Safely experimenting with larger transformers? - Page 1
Forum section: electronics / beginners

author: sausage
author_rank: Regular Contributor
timestamp: May 12, 2022, 11:32:50 pm
content: I have a nice chunky transformer from a Panasonic home theater system. It has multi-tap inputs and three sets of outputs.Rather than inject 240V into the primary, is there any reason why I couldn't use leads from a 9V AC power adapter into the primary side so that I can measure the AC coming out of the secondary windings? These voltages would give me a little more confidence for safety.The adapter is a 9VAC 1 amp.

author: thm_w
author_rank: Super Contributor
timestamp: May 12, 2022, 11:47:59 pm
content: You can do that.Measure the AC voltage of the adapter once its connected to the transformer primary, as it can be a fair ways off from 9VAC when its loaded/unloaded.

author: sausage
author_rank: Regular Contributor
timestamp: May 12, 2022, 11:57:14 pm
content: Ah good tips thanks. It is certainly 12VAC when unloaded.

author: james_s
author_rank: Super Contributor
timestamp: May 13, 2022, 01:46:17 am
content: I do this sort of thing occasionally when attempting to identify the windings of an unknown transformer. For high frequency transformers I use a signal generator.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 13, 2022, 01:47:53 am
content: Transformers still do work at a lower voltage then they are designed for, but 9Vac for a 240Vac transformer is extremely low. It less then 4% of it's rated voltage. Secundary windings will also have a lower voltage in the same proportion, so a 24Vac winding will have less than a volt of output, and if it is loaded by some current then the voltage drop over the resistance of the copper itself reduces the output voltage even more. You probably won't reach the nominal current, even if you short a secundary winding.

author: sausage
author_rank: Regular Contributor
timestamp: May 13, 2022, 02:52:36 am
content: Doctorandus_P, but in this case, to try and work out the winding ratios, any lower AC voltage should be fine?

author: edpalmer42
author_rank: Super Contributor
timestamp: May 13, 2022, 03:04:00 am
content: If you already know which windings are the primary, you could insulate them and just ignore them. Then inject your 9VAC into one of the secondary windings. This would allow to get the windings ratios with reasonable, but still safe voltages.But always assume that the thing is trying to kill you! Make all your connections with everything powered down and make all measurements without touching the unit while it's live. There could be a high voltage winding that's intended to power the display or something like that.I've always felt that more injuries are caused by us jumping like maniacs when we feel a shock rather than from the shock itself.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 13, 2022, 07:35:41 pm
content: As edpalmer42 already wrote, you can inject an AC voltage into any of the windings.The primary winding may also have multiple taps. You can find these by measuring the DC resistance or the beep mode of your DMM.Once I've identified the primary winding and verified that it is capable of 230Vac I usually solder wires to it and a fuse and then make sure I can't touch that part anymore. Put it in a box, use shrink sleeves, or temporary isolation tape, and I combine this with an isolation transformer I've got lying on my desk just to be sure.But to work safely is a combination of common sense, personality, and knowing what you're dealing with.If you put a 100Ohm power resistor in series with your 9Vac then you're already reasonably safe. In that case it can still hurt a lot if you accidentally touch both prongs of the primary winding at the same time, but because power is limited, it's unlikely to kill you. But how unlikely must it be before you deem the left over risk acceptable?It's not a very good learning environment for beginners.

author: DavidAlfa
author_rank: Super Contributor
timestamp: May 13, 2022, 08:00:59 pm
content: Yes, you could connect the secondary from another transformer to it and get 220V back.If unloaded (The DMM isn't a load), better to add a high value resistor like 470K in the 220V side, so there's a path for back-EMF spikes when disconnecting the primary.Also if your DMM is connected to the unloaded primary and you suddenly disconnect the secondary, you might damage the DMM!Simply poking it with a 9V battery will generate HUGE inductive spikes at the 220V side if unloaded, easily beyond 5KV.Ages ago I played with a small transformer, drove it with a transistor in flyback mode, switchign 12V into it.The secondary was unloaded, making sparks at least 5mm long... It lived for 10 seconds before arcing internally

author: james_s
author_rank: Super Contributor
timestamp: May 13, 2022, 11:04:53 pm
content: I remember when I was probably about 10 I took an old power transformer and 6V lantern battery to school and got my friends to hold onto the primary wires while I tapped the secondary wires across the battery, it would create quite a zap. If someone did that today they'd probably end up in jail but back then it was just fun.

author: themadhippy
author_rank: Super Contributor
timestamp: May 13, 2022, 11:41:55 pm
content: QuoteIf you had wired in the motor from your meccano set in series with the secondary you wouldnt have needed to keep tapping the wires onto the battery, strange how my school friends didn't trust me after i made that discovery.