+/-15 VDC Power Supply

Discussion in 'The Projects Forum' started by JCapron, May 19, 2018.

1. JCapron Thread Starter New Member

Jul 4, 2016
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I am looking for a circuit diagram for a dual output, +15/-15 VDC, 1.0 amp power supply. The power supply has to be low voltage input, either 24 VAC or 24 VDC. I am trying to replace the 120 VAC power supply that we have used in the past. I can find lots of examples of 120/240 VAC power supplies that all use center tap step down transformers. However, I cannot find a 1:1 CT 24 VAC transformer to adapt those circuits to a Class 2 application.

I also cannot find a circuit that uses a low voltage DC input that produces a dual +/-15 VDC output. Thanks for any advice.

2. AlbertHall AAC Fanatic!

Jun 4, 2014
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Do you need to isolate the outputs from the input if you are using 24VAC input?
If not then you can use this circuit to get ±33V and then use 7815/7915 to get the regulated outputs. Mote that the 33V is very close to the maximum input voltage allowed (35V) so how accurate is the 24VAC?

3. crutschow Expert

Mar 14, 2008
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Where is the 24VAC coming from?
Can you explain why you want to use that voltage instead of the 120VAC line voltage?

4. ebp Well-Known Member

Feb 8, 2018
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I think it will be nearly impossible to find an off-the-shelf transformer that is suitable.

I would suggest you look for a DC-DC converter. The drawback to a switcher is that the output noise will generally be higher than with a linear supply, which may be a consideration. External filtering can be added.

Something from the TDK/Lambda PXE30 series would do the job. They are not cheap, but quality should be excellent.

5. JCapron Thread Starter New Member

Jul 4, 2016
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With respect to AlbertHall, post: 1271028, member: 240904, "[Note] that the 33V is very close to the maximum input voltage allowed (35V) so how accurate is the 24VAC?"

The Class 2 transformers are not very accurate. I have seen the half wave peak as high as 39V under normal operation. These systems are installed in commercial buildings with backup generators, and the generator power is pretty sloppy.

With respect to crutschow, post: 1271032, member: 19451, "Where is the 24VAC coming from? Can you explain why you want to use that voltage instead of the 120VAC line voltage?"

Quite a bit of the building controls are powered by low voltage, power limited (24VAC) NEC Class 2 sources. This is a very common practice in heating and cooling systems in residential, commercial and institutional buildings. Class 2 eliminates the need for a lot of the installation standards that come with 120VAC, 240VAC or 277VAC that power receptacles, lights, etc. We use a lot of 24VAC to xxVDC power supplies for controllers, sensor, cameras, etc. In this case, we need +15/-15 VDC power supplies.

6. crutschow Expert

Mar 14, 2008
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So there's no problem with drawing a couple amps from that 24VAC source?

7. be80be AAC Fanatic!

Jul 5, 2008
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There good to about 1.5 amps the fuse blows at 3 amps but most times it about burns the transformer out too if you blow the fuse to many times.

But most are rated for 3 amps but then I do fix a lot that burn out and have a 3 amp fuse
I try to keep it around 1.5 if it me.

What happens is you add stuff to the sys they was made to run maybe 4 relays and a handful of senors 2 control boards most come in at 1 to 1.5 amps I wouldn't add something that pulls a amp on top of the load it takes care of.

Last edited: May 19, 2018
8. JCapron Thread Starter New Member

Jul 4, 2016
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NEC Class 2 is deemed to be 100VA for circuits that operate at 30V or less. The circuits are fused or otherwise protected. Two (or three) amps would not be an issue. We routinely install 75 VA transformers.

9. be80be AAC Fanatic!

Jul 5, 2008
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A 75VA transformer can't output but 2 amps then you have load's from what it was being used for.
Here the power company add a power saver board to the HVAC sys you see burnt out 75VA transformers all the time.
LOL

10. be80be AAC Fanatic!

Jul 5, 2008
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A 100 VA transformer max output is 2.6 amp @24 volts regulated DC now figure your total load to not use more then that but I would add headroom.
I wouldn't go over 2 amps

11. AnalogKid AAC Fanatic!

Aug 1, 2013
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Without knowing what the +/-15 V is used for, it is impossible to say for sure; but my *guess* is that a non-isolated converter circuit such as post #2 will cause problems.

ak

12. be80be AAC Fanatic!

Jul 5, 2008
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If he grounds it and the Virtual ground to the frame it will be a short.

13. JCapron Thread Starter New Member

Jul 4, 2016
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A 100 VA Class 2 transformer amperage is slightly over 4 amps (AC).

14. JCapron Thread Starter New Member

Jul 4, 2016
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I agree. The 120 VAC +/-15VDC power supply we use now is isolated. A new 24 VAC power supply output would be +15 VDC and -15 VDC with shared common, ±5%, isolated, 1000 mA, 50 mV P-P maximum ripple. The only hurdle I have is the 24 VAC input.

15. AnalogKid AAC Fanatic!

Aug 1, 2013
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The only way to get the flexibility of the fully isolated outputs in the original supply is to have fully isolated outputs in the replacement supply. To the original question - yes, there is an off-the-shelf transformer you can use. A small 1:1 isolation transformer will run just fine at 24 Vac instead of 115 Vac:

Drive the secondary with 24 Vac, and you get two *isolated* 24 Vac outputs you can go crazy with. \$16 at Digi-Key. Since the outputs are isolated from each other, you can use identical LM317 regulators for both the + and - outputs. Better regulation than the 78xx series, plus a higher input-voltage tolerance. ebay modules are \$2.

If you don't like that part, DK (Newark, Allied, whatever) has many active, in-stock alternatives to choose from:

https://www.digikey.com/products/en...ty=0&ColumnSort=0&page=1&stock=1&pageSize=500

With wire leads in and out, it cooks down to 8 models from 15 VA to 250 VA.

ak

Last edited: May 20, 2018
16. ebp Well-Known Member

Feb 8, 2018
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The problem with using a transformer designed for 115 V at nominally 24 V at the same V·A product is that the resistive losses in the windings are likely to make it run very hot, especially when the added issue of high RMS to average current ratio with capacitive filtering is added to the equation.

17. be80be AAC Fanatic!

Jul 5, 2008
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Your using 24 volts to come up with 4 amp that wrong you be lucky to get 2.5 amps from these
the output is figured at peek the same figures used in post 2 to get you a +- 15 volts

4 amps and you'll be seeing this
I been working with these for over 36 years there and seen 100VA burnout faster then 75 there cheap one's and good ones
They over rate these using 24 volts to do the math
a IC-0100-102 is rated at 4.17 amps on 120 volts it should be 3 amps but I've seen lots fail at less then 3 amps they get hot and short out.
The IC-0100-103 is .83 amps on 120 volts it's a 100VA @ 24 too
I would do some testing with what you have there not all what they say on case.
But good luck with this.

Last edited: May 20, 2018
18. JCapron Thread Starter New Member

Jul 4, 2016
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We must not be buying the cheap ones because we buy several hundred transformers every year with no issues. But thanks for the input.

19. AnalogKid AAC Fanatic!

Aug 1, 2013
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One concern about using a high voltage isolation transformer in a low voltage application is the wire gauge. A 60 VA transformer is designed to deliver approx. 0.5 A at 120 Vac. At 24 Vac, the core easily can shuttle the 30-40 VA needed for this application, but the windings might overheat trying to deliver 1 A. A different solution for this is to use two standard 24 Vac power transformers back-to-back. 24 Vac into the secondary of xfmr #1, #1 120 Vac primary connected to #2 120 Vac primary, #2 24 Vac secondary as the output. Center-taps / dual windings as needed. Twice the iron, probably less total cost, and each winding is designed for the current it is handling. Went there, did that, still working after 40 years.

ak

20. JCapron Thread Starter New Member

Jul 4, 2016
11
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The early (1980's) electronic DDC systems had 24VAC controllers that all came with onboard isolation transformers. They were 300 or 1200 baud TTL devices that were slow but bulletproof. Around the turn of the century, the manufacturers dropped the isolation transformers and switched to RS485. The new controllers were fast but finicky. I just need to find the source for the transformers used in those pre-2000 DDC controllers.

Thanks for everyone's input.

Last edited by a moderator: May 21, 2018