24v transformer question - Page 1
Forum section: electronics / beginners

author: shanezampire
author_rank: Contributor
timestamp: June 21, 2019, 02:07:25 am
content: This is just a curiosity question. I have the below transformer. I originally thought it was a 120v transformer and hooked it up to 120v. I was only getting half the 24v out. Actually around 14v give or take. I cant remember. Which is actually what I really needed and wanted in the first place. Question is, can it be hooked up and ran that way without harm or over heating?

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 21, 2019, 05:13:32 am
content: Looks like its rated for about 20W output or 20W / 24V = 830mA . Don't draw more than that and you should be fine.If it were the other way around and it was a 120V transformer and you wanted to drive it at 240V, that would almost certainly be a problem as the flux would rise twice as fast and almost definitely push the primary inductance well into saturation, drawing alot of current and overheating (or worse) from I^2 R losses in the primary winding.

author: bob91343
author_rank: Super Contributor
timestamp: June 21, 2019, 05:25:37 am
content: It's marked as being a 240V primary. You can run it at half that and expect half voltage output but the same current. So its power rating is proportionately lower. That's because the wire size determines the maximum current.Having said that, the practical aspect is that you can draw a bit more than half rating due to other losses being lower. I would limit the load to around 60% of rated power, or perhaps 20% more than rated current. That would make it top out at around 1 Ampere load current.If you try to get that out of it, be sensitive to its temperature rise and if it seems to be cooking, reduce the load.

author: shanezampire
author_rank: Contributor
timestamp: June 21, 2019, 06:22:28 am
content: Thanks for the replies! I hooked it to a Bridge Rectifier and hooked it to a pretty dead 12v battery for about 30 min to an hour and it was reading about 15.4v then dropped to 14.7 as the voltage started rising. Nothing got over room temperature. I was thinking of hooking it to the below (XH-M603) and setting the cut on voltage to 12.5 and cut off about 12.9 or something similar. Just as a little float charger for a battery I may have sitting for months. Just tinkering and learning.

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 21, 2019, 03:55:34 pm
content: Quote from: shanezampire on June 21, 2019, 06:22:28 amNope. Don't connect unregulated power sources to batteries. How did you know how much current that battery would draw when you connected it?

author: bob91343
author_rank: Super Contributor
timestamp: June 21, 2019, 04:49:09 pm
content: Unregulated is fine as long as the source impedance is high. Regulated voltage from a charger is inviting trouble due to too much current. You don't have a lot of control over the voltage; battery condition determines that. So unregulated is good provided it's got poor load regulation.That way, the current starts high into the discharged battery and then drops off as battery voltage climbs, just what you want.

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 21, 2019, 05:04:55 pm
content: Quote from: bob91343 on June 21, 2019, 04:49:09 pmBy unregulated I meant current. There is no telling what that transformer can deliver as far as current so its poor load regulation is pretty much unknown to the OP. Yes its small and might be have a high winding resistance but thats pretty far from safe EDIT: to assume.

author: ArthurDent
author_rank: Super Contributor
timestamp: June 21, 2019, 07:10:10 pm
content: One consideration on running a 240 volt transformer from 120 is that the windings have the same number of turns and the same wire size so the windings have the same resistance values on either voltage. If run on rated 240 volts, the 24 volt secondary might have 2 ohms of resistance (probably less). If you had a 1 amp load then the 2 ohms winding resistance would drop 2 volts and the output would be 22 volts under load or about 92% of the no load voltage. Transformers may be rated for full load current but the 'sag' will be proportionally greater when run at 1/2 voltage.If you run the same transformer on ½ the rated voltage of 120, giving 12 volts out, with a 1 amp load, the same 2 ohms will still drop 2 volts and the output will be 10 volts or 83% of the ‘rated’ voltage. The same effect happens on the primary where the greater resistance of the primary winding will have an effect on the voltage the primary sees but where the load voltage will be ½, the effect will be less. So the voltage out under load with 120 volts in might be 75-80% of the no-load voltage as opposed to around 90% with 240 volts in and the no-load to load regulation will be worse.Generally these losses won’t be a problem and I have used transformers at ½ rated voltage and they run nice and cool.