Variable voltage dropper for speed control of tattoo machine

[Gryd3]

Ill grab a multimeter tomorrow, how does one go about measuring? It will probably be easier than asking Roy, but ive messaged him, so the ball is in his court..

Ive been describing 2 different styles of machines in previous posts.. coil and the rotary, its the rotary that has the motor im looking to build my powerpack for..
I use about 4.5v for my rotary machine to do dotwork.

I see you already found out how to measure current with a meter. Some meters require you to plug the red lead into a different socket labelled specifically for measuring current. Don't worry though, they are lebelled with a big A right beside the socket for the red probe lead.
Rotary is just a small DC motor, it should have a consistent DC current draw, but will draw a little extra when it is working harder, or starting up.
If you end up measuring yourself due to a delay or Roy's message, can you get a couple readings?
Set the power supply at the 4.5V you want, and record the current draw.
Set the power supply to another voltage or two that you work with and record those current draws as well.

I'm not asking you to burn out your motor so only give us the higher end of the reading your after. If we get 2 or 3 sets of Voltage to Current readings, we can be sure to recommend an ideal, cost effective, easy solution for you.

 

[Kastrosama]

Ive gone over old messages with roy from when i was going to buy the machine, he said that at 9v it would push a certain needle size, so it will definitely take 9v.

So ill do a reading at 4.5 but ill also do a reading at 9v as its going to be the largest possible draw.
With that, if i get a pot that suits 9v, it shouldnt give me any hassles in future unless i wear it out from constant use.
Is it possible to buy a pot that also clicks into an off position?

 

[Arouse1973]

If you insist on using just a POT then yes you can get them with a on off switch. I have only seen low power versions, not sure if they do wire wound POTs with a power switch. You will have to Google it.
Adam

 

[Kastrosama]

Hi adam, the reason i want to use the pot is to vary voltage.
When you use a larger needle in the tattoo machine, there is greater resistance from the skin as theres a greater contact area.
To overcome this you turn up your voltage a small amount.. 0.5 or 0.6 of a volt makes a difference.
Larger needles (larger grouping of needles soldered together) will need larger voltage.
Hence making it variable to suite what needle is being used.

 

[Arouse1973]

Yeah I get that, but earlier suggestion of a voltage source would be better. This reduces the size and cost of the POT. That's what I meant.
Cheers
Adam

 

[Kastrosama]

Howdy, just back from the studio..
Ran the machine at 4.5v for a few mins, got a constant reading of .10
Then i ran it 9v for the same amount of time and got .12
No fluctuation in reading.

 

[KrisBlueNZ]

A good solution would be an emitter follower. This will act as a buffer for the potentiometer and supply the required current. This will enable you to use a standard low-power potentiometer without worrying about power dissipation in the potentiometer. And as long as the input voltage is regulated, the output voltage will be stable too.

You can use a standard power transistor or a Darlington transistor. I recommend a Darlington because of the high current gain. However they will drop at least 1.5V so your maximum output voltage will be 1.5V less than your supply voltage. If that's a problem, increase the supply voltage!

Suitable Darlington NPN transistors would be a BDX53 or any NPN in the TIP1xx family. For example, http://www.digikey.com/product-detail/en/TIP121G/TIP121GOS-ND/920326

Here's a connection netlist.

Components:
V_IN: Input supply voltage, up to 30V DC
VR1: potentiometer, rotary or slider, linear, 1kΩ~50kΩ
Q1: NPN Darlington power transistor e.g. BDX53, TIP120/1/2, TO-220 package
V_OUT: Output voltage, DC

Connections:
1. V_IN positive to VR1 clockwise end and to Q1 collector
2. VR1 wiper to Q1 base
3. Q1 emitter to V_OUT positive
4. V_IN negative to VR1 anticlockwise end and to V_OUT negative.

You may need a heatsink on Q1. For example http://www.digikey.com/product-detail/en/RA-T2X-38E/RA-T2X-38E-ND/2416488 or something larger.

 

[Gryd3]

Howdy, just back from the studio..
Ran the machine at 4.5v for a few mins, got a constant reading of .10
Then i ran it 9v for the same amount of time and got .12
No fluctuation in reading.

That is perfect, can you confirm that you had the meter set to 1A? that would mean you have a draw of 100 - 120mA which is easily manageable.

A good solution would be an emitter follower. This will act as a buffer for the potentiometer and supply the required current. This will enable you to use a standard low-power potentiometer without worrying about power dissipation in the potentiometer. And as long as the input voltage is regulated, the output voltage will be stable too.

You can use a standard power transistor or a Darlington transistor. I recommend a Darlington because of the high current gain. However they will drop at least 1.5V so your maximum output voltage will be 1.5V less than your supply voltage. If that's a problem, increase the supply voltage!

This is planned to be portable and run off a 9V battery. Seems as though a 7.5V limit is ok. Thank you for the quick simple circuit.

Kastrosama, the 9V battery will provide you with about 4-5 Hours of run-time. If you buy 'Lithium' 9V batteries, you can push that to a full day. If you buy rechargeables, many will only last a couple hours tops. Kris's circuit will do just fine for what you want. If you want to try a potentiometer anyway you know the risks. At 4.5V, the potentiometer will need to dissipate 0.5Watts which is very likely to cause a little smoke..
Alternatively, depending on how portable you actually want it, you could also use a 12V alarm/motorcycle battery. They are a little bigger, offer more headroom to play with the output voltage you want and are rechargeable. They will last 15-20+hours on a single charge, but due to size, would need to be set on a table or the ground like your current power supply at the studio. (But does not need to be plugged in)

 

[Kastrosama]

A good solution would be an emitter follower. This will act as a buffer for the potentiometer and supply the required current. This will enable you to use a standard low-power potentiometer without worrying about power dissipation in the potentiometer. And as long as the input voltage is regulated, the output voltage will be stable too.

You can use a standard power transistor or a Darlington transistor. I recommend a Darlington because of the high current gain. However they will drop at least 1.5V so your maximum output voltage will be 1.5V less than your supply voltage. If that's a problem, increase the supply voltage!

Suitable Darlington NPN transistors would be a BDX53 or any NPN in the TIP1xx family. For example, http://www.digikey.com/product-detail/en/TIP121G/TIP121GOS-ND/920326

Here's a connection netlist.

Components:
V_IN: Input supply voltage, up to 30V DC
VR1: potentiometer, rotary or slider, linear, 1kΩ~50kΩ
Q1: NPN Darlington power transistor e.g. BDX53, TIP120/1/2, TO-220 package
V_OUT: Output voltage, DC

Connections:
1. V_IN positive to VR1 clockwise end and to Q1 collector
2. VR1 wiper to Q1 base
3. Q1 emitter to V_OUT positive
4. V_IN negative to VR1 anticlockwise end and to V_OUT negative.

You may need a heatsink on Q1. For example http://www.digikey.com/product-detail/en/RA-T2X-38E/RA-T2X-38E-ND/2416488 or something larger.

Im not going to lie, its all well and good saying the above.. but im lost
Is there a possibility of a quick drawing? from there Im sure ill be able to figure it out.
Cheers lads

 

[Arouse1973]

A good solution would be an emitter follower. This will act as a buffer for the potentiometer and supply the required current. This will enable you to use a standard low-power potentiometer without worrying about power dissipation in the potentiometer. And as long as the input voltage is regulated, the output voltage will be stable too.

You can use a standard power transistor or a Darlington transistor. I recommend a Darlington because of the high current gain. However they will drop at least 1.5V so your maximum output voltage will be 1.5V less than your supply voltage. If that's a problem, increase the supply voltage!

Suitable Darlington NPN transistors would be a BDX53 or any NPN in the TIP1xx family. For example, http://www.digikey.com/product-detail/en/TIP121G/TIP121GOS-ND/920326

Here's a connection netlist.

Components:
V_IN: Input supply voltage, up to 30V DC
VR1: potentiometer, rotary or slider, linear, 1kΩ~50kΩ
Q1: NPN Darlington power transistor e.g. BDX53, TIP120/1/2, TO-220 package
V_OUT: Output voltage, DC

Connections:
1. V_IN positive to VR1 clockwise end and to Q1 collector
2. VR1 wiper to Q1 base
3. Q1 emitter to V_OUT positive
4. V_IN negative to VR1 anticlockwise end and to V_OUT negative.

You may need a heatsink on Q1. For example http://www.digikey.com/product-detail/en/RA-T2X-38E/RA-T2X-38E-ND/2416488 or something larger.

Thats a good one Kris. But if the supply changes a bit under load then this will be seen on the output ,dependant on wiper position. Is this ok l dont know?
Adam

 

[Gryd3]

This may or may not do it for you.

The potentiometer will have 3 leads on it.
The Transistor Kris mentioned will also have 3 leads on it and a tab.


There are more robust options, but this will get you going without needing a really beefy potentiometer.

 

[Arouse1973]

A good solution would be an emitter follower. This will act as a buffer for the potentiometer and supply the required current. This will enable you to use a standard low-power potentiometer without worrying about power dissipation in the potentiometer. And as long as the input voltage is regulated, the output voltage will be stable too.

You can use a standard power transistor or a Darlington transistor. I recommend a Darlington because of the high current gain. However they will drop at least 1.5V so your maximum output voltage will be 1.5V less than your supply voltage. If that's a problem, increase the supply voltage!

Suitable Darlington NPN transistors would be a BDX53 or any NPN in the TIP1xx family. For example, http://www.digikey.com/product-detail/en/TIP121G/TIP121GOS-ND/920326

Here's a connection netlist.

Components:
V_IN: Input supply voltage, up to 30V DC
VR1: potentiometer, rotary or slider, linear, 1kΩ~50kΩ
Q1: NPN Darlington power transistor e.g. BDX53, TIP120/1/2, TO-220 package
V_OUT: Output voltage, DC

Connections:
1. V_IN positive to VR1 clockwise end and to Q1 collector
2. VR1 wiper to Q1 base
3. Q1 emitter to V_OUT positive
4. V_IN negative to VR1 anticlockwise end and to V_OUT negative.

You may need a heatsink on Q1. For example http://www.digikey.com/product-detail/en/RA-T2X-38E/RA-T2X-38E-ND/2416488 or something larger.

Would you need a current limit resistor so when the POT is up the top of its travel to limit base current?
Adam

 

[Arouse1973]

This may or may not do it for you.

The potentiometer will have 3 leads on it.
The Transistor Kris mentioned will also have 3 leads on it and a tab.


There are more robust options, but this will get you going without needing a really beefy potentiometer.

Do you think a flyback diode would be prudent on the output?
Adam

 

[Gryd3]

Do you think a flyback diode would be prudent on the output?
Adam

Good call. It's driving an inductive load.
This is more of Kris' brainchild if I drew it correctly. I'd use an LM317 but mine might be overkill

 

[KrisBlueNZ]

Gryd3's diagram in post #31 is what I'm suggesting. The battery needs to connect with positive at the top.

Yes Adam, the output voltage will vary if the battery voltage varies due to changes in load. That's definitely not ideal but I'd say it's unlikely to be a problem in practice.

No, there's no need for a base resistor. It's an emitter follower circuit.

You could put a diode across the output and another one across the transistor. Probably more helpful would be some biggish capacitors. I can't draw a schematic at the moment but I'll post one in a few hours.

I don't think a PP3-type 9V battery will give very good performance. They're designed for pretty low current applications. You would only get a few hours maximum out of one of those. I would use a battery of C cells, at least - NiMH or some lithium chemistry - or a small SLA/VRLA battery.

 

[Kastrosama]

Only a few hours of use is perfect.. 1 or 2 hours is loads really.

 

[Gryd3]

Only a few hours of use is perfect.. 1 or 2 hours is loads really.

If you're happy with that, pack a couple spares. They will work, but the times quoted are essentially running them dry which may result in less run time. Its easy enough to hook up a new battery though

 

[Kastrosama]

true that! a new battery is much easier than battling and trying to expand battery life!
So, is that it then? the diagram above will do the job or does it need to be modified?
Im possibly going to have a PCB printed up to make it nice and neat... that is if I can figure this all out

 

[Gryd3]

true that! a new battery is much easier than battling and trying to expand battery life!
So, is that it then? the diagram above will do the job or does it need to be modified?
Im possibly going to have a PCB printed up to make it nice and neat... that is if I can figure this all out

You could always just use protoboard. If your going to invest a bit more to print a PCB, you might as well build a more robust circuit

The one you have will work, but the output will drift slowly downward as the battery dies. Other solutions will make the output more stable, so the battery can be half dead and you will still have the same voltage to your gun.

Other than that choice, the only other thing to consider is what battery you use.

A 9V will work, but not for prolonged periods, but you can easily hook up a bigger battery like a sealed lead acid 12V battery that Kris had mentioned above. They will pack well over 10 times the charge and can let you run almost all day and you can recharge em
Of course, lots of battery choices are available, and you can simply unplug one, and plug in another.

Have fun!

 

[KrisBlueNZ]

Sorry for the delay. Here's a design with all the features we've discussed.