Variable 0-15V dc supply

[Puno]

I would like to design a power supply that not only supplies between this range but also able to display how fast a relay is switching, more info. this power supply will supply a dc to a tattoo machine and the tattoo machine works like a normal relay operation except that when the two coils pulled down the armature bar with the needle attached to it, it breaks the circuit so therefore it springs back up completes the circuit and the needle is pulled in again and so on. So the idea is would like to know what sort of components involved in order to record this on, off cycle inorder to tell me how fast the needle is driven.

 

[(*steve*)]

You want 2 different things. A power supply will provide power. Some sort of frequency meter will count the cycles per second.

There are many designs for variable power supplies (most will go no lower than maybe 2 volts though. A very important consideration is the current required. You need a supply capable of supplying a little more than is required (to give it a bit of margin).

As for counting the operating frequency, this is less trivial to do, perhaps the cheapest option would be a multimeter with a Hz range.

 

[poor mystic]

Hi
I have designed power supplies for bonecarving tools which ran from 0V to about 80V. I used small variacs which were rated at 115V from memory.

I would advise first an isolating transformer (because if your customer gets a shock you could be responsible for their death).
Use a step-down transformer and a variac like http://cgi.ebay.com/POWERSTAT-N116B...64702?pt=LH_DefaultDomain_0&hash=item4cf6f681. Rectify the output using a 10 Amp full-wave rectifier.
The difficult thing to guess is how much smoothing capacitance to use - too little and the tattoo machine may not run smoothly, too much and the rectifying diodes will blow up. A bit of suck-and-see is required to get it right.

I think that you should get a professionally-registered biomedical technician to build this equipment. Although I am electrically registered I would not be prepared to take responsibility for building this gear myself, and I cringe to think that any blame could become attached to me through inadequately warning you in this post. Lots and lots can go wrong.

I have attached a rough drawing of the circuit I recommend.

 

[(*steve*)]

The first isolating transformer is almost certainly superfluous. The step down transformer is isolating.

It's a good way to vary the voltage. It's not regulated, but with a constant load that will not matter greatly.

 

[poor mystic]

Hi Steve
Of course you are right that the step-dow transformer provides isolation, it's just that where the consequences of failure of isolation might include death of a customer I prefer certified isolation over mere common sense. My comments are conservative because I do not wish to be in any way responsible or liable for such failure.
My comments are quite serious, including the one about a biomedical engineer, though I can easily believe that some people would think my caution excessive.

 

[Puno]

first thanks for the response, now the first part of my power supply unit is no problem in designing it as to output a 4A at least and between 3-15v or 3-18v output. i have forgotten about the multimeter with freq mode on it maybe i should try that one out first and see what happens. Cause if that works then the whole project is achieved just get a cheaperer DMM that reads freq in Hz and there you go, problem solved.
thanks to everyone for their inputs.

 

[poor mystic]

Thanks for your thanks, Puno!
You might need to put a resistor of say 10 Ohms in series with the tattoo machine. The current supplied to the machine will develop a voltage across the series resistor and you'll be able to monitor current pulses into the machine by measuring across the resistor.
Good luck!

PS Use a 15V transformer rated at at least 100W and that way your whole design will be nice and conservative with things never getting hot and consequently failing. A cool design is a good design... it won't cost much more to do it right.

 

[nbw]

A cool design is a good design... it won't cost much more to do it right.

Good advice - I follow the same when building amplifiers. Better to go one size up on the transformer so it's not running too warm or hot. I have a saying 'forty-nine is fine' - up to 49 deg C is ok for me, if it's running hotter than that at a normal load for the application, I start asking questions.

 

[Puno]

I think this topic with the transformer rating, to my knowledge as long the load stays within the VA rating of the transformer will be fine.

 

[(*steve*)]

If your load were simply a resistor (or something purely resistive, I would agree with you -- almost.

It's always worth designing in a bit of spare capacity for pretty much the same reason as you don't operate your car's engine at the red line constantly...

However, when you have a more complex load, it can be harder to determine exactly what it represents to the source of power driving it. In this case you have 2 options. Firstly you can do complex calculations, measurements, etc., and come up with a suitable power rating, or you can simple choose a device that will be big enough. If you need things to be as small as possible, then go the first route. Otherwise the second will be simpler and probably less prone to error. Poor mystic is suggesting the easy route.

nbw provides his heuristic to determine if there are problems by setting an arbitrary temperature limit of 49C. If your transformer gets that hot then maybe you need to reconsider whether it is large enough (of course, if your ambient temperature is 40C you may easily reach this)

So, whilst what you say is in theoretically correct, consider that: In theory there is no difference between theory and practice, but in practice there is.