Question regarding LM2575-12V Voltage Regulator

[zrlw05]

I recently purchased some LM2575 (5 volt) regulators from lightobject.

I noticed something I didn't understand. I am using a 15 volt fixed DC supply for the input, and driving several flashing LED circuits (astable multivibrators) for crossing signals on a model railroad. (very low power demand, maybe a switching supply was a bad choice?)

When I measure the input supply voltage, I normally see 15V. When I hook up the LM2575 circuit, I see 5V on the Vout side of the device, but if I check voltage across the input supply posts, my voltmeter now reads 21V. I simply do not understand what is going on here.

Can someone explain ? Is my meter lying to me ?

[Tech_Marco]

What kind of DC power supply you used?

It seems that your PS is not a regulated but a simple one with a half or full bridge diode. It may not come with a capacitor for filtering so once it hook up to the power module (or a capacitor), it give you higher voltage because of charging/discharing effect by the capacitor.

Well, as long as the output is 5V, you don't have anything to worry

[zrlw05]

The power supply is a Model Rectifier Corporation Tech II 2400. It has a single transformer, and circuitry to supply 22.5VAC, fixed 15VDC, and variable 16VDC(for train control). I have never opened the case on these powerpacks (I have 2 of them), but I figure the transformer secondary is 22.5VAC, and it uses LM317 to supply variable DC, and separate a linear regulator to supply 15VDC (both behind a rectifier and smoothing cap).

The reason I was alarmed, was that I intended to use the 15VDC elsewhere on the layout to drive signal circuitry, and the 5VDC to drive LED flasher circuits (with common ground) as well as the odd motorized scenery item. I want to use the 15VDC as a buss, and the regulators will be scattered around the layout, near their loads. Things that don't need 5VDC will use the 15VDC straight away.

I wanted to get a handle on why the apparent voltage on the supply side of the switching regulator was higher.

I connected a second 2575 regulator in parallel to the first, and observed the supply voltage seemed to climb from 21 to 22.5 when I did that.

Anyway, I changed over to using a dedicated wall wart for the 15VDC (1amp) supply. I checked, and see no such effect on line voltage. I'll stick with that...

Thanks very much.

Rusty