Question for the cast of CSI Electronics®

[DICKEYBIRD]

After my embarrassing gaffe with the contactor coil/ammeter yesterday I figured what the heck, I'm on a roll...ask more questions.

I won a Tamura 24VDC 2.4A power supply on ebay. It arrived today and I was surprised at the size & weight. The thing is almost 6 inches square and weighs 4 1/2 lbs! Why does it need the huge transformer? Why all the other electronics? I recognize the bridge rectifier & the big capacitor but what's with all the other electronics....and the power transistor on the bottom? Please reduce my ignance and educate me; the Cliff Notes version.

ps: Not everyone makes a killing on ebay. No other bids and it sold for $5 plus $10.15 shipping. The postage sticker on the box read $16.95. The fellow definitely gets positive feedback.

[MaxHeadRoom]

Its a linear supply, but regulated, much heavier than switching supplies, they often have a crowbar short circuit protection, also you notice there is 2 terminals for each output, if you have a long run to feed the load, you use 4 terminals instead of two and take a pair to the load to sense the voltage drop and the output will increase accordingly.
The 2SD797 is the series regulator transistor.
Max.

[DICKEYBIRD]

Thanks Max, enlightening as always.

[rdfeil]

Milton,

The transformer is typical of that in a linear power supply. Linear meaning that everything happens at line frequency, 60 hertz. At low frequencies you need more iron than at high frequencies. That is why a switching type power supply, like in computers, have much smaller transformers and capacitors as they first simply rectify the line voltage to get high voltage DC then switch that at a much higher frequency, 40,000 to 100,000 hertz, so that the transformers need much less iron to perform properly. The 'extra' electronics is for regulation. Without regulation the voltage will not be stable with varying load currents. Also, that power supply has remote sensing regulation. Notice the two extra terminals next to the output terminals labeled S+ and S-. Those terminals are used to provide remote voltage sensing for the regulator circuitry. For example, say you were to locate the power supply a long distance away from what it is powering. You would have losses in the wiring between the power supply and the load. Now if the load is constant you could simply adjust the power supply to provide the proper voltage to the load, but is the load varies the voltage would have to constantly be adjusted to provide the proper voltage. Enter the remote sense circuitry. To use remote sense you run two extra wires from the power supply to the load, they are normally very small wires as they don't provide power they just measure the voltage at the distant end. Now when you set the power supply for the proper voltage the remote sense will automatically adjust the power supply to keep the voltage at the load constant.

This is a little more than the cliff notes version but I hope it helps.

Edit: Max beat me with the cliff notes version.

[DICKEYBIRD]

Thanks Robin, even I could understand that explanation.

Looks like I got my money's worth; that's a pretty decent piece of equipment. Ya never know when I'll need precisely 24V way across the shop from where the supply is mounted.

[J Tiers]

One added point.....

The supply is really a 30 volt (or thereabouts) 2.4 amp supply..... the "regulation" occurs by holding the OUTPUT constant while the input varies.

Since 24V at 2.4A is about 58 watts, and 30V at 2.4A is 72 W, the transformer has to supply a bit more than 72 watts or so. The difference between that and the output wattage is dissipated in that large power transistor, warming up the chassis/heatsink.

The transistor functions as a "variable resistor" * which is "adjusted by" the electronics to provide the exact needed voltage at whatever current is being drawn at that moment..

Actual numbers may vary depending..... but the idea is the same.

* The very name "transistor" is, IIRC, derived from "transfer resistor", which was a descriptive term used in the very early days of development of the device.

[darryl]

The regulator ic in there, the 723, is a decent regulator. I've seen those used in power supplies with 20 or more amps of output, and you wouldn't even see the voltmeter budge between no load and full load. The worst problem I see in any adjustable power supply is with the adjustment pot itself. They tend to develop an intermittent, which will throw things out of kilter- enough at times to cause damage in the supply, not to mention whatever load is attached.

That thing in there called VR1- that's the voltage adjustment pot. If you carefully mark the position it's in, then rotate it a bit (power off and stored charges dissipated, or unplugged for the night) you will feel a point where the contact has made a tiny dent in the track. This is the point you want to return the control to when you are done applying contact cleaner and working the control back and forth a bit. You should find the output voltage varying quite smoothly when you turn the control. A cheap analog meter is probably best for this test.

Regardless, this is probably the best preventive maintenance you could give that power supply.

[oldhat]

Hi all
I have work in electronics for over 40 years, in aerospace for 9 years.

From what I have seen is that a linear power supply last minnie times longer then switching power supply and take power surges better.
Switching power supply from china are no good the caps are under rated for Temperature and power factor and voltage. Go bad in 3 to 4 years if not faster.

Bill

[DICKEYBIRD]

Welcome Bill!

I have little electronics knowledge but after feeling the weight of the thing, looking at size of the components and the hand soldering, I totally agree with you. This thing is built to last...and to be repaired if needed.

[browne92]

I think Max is right. It's crowbar protected. The two big precision power resistors off to the left are for that. Darryl: VR1 is marked O.C.P. Over current protection. VR2 is marked V. Adj. That will be your voltage adjust.

Careful how you mount this thing. The large chunk of aluminum is probably grounded. The case of that regulator is probably not. I'm guessing it has unregulated voltage on it.

Another thing you don't see much anymore...transformers with taps for different input voltages.

Nice find.