RMS Voltage and dips. - Page 1
Forum section: electronics / beginners

author: sureshot
author_rank: Regular Contributor
timestamp: July 31, 2018, 10:25:59 pm
content: I have a transformer recovered from a previous project. My intention is to wire it up in a case and rectify and filter it's output for testing linear power supply circuits. The transformer is 2 x115 Volts primary, and 2 x 15 Volts secondary, at a power of 300 VA. In its previous use it powered a linear power supply circuit with 4 x series pass transistors.The issue I have is the power supply it came from dropped at least 1.4 Volts under heavy loads 10 Amps plus. The capacitance of the filtered input was 28000 uf 35 Volt electrolytic capacitors, which I thought would handle the ripple current easily. The capacitors where through hole radial, but not high ripple snap in type can electrolytic capacitors.I have built a MOT power supply in the past with a rewound secondary winding of 14.70 Volts, that circuit uses 90000 uf of capacitance, a low dropout regulator and only dips 0.2 Volts at 10 Amps. So as I'm going to be using this reclaimed transformer for a testing device, I could do with out to much dropping under load tests. I'm not sure why it dipped in voltage in it's previous application, but it did. Anything I test with it I'm shooting for just over 20 Volts rectified and filtered. The only difference in the two power supplys MOT, and linear series pass units where the transistors. In the MOT they where MJ11015 pnp, and in the unit I reclaimed the transformer from they where MJ4502 pnp transistors.So I've got just over 15 Volts AC with this transformer that's reclaimed, I didn't want to go silly on the rectified capacitance filtering, but think I need about 50000 uf of high ripple snap in capacitance. So I figured the transformers maximum rating is 10 Amps for each secondary winding in parallel. A total of 20 Amps, it's a good quality toroidal transformer made by Vigortronix. Is it possible the previous application circuit it was in caused the voltage drop ? All wiring was rated at 20 Amps for cable in it's previous use. I'm just at a loss as to why it dropped that 1.4 Volts under a 10 Amp load.I know there is probably no definitive answer, but looking for help on possible probabilities to this issue.Thank you for reading, any ideas appreciated.

author: rstofer
author_rank: Super Contributor
timestamp: August 01, 2018, 02:31:55 pm
content: Capacitance will help reduce ripple between cycles but it's the transformer internal resistance that is dropping the long term voltage.Just add a load to the transformer without the rectifier and capacitance. See how much the RMS value changes from light load to full load.Capacitance won't overcome voltage drop in the winding.The other problem with excess capacitance is that the transformer has to provide all of the power required during a small conduction angle. You should research this, Google has plenty of info. Lots of extra heating due to I2R when I has to go crazy high due to a small conduction angle.

author: drussell
author_rank: Super Contributor
timestamp: August 01, 2018, 04:43:41 pm
content: Quote from: rstofer on August 01, 2018, 02:31:55 pmYes. This is the OP's main issue.The OP might also consider using a choke-input filter, or at least researching it to understand how thewaythe current is drawn from the transformer has many follow-on effects.

author: sureshot
author_rank: Regular Contributor
timestamp: August 01, 2018, 05:57:51 pm
content: Thank you for your replys, these are certainly things I was unaware of, in fact never heard of before. So yes I will look closley in to these processes when high current is drawn. I've always thought that as long as the RMS voltage was sufficient, a high level of capacitance would keep the output voltage stable. In the case of the MOT (microwave oven transformer) power supply I put together, I only managed to get windings on the secondary up to 14.7 Volts AC. With the capacitance I added (a guesstimate) 90000 uf it's doesn't dip even above 10 Amps of halogen lighting load. Although I did use a 12 Volt low drop out voltage regulator to maximise my chances of a steady stable out put under load.So my 300 VA 15 Volt transformer should behave the same way I would have thought, but it doesn't at around or greater than 10 Amps. All I could attribute it to was the MOT had very heavy gauge wire for the secondary winding, from memory that was 12 AWG cable. Maybe the wire diameter kept the voltage from sagging under heavy loads. But I will definitely check out your suggestions, thanks again.

author: sureshot
author_rank: Regular Contributor
timestamp: August 01, 2018, 10:41:03 pm
content: I've not run the MOT power supply for long in the past. But with a 14.70 Volt AC no load, after 3 Hours with a 10 Amp load it's at 14.30 Volts AC and the DC rectified is at 17.20 Volts. The regulated output to the load is 12.58 Volts steady. So even though this old MOT transformer is a repurposed rewound unit, it seems to hold up very good. Hope the 300 VA 15 Volt transformer does in a new circuit. Just for interest MOT was at 80°C after 3 Hours running with fan cooling, fan under volted. I can only imagine it's the 12 AWG secondary in the MOT transformer.