Source for transformer

[motocoder]

I've been prototyping a design for a programmable high precision current source. It all works, but I think I can improve it by lowering the common mode voltage range of the op-amp that is doing the current sensing. To do this, I want to move to a design that "floats" this op-amp's power rails on top of the output voltage. The easiest way to do that is to use a transformer that has a separate winding.

So what I would like is a transformer that takes 110/120V primary, and has secondaries with:

30 - 35V @ max 1A
9V - 12V @ a few millamps, center tapped (i.e. +/- 4.5V - +/-6V).

Any ideas where to source such a beast?

[mariush]

One suggestion would be to buy a toroidal transformer and just add your own insulated wire over the existing wire ... tap the center of that wire for your new ground and the ends of the wires are your +v and -v

You can use a couple of linear regulators to actually get the voltages you want afterwards.

For prototyping, it would do. If you do commercial product, you can custom order such transformers.

Here's some suggestions (but I'd suggest going for 50VA or more) :  http://www.digikey.com/short/3tq930

[motocoder]

One suggestion would be to buy a toroidal transformer and just add your own insulated wire over the existing wire ... tap the center of that wire for your new ground and the ends of the wires are your +v and -v

You can use a couple of linear regulators to actually get the voltages you want afterwards.

For prototyping, it would do. If you do commercial product, you can custom order such transformers.

Here's some suggestions (but I'd suggest going for 50VA or more) :  http://www.digikey.com/short/3tq930

Thanks, mariush. I think the local electronics store has some surplus toroidal transformers, and I have some magnet wire, so I'll probably do exactly what you suggest.

[dadler]

You may be able to find something here that will work for you:

http://www.antekinc.com/transformers/

I have purchased a few transformers from them, quality is good, locally made, prices are respectable.

[motocoder]

You may be able to find something here that will work for you:

http://www.antekinc.com/transformers/

I have purchased a few transformers from them, quality is good, locally made, prices are respectable.

Thanks, dadler - I will check them out.

[Paul Price]

Rather than spending $25-$50 more for a semi-custom or hard to find toroidal power transformer with an aux winding, realize you can spend $1 and use a small capacitor, two diodes and a zener, filter cap to create a low-voltage doubler to create a boost V++ supply that ground references itself to the V+ post-rectifier output of whatever single-winding transformer you choose to use. By using a small-valued cap from the power transformer that is in series with the junction of the two diodes mentioned, only a small amount of average current would be developed, but a cap value could be calculated to get this doubler circuit to develop sufficient power to provide a stable power rail required by your op-amp.

[floobydust]

I suggest a low power DC-DC converter module, like a CUI PEM1-S. These are 1W with 3kV isolation, SIP package. They are unregulated. Datasheet: http://www.cui.com/product/resource/pem1-s.pdf
A 1nF cap between in/out, the EMI is quite low.  I use these for high-side sense op-amp power.

[Paul Price]

The OP says he is looking for a transformer that delivers "30 - 35V @ max 1A"

Those converter chips are nice but limited to approx 26V Vin but Vin would hit >34 to >49V peak with the OP's desired xformer.

[bson]

You may be able to find something here that will work for you:

http://www.antekinc.com/transformers/

I have purchased a few transformers from them, quality is good, locally made, prices are respectable.

Nice!  I've been on the hunt for small, low-power (15-30VA) toroidals and these guys look a perfect source.  Just ordered a couple of dual 15V 25VA ones to try out.  $11/ea plus postage, that's excellent!

[dadler]

No problem!

They also sell on eBay, but the prices (with shipping) are generally better on their website.

eBay link: http://stores.ebay.com/Antek-inc/

[motocoder]

I suggest a low power DC-DC converter module, like a CUI PEM1-S. These are 1W with 3kV isolation, SIP package. They are unregulated. Datasheet: http://www.cui.com/product/resource/pem1-s.pdf
A 1nF cap between in/out, the EMI is quite low.  I use these for high-side sense op-amp power.

Hi floobydust -

I guess these are switching regulators. I don't want to introduce any switching noise into the circuit, as my experience has been that it's virtually impossible to eliminate once there.

But thanks very much for the suggestion - those modules will come in handy in other projects, I am sure.

[motocoder]

Rather than spending $25-$50 more for a semi-custom or hard to find toroidal power transformer with an aux winding, realize you can spend $1 and use a small capacitor, two diodes and a zener, filter cap to create a low-voltage doubler to create a boost V++ supply that ground references itself to the V+ post-rectifier output of whatever single-winding transformer you choose to use. By using a small-valued cap from the power transformer that is in series with the junction of the two diodes mentioned, only a small amount of average current would be developed, but a cap value could be calculated to get this doubler circuit to develop sufficient power to provide a stable power rail required by your op-amp.

Hey, that's a great idea. Thanks!