attached Files

BOW POWER CIRCUIT.pdf (51.1 KB, 17 views)

 

Yes your 24VAC is an RMS(root mean square) measurement. The peak voltage of an AC sine wave is SQRT(2)*Vrms. In your case
This is probaly different from your measured value because your line voltage is higher than average so that the AC output of the transformer is a bit higher.

 

Does anyone know why you need DC to cut foam?
BTW, you won't get 37.5V when you connect it to your Nichrome wire.

__________________
Ron (aka Rube)

 

Thanks for the quick reply!

I checked my AC voltage and it hovers around 121VAC. This transforms the voltage to 28.1VAC.

Then multipling

28.1*sqrt(2) = 39.7334VDC

then accounting for the voltage loss in the Bridge Recitifer

39.73 - 1.4 = 38.3VDC which is pretty close to what I am getting, but why am I getting 28 Volts out of my transformer when it says that it is a 24 Volt transformer?

 

I think transformers are generally rated at full load. That would account for some, or maybe all, of the difference.

__________________
Ron (aka Rube)

 

So, you are saying that if I hook up my nichrome wire across TB7 that should drop the voltage?

 

Hi jbchrist

That' correct, the output voltage of a transformer is always specified
at full load. The difference between no load and full load will be greater
with low power transformers, so don't be surprised if a 24 volts 5 VA
transformer gives you an output voltage between 36 and 48 volts.

on1aag.

 

The reason it will drop the RMS voltage is that it will cause more ripple (droop) on the capacitor. The RMS voltage will probably drop by a couple of volts, maybe more, depending on the resistance of the transformer.
If you really want 24 volts, get rid of the rectifier and the capacitor, and connect your fuse directly to the secondary of the transformer. I see no reason for DC in a foam cutter.

__________________
Ron (aka Rube)

 

The bridge rectifier and capacitor aren't even needed. You can run a foam cutter straight from the transformer, why someone would even think of running a foam cutter off DC if they had AC already I don't know.

__________________

 

The reason for the rectifier and the capacitor could be higher output
power. If you increase the effective voltage by 41 %, the effective
current will also increase by 41 %, doubling the power. A 24 volts, 6
amps transformer is easy to find, a 36 volts, 4 amps transformer might
not be readely available in local shops.

on1aag.

 

Which gets hotter, the nichrome wire or the transformer?

__________________
Uncle $crooge

 

I've never heard of a rectifier or capacitor setup that actually increased output power... The diodes are going to introduce loss and so is the capacitor, how does that increase power? It's heating element not a DC load where voltage regulation matters.

__________________

 

That's the only reason I can think of. I'm sure you could find a suitable transformer in the USA at Mouser or some other dealer. Maybe the DC solution was most convenient for the original designer. Maybe it was just ignorance.

__________________
Ron (aka Rube)

 

You can in fact get at least 30V DC from a transformer that only puts out 24V RMS AC. The transformer, however, needs a higher current rating than the DC output current by a factor of about 1.6 - 1.8 (I've seen both numbers).

__________________
Ron (aka Rube)

 

The reason I am using DC in the Foam cutter is the software and the circuit card will control the heat automatically from the software. That is why is needs to be converted to DC.