2nd Power Supply

[ericgibbs]

Mark,
You will most likely find that when working a project a number of supply voltages are required. This 'design' will only ever give one output voltage at anyone time.

I would suggest you reconsider trying to make ' a doing everything' psu.

Make up 3 bench supplies,
1. 30Vdc thru 20Vdc
2. 24Vdc thru 12Vdc
3. 12Vdc thru 1.5Vdc

you can also buy low cost wall wart 'dc' psu's for fixed voltages

 

[mark_3094]

Sounds like a good plan. This isn't just about getting a power supply though. A lot of it is getting experience and learning how it works, which is why I want to build one rather than buy one.

I'll start with the 1.5 - 12v.

I still like the idea of using the multitap transformer to cut down the input voltage depending on what's selected. Is there any good way to do that? Or should I just be content with using heatsinks to dissipate heat?

 

[ericgibbs]

If you used a centre tapped transformer, say 12Vac 0v 12Vac and a fullwave bridge, it would be possible to switch from the centre tap to one of the transformer ends.

That would give a ranges of 12Vac and 24Vac.

do you follow.?

 

[mark_3094]

I'm not familiar with centre tapped transformers. I assume there is three terminals, 2x 12v and 1x 0v, and the two 12v can add up to 24v?

I have two transformers with varying ranges, but they don't sound like that. They have several output voltages.

If I had 15v coming out of the transformer, and in theory I had the the voltage regulator set to 1.5v, would a normal heatsink made for a voltage regulator be enough to dissipate the heat?

 

[ericgibbs]

I'm not familiar with centre tapped transformers. I assume there is three terminals, 2x 12v and 1x 0v, and the two 12v can add up to 24v?

I have two transformers with varying ranges, but they don't sound like that. They have several output voltages.
Post what these voltages are.

If I had 15v coming out of the transformer, and in theory I had the the voltage regulator set to 1.5v, would a normal heatsink made for a voltage regulator be enough to dissipate the heat?
The 15Vac from the transformer would be rectified and smoothed to about 20Vdc.
So the 'off load' voltage would be 20Vdc, at 1amp into a load at 1.5V the
20Vdc would drop to about 15Vdc, so you would have 15V-1.5V across the regulator..

So thats 13.5V * 1amp = 13.5Watts!!!!!

Mark,
Do you know what the voltages are on the multi tap transformer also the VA or current rating of the transformer.??

 

[ericgibbs]

Mark,

Something like this dwg.

 

[audioguru]

Hi Mark,
Your 100 ohm resistor from the output to the ADJ pin of the LM317 has a voltage across it of 1.25V. Then its current is 1.25V/100 ohms= 12.5mA. Its heat dissipation is only 1.25V x 12.5mA= 0.016W so a big 1W resistor is not needed.

Like all other IC regulators, the LM317 has thermal shutdown when it gets too hot. It turns off then turns on again when it has cooled a little.

Use a two-pole switch. One pole switches the transformer taps and the other pole switches the voltage-setting resistor.

 

[mark_3094]

Thanks. I have removed the 1w resistor in favour of a regular one.

So I'd need a switch with two poles, but about six throw? (That is assuming I have six selectable output voltages)

I had thought of that, but then I thought that it would need to handle up to 1.5A of current through the switch if it's switching the transformer. Is that right?

 

[mark_3094]

The power supply seems to be working, but I still have trouble getting a small DC motor to work.
It is a small 6v DC motor that runs fine from a battery, but when I connect it across the output of this power supply, the output voltage drops to zero.
Is there special rules for how a motor should be connected to a circuit / power source?

 

[audioguru]

The power supply seems to be working, but I still have trouble getting a small DC motor to work.
It is a small 6v DC motor that runs fine from a battery, but when I connect it across the output of this power supply, the output voltage drops to zero.

Measure the resistance of the motor. Subtract the resistance of the meter's leads. The motor's current will be 6V divided by the resistance when it starts.
Measure the current of the motor when it runs from a 6V battery.
Measure the votage of the transformer when the motor tries to start to see if its voltage drops too low (must not not less than 9VAC).

The LM317 has an output current of up to typically 2.2A (1.5A minimum) only if the voltage from its input to its output is 15V or less and if it does not get hot. It must have a pretty big finned aluminum heatsink with thermal grease between.