AC test transformer

Thread Starter

timbaker0365

Joined Aug 11, 2020
32
I'm taking some classes and wanted to test some AC practice circuits on a breadboard, I was looking for a low voltage AC-AC transformer, but all I could really find a doorbell transformer, 8,16, and 24V I think, which I would use if it's OK. Do these provide isolation safety? Any other recommendations? Thanks.
 

tautech

Joined Oct 8, 2019
199
If you have a bucket full of old wallwarts like I do some of them are likely AC ones although many measure above their rated voltage unless they're loaded. For most projects you won't need much current and these can be simpler to work with as they don't require any need to work with the mains supply side, just plug them in and go!
 

panic mode

Joined Oct 10, 2011
1,880
isolation is between primary and secondary. often secondary side will have taps rather than isolated secondary windings. always check with multimeter to confirm presence/absence of continuity between different transformer terminals.

btw. you can connect two equal transformers back to back (primary to primary) to get 1:1 ratio if that's what you need.
even if not equal, (say 120:9 and 120:6) connecting them this way would give you "new" transformer with 9:6 ratio and perfect for playing. just insulate 120V terminals so they are safe and don't touch anything.
 
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Thread Starter

timbaker0365

Joined Aug 11, 2020
32
So after no luck finding an AC wall wart (a bunch of DC) I finally broke down and got doorbell transformer from HD. So I checked the primary-secondary for any continuity and it appears isolated. My plan is to set up a switch for the primary side (plug in to wall) and my question is, on the way back from the store I got thinking about the ground wire... Is it safer NOT to ground it to the metal box (which was my first plan) as to lessen the risk of any direct ties the line current? The transformer housing is def. grounded to green (checked with ohmmeter). I guess if was NOT planning on grounding the thing to box I should've bought a plastic box... but anyway. Thanks for any help.
DoorBell.jpg
 
Metal boxes should be grounded. In general, the stuff in it should be too.

As an example: a 3-prong electrical outlet in a metal box
The box gets grounded AND the 3rd prong gets grounded. In this case we don;t relay on the screws to make electrical contact to ground.

If a green is provided, use it. The output of the transformer would still be isolated from ground. it protects against a winding short to the metal transformer mount.

Doorbell and furnace transformers MIGHT be of an "energy limiting design". If they are, they will survive a short circuit.
e.g. http://www.hammondmfg.com/ba.htm
 

Thread Starter

timbaker0365

Joined Aug 11, 2020
32
If a green is provided, use it. The output of the transformer would still be isolated from ground. it protects against a winding short to the metal transformer mount.
Yes, I do have a basic understanding of the grounding thing, etc., is why I asked in the first place, and thank you. My only original concern was thinking something of the sort if I went to shut the switch off to the transformer and touched the metal box (ground) and was somehow in contact with the secondary power (like with my other hand, not paying attention or whatever) would that somehow now tie me back to main through ground, or "unisolate" me, but I guess you're right even if that were to occur the secondary is still directly isolated so that would not complete the circuit (through me!)... correct? Thanks...
 
You pretty much have the right idea. For your bench you could get a GFCI or cambination GFCI/AFCI. Th e latter looks for a signature of an arc like what might happen with a frayed extension cord.

You could be using a grounded piece of test equipment that grounds the secondary, but it still isn't a big deal. NEC Class II wiring is power limited to 100W and below 25V and that's why you don't need conduit. It's not considered dangerous.

Dangerous is a place you don't want to go. I worked on 0-100 kV DC at 0.1A power supplies for an X-ray generator and a 30 kW "picture tube" basically, an e-beam evaporator. That power supply was regulated and around 15 kV DC at 1.5A. Then the 1000 W RF power supply with 3kV power supplies in it. or the 3000 A 6.3V AC power supply. How about the lamp that required 22V 40A to run and 40 kV pulse to start.

Then I was also at the other end of the spectrum measuring a few pA or 1E-12 Amps at +-100V.

And in the grey area of +-1.5V, and < +-25mA.

The deal with the "green wire" might be a better connection. Sometimes varnish is use don the transformer and it gets on the case.
 

Thread Starter

timbaker0365

Joined Aug 11, 2020
32
Speaking of GFCIs, I've been thinking lately, my basic understanding of a GFCI is it detects a difference in amps going out to amps coming in and kill switches the circuit if more than say 5 mA difference, being that there is a likelihood that difference is going through a person to ground. So I was wondering would it really offer much protection if the person became part of the circuit- say I grabbed the hot with one hand neutral with the other and a few amps pumped through me (right through my aortic valve or whatever), and all went right back through the circuit, no amps lost - amps going out = amps coming in, no trip of circuit my aortic valve continues to boil until my GF comes back from grocery shopping and finds the corpse calls the coroner to remove the body. Am I correct on that?

Anyhow I ask because I thought about putting in a GFCI but the only real protection that offers is ground fault, correct? I am still considering it but if it was some sort of failsafe we probably wouldn't be talking about isolation transformers, etc... right or am I missing something? I guess I just don't want to get some false sense of security about it all and want to try and understand more before I get too far or careless and blow something up, or myself. Thanks again...
 

AlbertHall

Joined Jun 4, 2014
11,210
say I grabbed the hot with one hand neutral with the other and a few amps pumped through me (right through my aortic valve or whatever)
But this would be a rare circumstance. You are much more likely to get a shock through one hand, directly from the live connection or from unearthed metalwork which has become live due to a fault.
 

SamR

Joined Mar 19, 2019
3,236
What you have is fine. You need a cord with a 3 prong plug, H black, N wht, GND grn. I see you have a switch and keep it handy in case the magic smoke starts to appear. Add an inline fuse between the switch and XFMR. I like to use a 1/2A and keep plenty of spares. Ground the metal case to the plug wire ground. You have an interesting tri voltage secondary transformer. Only 2 wires will come from the secondary at the selected voltage and there is no center tap to experiment with so that will limit you a bit playing with the rectifier designs. Have fun!

EDIT: you might want to look for a Triad Magnetics F-25X XFMR which is 120/12.6V @ 1.5A center-tapped which can be found on Amazon or eBay for less than $20. That will allow you to explore all of the various rectifier models. You might also consider fusing the secondary wiring.
 
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Thread Starter

timbaker0365

Joined Aug 11, 2020
32
But this would be a rare circumstance. You are much more likely to get a shock through one hand, directly from the live connection or from unearthed metalwork which has become live due to a fault
Plausible and agreed. So was your reply for or against (or neither) my thinking on the GFCIs offering much protection (only GF). If I get a shock through one hand do you mean from that hand running through another part of my body back into the circuit, which would more or less be the case I described and therefore GFCI wouldn't be much good... am I correct there? I'm not asking this as a rhetorical question, I'm asking this so I understand if I'm correct or not. OR when you say "get a shock through one hand" do you mean a live pos. and neg. both touching my one hand and traveling through and out of that hand, which I'd likely still be alive (or not if my hand started smoking and turned to a skeleton and I had a heart attack upon seeing it). If that is the case, again the GFCI wouldn't do a lot of good if it works the way I presume it does, correct?

Let me say, I'm not against the GFCI thing, in fact I planned on installing one or two - I have two outlets in my shop, both with double receptacles, both on their own circuit. I can't remember if when you install a GFCI either at the beginning of or end of the circuit it protects the entire circuit, would have to look it up. Anyway, I just kind of forgot to grab it (them) when I was at HD as when I thought about how they worked they weren't really at the top of my list anymore... Thanks for the input...

-Tim
 
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Thread Starter

timbaker0365

Joined Aug 11, 2020
32
Add an inline fuse between the switch and XFMR.
Hi Sam, thanks. I've read several of your posts recently - always informative. In fact I did grab an inline fuse link last week even before I decided to go with this transformer idea. At the store the smallest they had was 1A so I grabbed a box but will look for some smaller ones... thanks again.
FuseLink.jpg
 

Thread Starter

timbaker0365

Joined Aug 11, 2020
32
My only original concern was thinking something of the sort if I went to shut the switch off to the transformer and touched the metal box (ground) and was somehow in contact with the secondary power (like with my other hand, not paying attention or whatever) would that somehow now tie me back to main through ground, or "unisolate" me, but I guess you're right even if that were to occur the secondary is still directly isolated so that would not complete the circuit (through me!)... correct? Thanks...
Well wait a second... after thinking and re-reading my quoted post here, I'm thinking I was correct. If I did go to switch off the transformer and touched the metal grounded box and my other hand or something was in contact with power from the secondary, wouldn't that complete the circuit and cause potential shock? I mean, let's start with the hand that's in contact with the live power on the secondary, say I'm grabbing one of the wires, that won't do anything since the electrons are looking for a place to go - nowhere to go, but if now my other hand touches a direct path to ground (the box, even though it's on the other "isolated" primary side), that would in fact give the electrons a path to flow (through me!), correct?
 

Thread Starter

timbaker0365

Joined Aug 11, 2020
32
Anyway, I guess I'll be fairly safe at 8V. After looking up the average hand-to-hand resistance of the human body and using Ohm's law it's likely not enough amps would pass pass through me to do much harm. I'll check myself from time-to-time with the ohmmeter to make sure my resistance stays high :)
 

SamR

Joined Mar 19, 2019
3,236
The metal case, both the switch and XFMR are connected to the plug cords safety ground, grn. The XFMR secondary is floating, ie ungrounded. You can test for that by checking continuity from each of the 3 secondary terminals to the case and each primary terminal. What you have is essentially a step-down isolation transformer. The XFMR secondary available current (depending on the XFMR capacity) might be as high as ~1.5A but at 8-24VAC depending on which terminals you connect to. The 1/2A on the primary will blow if you short the secondary circuit but the secondary can put out ~1500mA unless you fuse it for less (which is enough to burn wires and put out magic smoke). That is why some fuse the secondary also. I buy 5x20mm glass fuses from china cheap along with various types of fuse holders. It only takes 3 months to get them LOL! So I buy at least 20 at a time in each of several values.
 

Thread Starter

timbaker0365

Joined Aug 11, 2020
32
That is why some fuse the secondary also.
Ok. My original intent was to fuse the secondary side and not the primary, for just the simple reason of neatness I guess on the primary since I'll be using a plug-in cord, didn't want to tap into it. Are you saying to fuse the primary with 1/2A instead, or maybe both sides? I guess my question would be if I had the secondary side fused with 1/2A or so, what's the purpose of fusing the primary side? Doesn't the secondary fuse basically cover the primary circuit as well? Thanks.
 

SamR

Joined Mar 19, 2019
3,236
I fuse the primary @ 1/2A on most of my small XFMRs, which is low. The actual XFMR probably doesn't need to be protected that low but you will need to find the PDF to find out what the true needed value will be. @ 1/2A on the primary, a dead short on the secondary will blow it (depending on the XFMRs capacity VA rating) and I am using fast-blow fuses. Depending on your XFMRs rating the secondary can still be high for some circuits if a fault occurs. I recently built a PSU and calculated that I needed 0.2A on the secondary. 200mA is still a good bit of load for some circuits and I had one small XFMR that burned out @ 175ma load but finding fuses lower than 0.2A is not easy. It all depends on what you are doing and what your current needs are and what the circuit needs to be protected for. With 2A fuses on the 120VAC primary, I have had wires on a 12VDC secondary burn and glow red without blowing the fuse.
 
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