# multi-tap transformer relay for LM317 tips

Example circuit:

Since LM317 is a linear regulator it will get very hot when i try to draw around 1 amp @1.25v with a 24v input. The transformer i have at home have these taps: 24v 12v 5v

Is it possible to add some kind of relay to make the transformer switch tap when the voltage gets over/under a specific voltage? I would prefer that the transformer automaticly switches tap. But if that is too advanced for me i can go with a manual a way (a switch) that changes the tap.

LM317 Datasheet

Thanks!

• Yes. And it's easy if you throw in a MCU. Commented Jul 9, 2016 at 6:43
• Why can't you simply use the 5v tap all of the time?
– Mark
Commented Jul 9, 2016 at 6:59
• Never used an mcu before. I guess mcu = microcontroller? Could you show me an example circuit?
– Xane
Commented Jul 9, 2016 at 6:59
• @mark because i want an adjustable power supply from 1.25 to 24v
– Xane
Commented Jul 9, 2016 at 7:00
• OK, I see. So you will be manually adjusting a pot from 1.25v to 24v? Then something that changes to the next higher tap when the output approaches the current tap might be appropriate? I will have to think . . .
– Mark
Commented Jul 9, 2016 at 7:05

simulate this circuit – Schematic created using CircuitLab

Figure 1. Transformer tap selector.

This isn't a complete answer but will give you some ideas to work on. Whatever electronic solution you come up with is going to need a steady voltage to work with. This makes it awkward to derive from the transformer as you have a full-wave bridge rectifier on it. A commercial product would add an auxiliary winding to the transformer to give, for example, a 12 V supply that doesn't change with the tap selector. Since this is a one-off project we can add a second transformer (ex wall-wart) to generate this.

How it works

• The main supply is derived from XFMR1 via two tap-selecting relays. If both are off the 5 V tap is selected. If MED is energised the 12 V tap is selected. If HIGH is energised the 24 V tap is selected.
• XFMR2 and the bridge form a secondary supply which remains steady.
• R3 and 4 divide the output voltage down to < V+AUX. A 4:1 ratio would be fine. CMP1, a comparitor, compares the divided down output voltage with the reference set by R7. When Vout > Vref the comparitor output will switch low energising the MED relay.
• A second comparitor circuit (not shown) is required for the HIGH relay.