Dropping 24V to 6V with a regulated power supply?

[mmaaddmmaann]

Hey guys, quick electrical question for you. I'm learning a bit about electronics, but I'm having trouble wrapping my head around how to apply ohm's law with a regulated power supply. Basically I have a 24v 1.5Amp regulated power supply, and a sensor that I need to power that draws 6v and 200ma. What I'm trying to do is build a basic circuit that takes my regulated voltage, and drops it to 6v. I'm not sure if passing the voltage thru a resistor will change the fact that the voltage is regulated but not the the current is on demand or not. Also, what If I needed to hook up two of the sensors, will putting them in series after my resistors in the circuit be fine. Cheers for the help if anyone can. mmaaddmmaann

 

[dknguyen]

Hey guys, quick electrical question for you. I'm learning a bit about electronics, but I'm having trouble wrapping my head around how to apply ohm's law with a regulated power supply. Basically I have a 24v 1.5Amp regulated power supply, and a sensor that I need to power that draws 6v and 200ma. What I'm trying to do is build a basic circuit that takes my regulated voltage, and drops it to 6v. I'm not sure if passing the voltage thru a resistor will change the fact that the voltage is regulated but not the the current is on demand or not. Also, what If I needed to hook up two of the sensors, will putting them in series after my resistors in the circuit be fine. Cheers for the help if anyone can. mmaaddmmaann

Regulated voltage means "Same voltage regardless of current changes...within reason of course". SO adding a series resistors after a regulator to drop the excess voltage will not keep the voltage regulated because now the output voltage will change with the current draw (since voltage drops across resistors are current dependent).

Something similar also happens if you try to use a resistive divider.

 

[mmaaddmmaann]

ok, that makes sense. So what do you think the best way for me to hook up this type of sensor is. 24v 1.5a regulated power supply, to a 200ma 6v load? How many ohm resistor would I need? Cheers

 

[BrownOut]

You should use a unity gain op-amp with the resistive divider that drops the 24 volts to 6V at the amp's "+" input. You can use any combination of resistors that give you 3:1. ie, 100K for the high side and 33K for the low side.

 

[MikeMl]

You should use a unity gain op-amp with the resistive divider that drops the 24 volts to 6V at the amp's "+" input. You can use any combination of resistors that give you 3:1. ie, 100K for the high side and 33K for the low side.

However, not many opamps can deliver the OP's required current of 200mA. You either need a "power opamp", or use a series regulator power transistor (or power Fet) inside the feedback loop of a conventional opamp to get the required 0.2A.

Another reality check. Dropping 24V to 6V means that 18V is dropped across the regulator element (what ever it is). 18V * 0.2A =3.6W, meaning that some sort of heatsinking will be required. This is beyond the capability of a "small-signal" TO-92 transistor, a typical opamp, or a LM317L.

Attached is a circuit that would work. The simulation shows the voltage regulation (red trace) as the load resistance changes from 25Ω to 35Ω. The complex expression shown in dark green is the power dissipation in the 2n3055 power transistor. The blue trace is the current through the simulated load resistance (your sensor).

 

[crutschow]

You could use an LM317K (TO-3 case) or an LM317T (TO-220 case) attached to a heat sink with a low thermal resistance (<15C/W). It's simple with the advantage of having current and thermal protection.

 

[kchriste]

The easiest would be to use a LM317. The LM317 will disspate 3.6W at a 200ma load so be sure to heatsink it. You need a mica washer, insulated screw washer, and heatsink compound between the LM317 and the heatsink because the tab will have 6V on it.:

 

[Hero999]

Yes I agree, use the LM317, it's certainly the easiest solution.

If you want fun you can build an SMPs, it's more difficult but it's more efficient and requires no heatsink.
2-transistor Black Regulator

 

[BrownOut]

Easiest seems to be mostly a matter of personal preference. I count 5 components on MikeMI's circuit, not including a power supply bypass cap, which you'll probably want to include, and 7 components on the 317 curcuit. Of course, in the 317 case, you don't need to make any component choices, as they are already specified. Just as an interesting aside, if you crack upen a 317, you'll find a circuit almost identical to MikeMI's one.

 

[crutschow]

Just as an interesting aside, if you crack upen a 317, you'll find a circuit almost identical to MikeMI's one.

Without the overcurrent and overtemperature protection of the 317.

 

[Hero999]

Or regulation, the LM317 uses a bandgap reference, not a potential divider.

 

[BrownOut]

Correct, the reference is assumed to be in the regulated 24V.

 

[crutschow]

Easiest seems to be mostly a matter of personal preference. I count 5 components on MikeMI's circuit, not including a power supply bypass cap, which you'll probably want to include, and 7 components on the 317 circuit. Of course, in the 317 case, you don't need to make any component choices, as they are already specified.

For the 317 circuit I count two resistors, two decoupling capacitors, and the 317, for 5 total. What are the other two components?

 

[audioguru]

The datasheet for the LM317 shows optional extra parts and explains what they do.

 

[crutschow]

OK, I should have said "basic" 317 circuit.