Problem commoning 0v,s  + psu that goes down to zero volts circuit - Page 1
Forum section: electronics / beginners

author: multime
author_rank: Regular Contributor
timestamp: May 01, 2025, 01:08:48 pm
content: I have put together the following circuit from "Practical Electronic October 1978" . I chose it for its simplicity and most of all because its output goes to zero (and it does).The circuit is powered by a 2 x 12v toroidal transformer for a 24v output, I wanted a higher current output and achieved this by using a seperate 300va 2 x 15v toroidal transformer who,s windings are switched by a relay in parallel up to 13.5v and in series after this. I have commoned the 0v of both transformers.I now need to use a third transformer ( i know transformer city) a 2 x 12v toroidal which is going to power a 12v relay for winding switching and 12v heatsink fan when needed. Controlled by a dual comparator (Lm393, powered from same source as LM741,s). This circuit is on the same board as the Lm741,s.The problem i have is when i add the 0v of this cicuit to the two that are commoned things start to go wrong. The voltage from the zener that regulates the negative voltage, drops by around 1v and resistor R3 that feeds it gets very hot, to hot to leave things switched on.I have enclosed the cicuit board i designed it was hand drawn and there are errors that i have corrected but corrections are not shown. I have promised myself that this will be the last hand draw i do, and i will learn to use KiCad.

author: jwet
author_rank: Frequent Contributor
timestamp: May 01, 2025, 01:42:03 pm
content: Quote from: multime on May 01, 2025, 01:08:48 pmDon't do that. Keep them isolated. You can tie the negatives terminal together at the final output but don't common the Xfrmr. That little diode capacitor pump at the bottom is a kind of negative supply that takes a bit of the AC from the transformer and rectifies to make a negative supply. This is a classic design that can work well but this is a quirk.Good luck

author: multime
author_rank: Regular Contributor
timestamp: May 01, 2025, 03:39:29 pm
content: Hi, thanks for your reply.I have uploaded another image of my circuit board showing the current sense resistor r7 in place and were it connects to the board and 0v it connects to. That being the strip you see round the edge of the board which is connected to the 0v of the capacitor that smooths the 300va transformer. At the top left of the board there are four Lucar connectors soldered in place and from the left the the third and fourth are were the 24vac come in to be rectified by br1, and the 0v from the bridge is connected to the smoothing capacitor previously mentioned via the 0v strip round the board. Back to the Lucar connectors and from the left again the first connected to d14 is were i was hoping to connect to the joining the two 12v secondaries or ct. That was a no go though as when connected it gave the same problem witf the zener and feed resistor. Had it of worked it would have negated the need for the third transformer. But as it stands i need to bring 12v on to the board and its 0v lead as commoning it to the board induces a fault i am at a loss as how to do it. As you can see a lot of the components for the switching circuit and ic4 the comparator connect to the boards 0v. The only thing i could try would be to take the charge pumps feed from the 300va transformer instead, would it work i dont know.

author: jwet
author_rank: Frequent Contributor
timestamp: May 02, 2025, 04:53:47 pm
content: I can't really follow all that. The advantage of store bought power supplies is they have enough qty to make custom transformers and you can have all the secondaries your want. Unfortunately you need about 1000 units for a Xfrmr guy to talk to you. All these supplies use multiple secondaries and make aux supplies that float on others. The final product is isolated from ground as well. Important for safety and much more flexible.

author: multime
author_rank: Regular Contributor
timestamp: May 02, 2025, 10:16:43 pm
content: Thanks again for your interest.I will persevere with my build, there's always a solution to these problems. The reasons i didn't buy a power supply 1. The cost 2. Manufacturers telling me 0v to whatever voltage when 0v is only a figment of there imagination 3. Were is the fun and sense of achievement in buying one