Ac Mains rms Regulator P.Fry 27 th Feb 2015 Mains voltages may induce ventricular fibrillation leading to death. This only takes a fraction of a second with currents as low as 30 ma. This article describes a mains connected power supply that should not be reproduced unless by persons skilled or qualified to do so in accordance with mains voltage safety practices. Overview The following describes the development of a 240 volt rms regulator prototype built specifically for the HRO operating in the 60 to 100 Watt. Mains in this location varies from around 236 to 244 and often moves around those limits in minutes and whilst the HRO is remarkably stable this does produce an annoying frequency drift on cw and sideband especially when trying to use the Xtal filter. Based on my observations during use as opposed to laboratory analysis HRO drift falls into three categories. 1. Fast < 2 second response to HT shifts. 2. Slower < 30 seconds due to heater voltage thermal changes. 3. Longer Hours warm up drift. Causes 1 and 2 don t necessarily move in the same direction immediately giving rise to a jump in frequency followed by a re stabilisation back slightly as a new thermal equilibrium is reached. I wanted stabiliser design that did not modify either the HRO or its power supply stabilisation being achieved by varying the mains voltage. This proved more difficult to achieve in practice than my first sketches back at the beginning of January implied. At one point I was going to give up and buy a 100 200 VA constant voltage transformer but having used them at work I know they are heavy expensive and get quite hot. However perseverance seems to have paid off and I now have a circuit that stabilises the rms value fed to the main transformer and gives the best stabilisation I have achieved to date with no modification to the HRO or its power supply. Background. Initial attempts used a circuit to model the HRO HT level and then use that as feedback into various saturated core magnetic amplifier transformer arrangements. That is I arranged 240 to 12 volt transformers so that the 0-12 volt side modified the mains voltage, control was achieved by varying the saturation limit of combinations of other voltage steering transformers. Control current to the arrangement was DC output from a proportional Plus Integral controller. PV being the voltage from my modelled HRO HT circuit. This worked enough to warrant further work but I had difficulty
getting the required voltage range to cover all the mains variations also my HRO HT model circuit which consisted of a small transformer and full wave rectifier was not tracking very well resulting in HRO drift even though the voltage appeared to be stabilising. ( I had not realised the thermal heater drift significance at this point) It was whilst trying to use a boost arrangement for the voltage using 0-12 volt transformers that I realised a VERY Significant safety issue in that if any of my control transformers became disconnected or I had a high current fault then the transformers could generate voltages way beyond design. Remember emf is proportional to webers per second so if you push a transformer from Iron saturation NORTH to Iron saturation SOUTH the transition time is what gives the voltage and the more you saturate the core the shorter the transition time becomes and hence higher voltages. Under very controlled safety conditions I tested this theory on one of my control transformers the voltage rose to 350 volts on the 240 volt windings. These circuits are published on the internet with NO warnings so take care. On the grounds of safety I scrapped my whole design and went back to the drawing board at this point for that very reason. Magnetic Amplifiers and saturated core reactors. Further internet research showed that the 3 limb saturated core reactor could be used in the type of circuit I had in mind. These overcome the difficulties of high voltages being generated in the control windings as the magnetic flux cancels in the central limb allowing DC control current to regulate the inductance and hence the power supplied to the load. However I could not find one or anyone willing to sell me one or the cores so back to basic scrap box raiding design. Standard E and I construction does not give enough window space to fit on the required windings. However I found a 50 VA Chinese transformer that had different shape stampings allowing me to wind my first prototype as shown in the photo and log book notes.
First saturable core prototype. I used Blue PTFE wire for the mains winding for insulation as I was winding direct on the tape covered limbs in close proximity to the tape covered central core DC control winding and did not trust enamel in the situation. Core results. First learning should have put on more turns of thinner wire for the control but I did not do this as I was still mindful of the high voltages developed in my MK 1 prototype. However on testing the balance was very good with just a few mv ac appearing on the control winding.
This core slotted straight into my test rig but two problems arose : 1. I could only get about 6 volts regulation, not enough to cover the mains variations I was experiencing. 2. The regulation was not stable even though all the op amps checked out there was still a poor correlation between my HRO HT simulator and the measured voltage. But this was not fully explaining the cause as there seemed to be some seemingly random influences affecting the regulation. ( Post script Found the anomaly my cheap plug in voltage indicators were spiking the mains!!)
Toroid s I found another design using toroid s but plotting the BH curves showed that the ones I purchased 65 X 35 X 20 had were not big enough for the HRO supply current. The design needs enough inductance to reduce the voltage but must not saturate at normal operating currents. These conflicting requirements ment my cores were not big enough. Final design. I then found another picture that looked promising,no design data just a concept, but In keeping with the safety requirements I realised if I used a stock 50 VA transformer with insulated bobbins I could segregate the mains and control windings.
Disaster I could not find any matched 50 VA transformers so I bought 2 new ones however when I took off the casing to dissemble them I discovered to my horror that the E and I s were not interwoven but were all stacked on each other with the I s welded onto the top. 4 hours later I had machined out the welds and hand deburred all the laminations. A quick BH curve trace showed that once re built the transformers 0-12,0-12 windings would achieve what I wanted up to about 150 watts load. Back on track Lucky escape!! Transformer test 1, 25 Watt 2, 60 watt, 3, 100 Watt, 4, 150 Watt
Log book illustrating transformer construction.
Re built and re wound transformers. Note the DC control winding is round both cores so that the ac flux cancels PER TURN DO NOT BE TEMPTED TO USE THE ORIGINAL 240 Volt WINDINGS IN A VOLTAGE CANCELLING MODE it works but can generate huge voltages. The red thick wire is my idea to balance the inductors and give a flux short turn should one inductor fail, belts and braces against inducing voltage in the DC control winding. Final Testing With mains at 240 I get a full 220 Volt to 239 variation but on closed loop I still get some voltage wandering. Time to address the HRO HT model. Found part of the problem after a night sleep. The 3 pin voltage units I was using to monitor the voltage draw spikes of current through the indictors so that when they are throttled back voltage spikes appear. Because my HT voltage model was full wave it was responding to the voltage spikes and causing the, up till now, not understood voltage variations.
The culprit my handy mains monitors causing spikes. I had been keeping to vintage components, 741 op amps being vintage in my mind but now it was time for some advanced silicone, the AD 737 rms to DC chip. I replace my HT simulator model with an rms to DC chip miraculously all instability problems disappeared and I could now dial in any mains voltage I wanted fro 220 to current mains voltage -1 volt. Running my HRO on rms stabilised voltage is a transformation. The only drift is the warm up drift ALL random mains induced drifts up and down are gone. SSB is stable and the Xtal filter is now usable on CW. Now the hard bit Tidy up, write up, Box up, consolidate All the op amps into two or three, I used an opp amp for every function including a few to inverts signals to keep them positive going. This is good for design trouble shooting but not functionally required. The photo below shows the block diagram and complete control board.
Block Diagram Rms to DC, AC Filtering, Set Point compare, Proportional Gain, Integral gain, buffer, and output transistor Development board. Note because of the magnetic amp gain the 2N3053 runs just warm and is able to control all the mains power.
General Comments 1. All Control circuitry including mains rms detection is transformer de coupled and referenced to mains earth. 2. Mains voltage isolation is via the insulated bobbin I will put back the plastic shields post prototype to give full as manufactured winding cover. 3. Earth leakage trip action is not compromised because there is no transformer isolations of mains live. 4. Proportional plus integral control ensures stability as delays in the rms to DC and in the saturated core device prohibit direct high gain feedback. P + I Terms can be trimmed as required for optimum settling time. 5. There is no electronic noise generated by this circuit. In short it works until proven otherwise.