Variable voltage, variable current, *AC* power supplies?

[Chris Wilson]

I have one or two commercial variable voltage, variable current, current limiting DC power supplies, but I have never seen anything similar for AC. Do they exist? I ask as I am about to test some TRIACS and realized such a supply would be quite useful. Thanks.

[kizzap]

The closest I am aware of that is similar is a variac. although I don't believe that they can limit current, but they allow you to vary the voltage amplitude.

[han]

variac series with light bulb / heater ?

[Chris Wilson]

I have a small and a large Variac, and for sure they will give variable AC voltage out. But compared to my Farnell and another DC supply of Japanese origin whose make I can't recall, it's a bit of a brute force way of doing things with little in the way of protection. hell, it's not even isolated from the mains unless you run an isolation transformer before or after it. No, I was thinking something like the Farnell, nice digi display of volts and mA / amps, fully isolated, over voltage and over current protection. I thought I was just looking in the wrong places, or with an incorrect search string, but maybe there's little call for something like that, and it plain doesn't exist?

Thanks for the replies.

[SArepairman]

i have seen isolated variac boxes with protection circuits ebay before. I put a variac + isolation transformer in a wooden box with a digital current/voltage display, fuses, and a ground isolation switch, so it could do whatever. I might mod it so it has a small 1:1 transformer in it for a low current limit setting. i had fun making it.

search for "ac power supply -dc" to prevent DC supplies from showing up.

I swear I saw one once that had a current limit. but I can't find it.



how do you do AC current limiting?

http://en.wikipedia.org/wiki/Saturable_reactor ?

otherwise you need a expensive transistorized supply.

[SeanB]

You do get AC power supplies ( Dave has one that he uses to make 115VAC 60Hz for US equipment if needed) but they are not common. Making them is difficult as you need to have a control loop that handles AC and limits voltage when you start to get into overcurrent. Means a difficult control loop that will distort the voltage waveform as it does current limiting, and a second control that makes the AC voltage using a sine wave reference. Basically a really big audio amplifier, and with supply rails greater than the peak of the output AC voltage, and capable of providing enough current. Not cheap to make.

[Rerouter]

if the frequency is not high, look for a 4 quadrant programmable power supply, you then feed in the AC setpoint at whatever frequency you want / that the device can keep up with,

[Bored@Work]

I thought I was just looking in the wrong places, or with an incorrect search string, but maybe there's little call for something like that, and it plain doesn't exist?

Oh, they do exist. Just call, e.g. Agilent, hand over a bundle of money and have fun.

[SArepairman]

I wonder if its possibly to modify a variac with some extra windings to act like a saturatable inductor..

or perhaps more easily your isolation transformer. then you just need a high current DC supply to control your AC current limit... 

[iloveelectronics]

A Chinese made product: http://item.taobao.com/item.htm?spm=a230r.1.14.341.1BsXLx&id=15663556862&_u=aoemap3ffe6

It's all in Chinese but the interesting thing is that the seller included a couple of photos to show us the inside!

[Chris Wilson]

That's exactly the sort of thing I had in mind, I wonder if advertises elsewhere, in English? Thanks for the link!

[iloveelectronics]

That's exactly the sort of thing I had in mind, I wonder if advertises elsewhere, in English? Thanks for the link!

I tried looking for it but couldn't find anything in English. The lowest rated model (500VA) weighs 25kg though so I guess they don't ship a lot overseas...

[Fsck]

You do get AC power supplies ( Dave has one that he uses to make 115VAC 60Hz for US equipment if needed) but they are not common. Making them is difficult as you need to have a control loop that handles AC and limits voltage when you start to get into overcurrent. Means a difficult control loop that will distort the voltage waveform as it does current limiting, and a second control that makes the AC voltage using a sine wave reference. Basically a really big audio amplifier, and with supply rails greater than the peak of the output AC voltage, and capable of providing enough current. Not cheap to make.

^. the only ones I've ever seen (name branded, only looked at "western" models) are in the 3k$ and up range.

[robrenz]

Depending on your power needs one of these might be the ticket. You will need a FG to set the frequency and amplitude for an AC output. Very handy bit of kit since it can source and sink DC also.

[kripton2035]

dave used a VFC500 AC supply in one of the video episodes I dont remember which one
in europe there are the EA4xxx series of ac power supplies
no idea of the price but it must be some €€€€€

Edit:
EA4xxx ac power supplies goes from 470€ for 800VA to some 950€ not so expensive.(+VAT...)
seller here : http://www.datatec.de/EA-STT2000-Stelltransfomator-1200VA.htm

[theatrus]

Chroma makes a whole host of AC supplies. For the smaller ones you're looking at 0-300V and 16Hz-1500Hz (airplane), and options for all sorts of distorted waveforms.

They are basically a very high voltage closed-loop audio amplifier.

For example, this is the low end model I've used before:
http://www.chromausa.com/pdf/Br-61500-acsource-022014.pdf