View Full Version : Adjustable DC output for a commercial AC-to-DC power supply?


deniss
09-12-2012, 10:31 PM
A while back, I purchased a Samlex RPS-1204 DC linear power supply and now wondering if it's possible to make its DC output adjustable. The PS has a transformer inside. The PS is rated for 4-6 Amp output, and puts out 13.8 VDC regulated.

Here's the spec sheet (there is no schematic online):

http://www.samlexamerica.com/documents/product-specs/RPS-1204CPBT_Samlex_Specifications.pdf

I haven't had the need to use a DC supply so far in my very low-level adventures into audio repairs, but I figure the time for that will come eventually.

So the questions is, is it possible to take a commercial supply like this and modify it so as be able to dial in the desired output voltage, instead of the fixed 13.8 VDC?

I'm sure that I will need to supply more information about the supply so that this question can be answered -- just let me know what kind of picture(s) I could take inside the supply, since there is no schematic.

BinaryMike
09-12-2012, 11:40 PM
Linear Technology makes a regulator IC that is very well suited for making lab supplies that can be dialed all the way down to zero volts output. Look this one up: LT3080

deniss
03-22-2013, 10:38 PM
Linear Technology makes a regulator IC that is very well suited for making lab supplies that can be dialed all the way down to zero volts output. Look this one up: LT3080

This is one cool device.

I've been studying its spec paper, which contains very detailed descriptions of the device, its caveats, and possible applications, and this looks promising. The PDF is here (http://cds.linear.com/docs/en/datasheet/3080fc.pdf).

The first picture I attached shows the device pinouts.

I'd like to ask for a little help on applying this device to my Samlex RPS-1204 linear DC supply, which is rated for 4A continuous and 6A surge, with a constant output of 13.8 VDC.

LT3080 is rated up to 1.1 Amps, so I would either need to use several of these devices in parallel or use a bypass transistor as suggested in the application sheet.

The second picture I attached shows the LT3080 in a high-current application: with a PNP bypass transistor to supply the additional current, while LT3080 with a particular R_set (= 332 kOhm in the picture) sets the V_out to a particular value.

According to spec sheet, V_out of the LT3080 device is determined by (R_set * 10 microAmps). In the picture, R_set = 332 kOhm... In my case, R_set would be a 1.2 MOhm pot (or a combination of a voltage divider and a smaller pot, as the application sheet shows) to produce an adjustable V_out between 0 and 12 VDC.

Am I on the right track if I use that configuration with the PNP transistor as shown? Am I correct in saying that the external PNP transistor shown in the diagram just channels the current from the input supply to the output? I'm still not too well versed in transistor electronics, so I don't really understand how the LT3080 is able to set the V_out while the external transistor pumps the extra current.