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 Post subject: Re: Isolation Transformer
PostPosted: Nov Mon 28, 2016 2:33 pm 
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Two 50-watt devices in series might not behave as expected. The current vs. voltage characteristics are almost certainly not linear.

If they WERE linear, then the net load would be 25 watts ( 200mA at 125 volts)

At the risk of being redundant.......a much better solution is a Variac and an ammeter. No fuse needed, except what is in the Variac**

A light bulb in series is mostly just acting as a fuse. You don't get any quantitative information.



** to protect the Variac

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 Post subject: Re: Isolation Transformer
PostPosted: Nov Mon 28, 2016 3:33 pm 
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pixellany, I will take your advise. It is pretty hard to ignore with your no nonsense avatar over there just daring me to. :)


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 Post subject: Re: Isolation Transformer
PostPosted: Nov Mon 28, 2016 3:36 pm 
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People ignore me 100s of times every day......;)

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 Post subject: Re: Isolation Transformer
PostPosted: Dec Mon 19, 2016 9:41 pm 
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I am still working on this project but am waiting for parts. I order parts from Asia at a low price but it is a long wait as free shipping is by ship. I found these really cool panel mount AC power input type IEC320 C14 sockets with a rocker switch as well as a fuse holder for just $1.15 (after shipping) $6 on Amazon. IEC320 C14 cords are a buck at Goodwill.

I decided to add an ammeter and voltmeter. They are small (2"X2 1/2" ) but cheap at $12 total for both. I am still waiting on the single panel mount AC 3 prong power output socket which was about $2. Edit: just got this item after a 28 day wait.

I did finally receive the small 6"X6" cube GE radio (photo below). Funny story; the first one I ordered failed to arrive because the owner lost it at home but sent a refund. Said he will send it free if it turns up. I ordered another (same model) $21.35. It has a wood cabinet and is a great enclosure for this project. I stripped out the old transformer and speaker which gives me enough room for the new larger isolation transformer (2"X2"X3") as well as the meters. The two meters, output socket, and the input socket/fuse/switch will mount on the back of the cabinet. I may replace the plastic back with hardwood ply.

I calculated the total cost of this project and it was more than expected but I have had a great deal of fun with it and it will be functional so is worth it. Total cost $55.26

EDIT: I read reviews on Amazon which show a high incidence of switch failures on the IEC320 C14 socket combos so I will be sure to use the minimum required fuse (fast acting) and limit use of these to small current draw projects.
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 Post subject: Re: Isolation Transformer
PostPosted: Jan Fri 06, 2017 2:23 am 
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Got the parts finally and mounted them in a wood panel I fabricated from a 99 cent artists palette I got at Goodwill. I still have to wire it up but it looks good so far. The power connectors are very solid so there is no flex when the plugs are inserted in the brand new and very snug receptacles. I wedged a square piece of wood I cut from the palette in against the radio electronics so there is no metal near the isolation transformer.
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 Post subject: Re: Isolation Transformer
PostPosted: Jan Sun 08, 2017 1:04 am 
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Hate to spoil the fun, but you might have a meter problem here.

If you look underneath the "V" on the voltmeter, you see a little sine-wave symbol. That means it's an AC voltmeter. It should work fine.

But if you look underneath the "A" on the ammeter, there's a solid line and a dashed line. That means DC. It probably will not budge when you put AC through it, or if it does, it's likely to be inaccurate.

One other thing. Depending on the kind of radios you intend to work on, this meter might be hard to read even if it works. If your isolation transformer is 50 VA, it means that you cannot draw more than 0.416 amps (at 120 volts) without overloading it. In other words, it's all over before you get to the first major division on the meter. That's not a problem for what you intend to do, BTW, because most AA-5 type radios draw around 30 watts, which is like 0.25 amps or so. But that's only halfway between zero and the first major division.

Now it is actually not that easy to find a 0.5-A full scale AC panel meter, but what used to be very common were 500-mA AC panel meters. 0.5-A = 500 mA, so you can take it from there.

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 Post subject: Re: Isolation Transformer
PostPosted: Jan Sun 08, 2017 1:37 am 
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Chris108 wrote:
Hate to spoil the fun, but you might have a meter problem here.
Now it is actually not that easy to find a 0.5-A full scale AC panel meter, but what used to be very common were 500-mA AC panel meters. 0.5-A = 500 mA, so you can take it from there.

GOOD EYE! That ammeter photo has been posted in this thread for quite some time and you are the first to spot the problem. Thanks a lot. I order parts from China so obviously I am in no big rush to complete this project. Still, I do wish to finish. I am searching for the 500 mA (thanks for that) AC ammeter right now and came up with the one pictured below for $5.65 but I am still looking. EDIT I purchased this type for $4.75 which includes shipping from Hong Kong. Shipping from HK is much faster generally than from China mainland. The DC meter this replaces was only $3.65 total so no great loss.

I have a 500 miliamp fast acting fuse (plenty of spares too) as this is to power newly acquired radios and I do not wish to blow up my meter in the event of a shorted transformer or some such disaster. That would peg this one instantly.
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 Post subject: Re: Isolation Transformer
PostPosted: Jan Sun 08, 2017 4:11 am 
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Well, just set the DC meter on the side. I've never found it a bad thing to have a variety of meters that can be used for different purposes. There are lots of places a 2-amp DC ammeter could come in handy, especially if you go into solid state gear in the future.

What you may discover is that the inrush current of an AA-5 may draw half an amp or more for an instant when you first turn it on cold. This could peg the AC ammeter momentarily. It will not happen if you bring the radio up slowly on a variac. But since I don't always use a variac in the first place, it may be helpful to throw a SPST switch across the ammeter to short it out when you don't want to use it. If you really want to get fancy, one could find the resistance of the meter, which is typically somewhere around 0.9 ohm or 1-ohm for this class of instrument. (The 2.5 on the face of the meter refers to the accuracy class).

If you shunt it with the same resistance as it has internally, the scale will double to 1 amp. Then you could combine the two ideas and put the switch in series with the resistor, and place them across the meter. When the switch is open you'd have 0.5 A full scale for monitoring radios. and when the switch is closed, the scale would be X2 for bringing them up.

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 Post subject: Re: Isolation Transformer
PostPosted: Jan Tue 10, 2017 4:30 am 
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Chris108 wrote:
If you shunt it with the same resistance as it has internally, the scale will double to 1 amp. Then you could combine the two ideas and put the switch in series with the resistor, and place them across the meter. When the switch is open you'd have 0.5 A full scale for monitoring radios. and when the switch is closed, the scale would be X2 for bringing them up.


That sounds like a good idea. I installed the switch already. There was not much room but it fit ok right in the center of the panel. I will put a 1 ohm resister in series with the switch across the meter too when the slow boat gets here. I have not wired anything yet but I have the elect. diagram for the transformer and the combo switch/fuse/mains is pretty simple.


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 Post subject: Re: Isolation Transformer
PostPosted: Jan Tue 10, 2017 12:07 pm 
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Just be aware that I am not certain about the one-ohm resistance of the meter; I'm going by what's typical for similar instruments. If the shunt resistor is off by a tenth of an ohm, it will still work but the reading will be off by 10%. In any case, a 50-VA transformer should be able to carry the initial surge but it will not be able to deliver an amp of current for very long, so you will not be using the meter on the "high" setting for lengthy measurements.

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 Post subject: Re: Isolation Transformer
PostPosted: Jan Wed 11, 2017 2:27 pm 
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I am still hoping to get the exact resistor for a 1 amp display but it is only to protect the meter initially. I plan to use the variac each time anyway and if there is a problem the 1/2 amp fast acting fuse will blow. In any case I will heed your advise to not overload the transformer.

The switch module was fun to wire but I see that many people online had problems with it. First, there are variations in design so each one must be analyzed individually with a multimeter. It is fairly simple if the basics are known, The neutral at the wall being the large slot is important. The fuse and switch being in the hot circuit is too. Last, the LED pilot in mine was easy as there was just one pin (ground). I would reemphasize what I said earlier about this modules safety being suspect. It is rated for 10 amps @ 250 volts but the cheap rocker switch is not capable of that. I advise using this in low amperage circuits. This project is an appropriate example of low current so this module should work great in this application.

There are two types of this IEC320 module. The best choice is the one which snaps into the panel as well as having the capability of being bolted or screwed to the panel as the one in the photo. I could not find a photo of the 3 pin switch so here is the 4 pin type. Note the fuse is in the center between the socket and the switch, Its cover pops off easily for access.

EDIT: I am still waiting on parts, Jan 23, 2017, and have changed it. I made a new panel and am only installing an Ammeter but it is a vintage (NOS) Japanese meter from an Ebay seller in Malta $12 total. I am putting banana plug sockets so I can attach a multimeter to monitor voltage. With this ammeter there is not much room for a second meter. It is 3" X 3". Photo of the meter below.
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