Using the Velleman K8042 with an AC-AC Wall Transformer
I just built a Velleman K8042 bipolar power supply that I got from Jameco: http://www.jameco.com/1/1/46848-k8042-s ... y-kit.html
The Velleman K8042 is a simple, easy-to-use and based on the popular LM317/LM337 series regulators.
Using the K8042 with a center-tapped or dual secondary transformer is pretty straightforward.
Transformers for conventional center-tapped full-wave rectification:
The Hammond 36VCT 30VA 1182-series toroid (PN 546-1182H18) is ideal for most light-duty bipolar supplies: http://www.mouser.com/ds/2/177/1182-736858.pdf
Triad toroid transformers (PN 553-VPT36-690) are also offered at Mouser: http://www.mouser.com/ds/2/410/VPT36-690-781808.pdf
Sometimes an external supply wall-mounted supply is required and a center-tapped transformer isn't available or practical.
I decided to see how well the K8042 performs using half-wave rectification using an untapped transformer.
Connecting an AC-AC "wall-wart" supply to the K8042 is simple: Only one pair of AC input terminals are used.
The photo shows the K8042 bipolar supply connected to a 2.1 mm power jack which provides bipolar output using a single transformer winding.
A selection of 120V-24V AC-AC transformers (or 20VAC which requires less heatsinking) can be found here: http://www.mouser.com/Search/Refine.asp ... rd=553-wau
These are the 20V AC-AC supplies from Triad: http://www.mouser.com/Power/Power-Suppl ... au&FS=True
240V to 24V AC-AC are available for international applications from other vendors.
The Velleman K8042 Bipolar Power Supply can be connected to a 24VAC wall transformer to provide +/-15VDC.
The output ripple and noise is typical for the LM317/LM337 <<500 uV.
Low-line performance was excellent when loaded by the Phono Transfer System.
The trade off by using half-wave rectification is efficiency. Transformer secondary current increases significantly.
Using the Phono Transfer System as an example, the DC load current is about 142 mA for both boards with all relays engaged.
The measured secondary current is several times that, approximately 430 mA RMS.
The rule of thumb is about 3.5 times the load current. See http://www.proaudiodesignforum.com/imag ... Design.pdf
If the positive and negative supply rails are unequally loaded, half-wave rectification can also cause DC current to flow in the transformer secondary.
If the imbalance is large, the DC current can increase transformer dissipation.
The Phono Transfer System again serves as an example:
The PTS' 12V relays are powered from the 15V rail through a 12V regulator. (They can also be isolated using jumpers.)
The +15V rail draws about 74 mA with the relays off, with the relays on, the current is about 140 mA.
When the relays are on, the DC current flowing in the secondary was confirmed to be about 70 mA.
70 mA (DC) is relatively small compared to the 430 mA RMS (AC) secondary current.
The heatsinks shown in the photo are about right for the PTS operating from a 18-22V secondary transformer.
The temperature of the positive regulator under full load is about 135 degrees F.
At 24V I would use a larger off-board heat sink, chassis mount the regulators (through insulators) or use the Velleman HS4003. http://www.mcmelectronics.com/product/V ... -/28-12972
For more information on heat sinking please see: viewtopic.php?f=12&t=448
The Velleman K8042 is a low-cost DIY bipolar linear supply that is ideal for use with both internal and external transformers. When using it in a half-wave configuration make certain that the transformer is sufficiently over-sized.