555 pwm controller.

[HellasTechn]

Hello friends.

A colleague gave me a nail drill that was damaged (rpm stuck to max). I found the controller chip shorted and can not find a replacement anywhere (local stores-ebay-aliexpress).
It uses an 1Amp transformer to bring 220V down to 18 and a bridge-capacitor circuit to make it DC.

I thought of using an LM317 but that dissipates way too much heat to be used in such small box.

So i came up with the idea of using a 555 in free run mode as pwm controller and feed the output to a power transistor that would then drive the motor.

Any thoughts , ideas ?

 

[Minder]

For one, you rarely need regulation for PWM/motor control, the demand cannot be that high if only a 1 amp Txfr?
The drill motor must draw way more than that.
A little more explanation of what you have and wishing to control is needed.
You can obtain 555 controllers on Ebay for a couple of $$'s.
M.

 

[HellasTechn]

For one, you rarely need regulation for PWM/motor control

i was thinking to build the circuit pictured below.

The drill motor must draw way more than that.

not really. Anyway the transformer is indeed rated 1A.

You can obtain 555 controllers on Ebay for a couple of $$'s.

Yes i already have a few.

A little more explanation of what you have and wishing to control is needed.

To put it simple i need to control the speed of a motor that works with 18Vdc using a potentiometer. The current consumption will vary depending on the motor load (since it is intended to work on nails) but it will not exceed 1A since the transformer is rated 1A.

The 555 pwm looks a simple and cheap solution for me. i guess that the power transistor heat dissipation on a small heat sink under 1 amp will be ok for use within a small plastic box.

 

[Minder]

Using a 1amp supply for a device that may draw 1a or more is using an undersized supply IMO.
This is not a large supply, I would have used at least a 5a supply for margin.
If you already have some of the suggested PWM controllers then it will have a power Mosfet in it.
Also they normally come with smoothing caps.
So basically all you need is a Txfr and a bridge.
Basic PWM. http://www.discovercircuits.com/DJ-Circuits/simplepwm2.htm
M.

 

[HellasTechn]

Using a 1amp supply for a device that may draw 1a or more is using an undersized supply IMO.

The original power supply was 1A so that makes me think that it will not draw more than that. (note that the motor is also rated 18Vdc)
What i do not know is what impact will the new circuit have on current being drawn by the motor.

So if i understand correctly your advice is to use an actual pwm controller not a 555 as a pwm controller.

So basically all you need is a Txfr and a bridge.

what is Txfr ?

 

[Minder]

The one i linked to IS a 555 PWM controller, one most common way of implementing it, as does the cheap ebay versions.
Txfr shorthand for Transformer.
Incidentally What was the original IC,?
M.

 

[HellasTechn]

The one i linked to IS a 555 PWM controller

Ho i am sorry. i didnt really pay too much attention on the title.

Incidentally What was the original IC,?

I dont remember it right now but tomorrow i will let you know since i am away and dont have it with me.

This circuit you linked should be fine with the about 22V that DC from the bridge-capacitor right ? I know that the 555 accept 18V max. should i add some resistor load to stabilize the supply voltage to 18V ? Or better use an 7805Vreg to feed the 555.

 

[kellys_eye]

The original power supply was 1A so that makes me think that it will not draw more than that.

No. The motor will try to draw what it needs even to the detriment of the transformer (overloading it, blowing any internal thermal fuse etc).

If you have a transformer that has an output that EXCEEDS the drill requirement THEN the drill will draw 'what it needs' and the transformer will survive.

 

[(*steve*)]

Assuming that this transformer is what was supplied with the drill, then a simple PWM controller based on a 555 should be fine.

When you say "nail drill", I'm assuming it is some sort of device used for cosmetic purposes by a "nail technician". I can quite easily see that something like this would be low power.

 

[WHONOES]

Try the attached circuit for a very simple 555 PWM motor driver using an N channel MOSFET. In the circuit diagram, the motor is represented by R1. It should have diode across the motor with the cathode connected to the + supply.

 

[Minder]

This circuit you linked should be fine with the about 22V that DC from the bridge-capacitor right ? I know that the 555 accept 18V max. should i add some resistor load to stabilize the supply voltage to 18V ? Or better use an 7805Vreg to feed the 555.

Because the 555 circuit will be fairly low current and stay steady enough, You can use a simple series resistor a cap and and 15v zener to drop the 555 voltage.
M.

 

[WHONOES]

Because the 555 circuit will be fairly low current and stay steady enough, You can use a simple series resistor a cap and and 15v zener to drop the 555 voltage.
M.

Up to a point. But you must bear in mind that the FET gate pulse current will have a fairly high amplitude which would necessitate a fairly low impedance volt reg circuit. For my circuit, I would recommend, for a 24V supply, a 100Ω resistor and 3 to 5W 12V zener and at least a 220μf cap.

 

[Bluejets]

Beats me why you don't simply replace the original crook chip.

 

[WHONOES]

Beats me why you don't simply replace the original crook chip.

I think that HellasTechn claimed that he had tried to find a replacement but could not. You must be an Aussie with that turn of phrase!

 

[Bluejets]

I think that HellasTechn claimed that he had tried to find a replacement but could not. You must be an Aussie with that turn of phrase!

Yes, but not a convict decendant, plumber calls me the Viking

 

[WHONOES]

Yes, but not a convict decendant, plumber calls me the Viking

I've got rellies in Oz.
Like your attitude.

 

[HellasTechn]

Assuming that this transformer is what was supplied with the drill, then a simple PWM controller based on a 555 should be fine.

Yes it is the original transformet. the one that came with the device.

When you say "nail drill", I'm assuming it is some sort of device used for cosmetic purposes by a "nail technician". I can quite easily see that something like this would be low power.

Exactly.

 

[HellasTechn]

Beats me why you don't simply replace the original crook chip.

I can not find it anywhere !

The thing is that the person who asked me to fix it, does not want me anymore to do it because he found a used one for like 15$. I am going to build the circuit anyway because pwm controllers are new to me and i want to play around a little bit.

THANK YOU ALL FOR YOUR HELP !

I'll get back here for help later on !

 

[Minder]

So what was the original part No.?
M.

 

[HellasTechn]

So what was the original part No.?
M.

I have not forgotten about that. I will take a look tomorrow at work and let you know on friday.