Cheap Variable Power Supply

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Rob Thomson
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Cheap Variable Power Supply

Post by Rob Thomson » Thu Jan 26, 2012 2:10 pm

Recently I was out looking for a variable DC power supply.

Something that would give a power output in the range of 1.5 - 12volts would be ideal.

While searching around I spotted on ebay a number of potential candidates. Surprisingly they are being sold as 'Tatoo Power Supplies'. Some very economical ones can be found for as little as $15 - $20.

I quickly purchased a suitable looking candidate.

On receiving the supply I was initially struggling to get anything out of it.
outside.JPG
As can be seen from the picture - the unit had three audio style jacks on the front, and 1 switch. These where labeled.

LEFT | RIGHT | FOOT

Having no documentation to hand I whipped open the case and started to trace the circuit on the board.

In the end the fix was really easy. In the 'Tatoo' world they no doubt use a foot pedal to turn power on & off. The socket labeled as 'FOOT' no doubt was designed to have some sort of switch attached to it.

I solved the issue by simply removing the audio socket and hard wiring a jumper across the polls.
inside.JPG

You can see on the inside shot above where the blue jumper bridges the pins.

Not content at this point I then modified the 'lead' that they provided as standard. The stock system had a rather odd adapter on the end that I can only assume is known in the Tatoo World!

I simply took that off and wired in two rather more useful leads. (JST & Servo)
lead.JPG
Tests with the supply using a meter have shown the onboard display to be accurate to within 0.1 of a volt.

All up.. I am happy with the unit. It is cheap - simple - and saved me spending more than $100 on an 'electronic stores' one!

Rob
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erazz
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Re: Cheap Variable Power Supply

Post by erazz » Thu Jan 26, 2012 2:21 pm

Nice find!
This is a must for electronic tinkerers... I splurged and bought a digital PS but for the low AMP stuff that I need this could have been just as good.

You might consider adding a couple of capacitors on the output to reduce ripple and make it really good!
Z

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Rob Thomson
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Re: Cheap Variable Power Supply

Post by Rob Thomson » Thu Jan 26, 2012 4:10 pm

Good idea! Any thoughts on the size?
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Re: Cheap Variable Power Supply

Post by SkyNorth » Thu Jan 26, 2012 7:32 pm

Do not put a large caps on a regulator output , put them on the input.

From the picture it uses a 3 terminal LM317 regulator.

Linear regulators do not like large capactive loads , it can cause the voltage on the output of the regulator to be larger
than the voltage on the input to the regulator when the power is switched off , this can cause a failure in the Regulator.

A larger cap on the input side of the regulator will help a bit.

A cap in the range of 2200uf /25V would work well on the input side of the regulator.

-Brent

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Re: Cheap Variable Power Supply

Post by Flaps 30 » Fri Jan 27, 2012 11:47 pm

SkyNorth wrote:Linear regulators do not like large capactive loads , it can cause the voltage on the output of the regulator to be larger
than the voltage on the input to the regulator when the power is switched off , this can cause a failure in the Regulator.
Which is why when I am putting together a linear regulator PSU, that I fit a diode across the input and output pins of the regulator (diode anode to the output pin on positive regulators) to prevent that from happening. That is in addition to a crowbar circuit in case the regulator should go short circuit, but that is another story.


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