Help Understanding the Spec's of a Power Transformer - Page 1
Forum section: electronics / beginners

author: cvmccoy
author_rank: Newbie
timestamp: April 04, 2021, 03:45:43 pm
content: I recently retired and after 45 years, I'm starting to get back into electronics, a hobby I enjoyed as a teenager. My first project is a 6 Volt DC Power Supply.I'm using the Hammond Manufacturing 164H20 (HM4331-ND) power transformer to step down the line voltage to about 10 VAC (I'm using the transformer in parallel mode). With just the transformer in the circuit, when measuring the secondary voltage across pins 5 and 8, I am getting 12.56 VAC when suppling 120.5 VAC on the primary. I tested the transformer in series mode and got 25.14 VAC. Based on thespecsfor the 164H20, I would expect to get around 10.5 VAC in parallel mode and 21 VAC in series mode (adjusting the values about 5% for the higher supplied line voltage).I had ordered two of these transformers and each one exhibited the same behavior. The box and the sticker on both of the transformers I received indicated that they were indeed the 164H20. However, I’m wondering if these units have possibly been mislabeled/mispackaged. To me, the voltage reading would seem to suggest that the units are actually a 164H24 (12/24 VAC in parallel/series mode) instead of a 164H20. Or am I possibly misunderstanding how transformers are spec’d. I contacted both the manufacturer and the supplier of the transformer with my concerns and neither have yet to respond for clarification.Some addition information: When I add the Bridge Rectifier into the circuit, I measure 11.0 VDC across pins 1 and 4. When I add in the C1 and C3 capacitors, the voltage reading is 15.74 VDC. I'm using a True RMS DMM to measure the voltages. I do not yet own a scope to see what is really going on.Any insight or explanation would be appreciated. Thanks.

author: Benta
author_rank: Super Contributor
timestamp: April 04, 2021, 04:01:37 pm
content: The rated output voltage is specified at nominal load. Without load, the output voltage will be higher (think winding resistance).

author: David Hess
author_rank: Super Contributor
timestamp: April 04, 2021, 04:28:19 pm
content: Quote from: cvmccoy on April 04, 2021, 03:45:43 pmThat is all consistent. 12.56 volts AC is 17.76 volts peak, minus 2 volts from the forward voltage drop of the diods is 15.76 volts peak which is what you measure when the capacitors are added. Under load, that will drop because the capacitors will discharge between half cycles of the power line and from the resistance of the transformer.Without the filter capacitor, the multimeter is measuring the pulsing full wave rectified output from the rectifier.

author: tunk
author_rank: Super Contributor
timestamp: April 04, 2021, 07:17:12 pm
content: Your "5V" for the fan will not work.Either use a separate 5V regulator or a suitable resistor in series with the fan.

author: cvmccoy
author_rank: Newbie
timestamp: April 04, 2021, 08:20:13 pm
content: Benta, so adding a load close to the maximum rated current of the transformer (2.0A in parallel mode) would mean a voltage closer to the rated output voltage of 10.5V (parallel mode). Is my understanding correct? Currently, I'm only able to create about a 200mA load and I do see a a voltage drop of about 200mV measured at the secondary terminals (parallel mode). If that means a 2 amp load would thus produce a 2 volt drop, then the rated output voltage would line up.I appreciate your insight. It is most helpful.

author: cvmccoy
author_rank: Newbie
timestamp: April 04, 2021, 08:24:23 pm
content: David Hess, I very much appreciate the explanation. I hope to have access to a scope soon and will enjoy seeing this in action. Thank you.

author: cvmccoy
author_rank: Newbie
timestamp: April 04, 2021, 08:56:01 pm
content: tunk, the fan was a late additional and I hadn't actually breadboarded it yet. And you are right, it doesn't work. I felt a 5V regulator was overkill for my needs. But a resistor in series with the fan seems doable. Using a 22ohm resister with a 40mA fan or a 4.7ohm with a 180mA fan seems to work well. I appreciate your help and suggestion.