Step down adjustable power supply

[michael howarth 1Participant @michaelhowarth1]

I had one of these pictured delivered today from Hong Kong. I can see the power in and power out terminals quite clearly but where is the adjustment? Is it the brass screw on top of the blue component? Or do I need a potentiometer somewhere in the circuitry?

**LINK**

Mick

[michael howarth 1Participant @michaelhowarth1]

[BazyleParticipant @bazyle]

Yes it will be the brass twiddly bit on the blue whatsit.

Keep an eye on how hot it gets if you are taking any significant power out of it.

[michael howarth 1Participant @michaelhowarth1]

Thanks Bazyle. I am putting in 12V DC and powering an electric screwdriver motor that I have adapted to power a filing machine. The screwdriver originally had a 3.6V Nicad but I believe the motor is a 3-12V Johnson. Does that sound OK? What can I do about heat…..a heat sink I presume?

Mick

[Tony Pratt 1Participant @tonypratt1]

OT I know, but where is the profit in this, I think it is 99p with free postage?

Tony

[michael howarth 1Participant @michaelhowarth1]

I was intending to build (or try, more like) one myself, but as you say Tony, the price is ridiculous and it only took just over a week to get here. I actually went as far as trying to buy the components from a well known electronics supplier but the sales man was less than helpful and I was embarrassed by my lack of knowledge so I abandoned the idea. Good luck to the man in Hong Kong.

Mick

[Les Jones 1Participant @lesjones1]

Hi Mick,
I do not think the current rating of the regulators is high enough to power the electric screwdriver. Even at its peak rating of 3 amps with a voltage of 3.6 volts that is only about 11 watts and with losses would result in about 1/100 hp

Les.

[Michael GilliganParticipant @michaelgilligan61133]

Mick,

I agree with Les … That chip is unlikely to handle a Screwdriver Motor.

I have some similar boards with on-board meters, which are great for LEDs … but the current-handling spec is optimistic to say the least !!

You would probably have more success using one of the ubiquitous "8Amp Dimmers" … which appear to be a standard PWM circuit.

[ I had planned to do some tests, then submit it to Neil for "Tip of the Month" … but I owe you, for the Drill-Powered Filing Machine idea ]

MichaelG.

[michael howarth 1Participant @michaelhowarth1]

Thanks MichaelG and Les……I will try a dimmer switch but what about this bit of circuitry **LINK**

Mick

[Neil WyattModerator @neilwyatt]

> OT I know, but where is the profit in this, I think it is 99p with free postage?

Probably a few tens of pence, but that's not the point – it's a way ebay sellers can rapidly build up positive feedback.

Neil

[Les Jones 1Participant @lesjones1]

Hi Mick,
I have just dismantled an old Black and Decker and checked the current. It took less current than I expected.
About 1.7 amps off load which increased to over 3 amps on load. I could not put much load on it as I was trying to hold the motor to the gearbox and hold the connections on at the same time. I think you should measure the current yours takes before ordering a different regulator. I do not think Michael's idea of using a dimmer switch (Assumption made. He is referring to a mains dimer switch.) will work for a few reasons. 1 They need to be fed from AC or at least totally unsmoothed DC so that the voltage passes through zero cyclicly to switch off the SCR or triac. 2 The diac which is normally used in the trigger circuit probably has a higher threshold voltage than your supply. It might work well enough if it was in the mains feed to the primary of the transformer in you power supply. (A real transformer – not a switched mode power supply.)

Les.

[Ian PParticipant @ianp]

Posted by Neil Wyatt on 15/08/2014 18:41:19:

> OT I know, but where is the profit in this, I think it is 99p with free postage?

Probably a few tens of pence, but that's not the point – it's a way ebay sellers can rapidly build up positive feedback.

Neil

Yes, but what does having a high feedback rating do in terms of making money?

If the idea is that once they have a high feedback they can put the prices up, if so I dont see how it can work. When the time comes for them to increase prices there will be competition from another seller just starting out and offering low prices!

Ian P

[Tony Pratt 1Participant @tonypratt1]

Posted by Ian Phillips on 15/08/2014 19:10:38:

Posted by Neil Wyatt on 15/08/2014 18:41:19:

> OT I know, but where is the profit in this, I think it is 99p with free postage?

Probably a few tens of pence, but that's not the point – it's a way ebay sellers can rapidly build up positive feedback.

Neil

Yes, but what does having a high feedback rating do in terms of making money?

If the idea is that once they have a high feedback they can put the prices up, if so I dont see how it can work. When the time comes for them to increase prices there will be competition from another seller just starting out and offering low prices!

Ian P

None of this makes sense, he has been a Ebay member since 2005, and good feedback does not pay the bills.

Tony

[Michael GilliganParticipant @michaelgilligan61133]

Posted by mick H on 15/08/2014 18:30:03:

Thanks MichaelG and Les……I will try a dimmer switch but what about this bit of circuitry **LINK**

.

Mick,

I think that one is very similar to your first.

… This is the type that I was suggesting.

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Les Jones 1 on 15/08/2014 19:06:48:

I do not think Michael's idea of using a dimmer switch (Assumption made. He is referring to a mains dimer switch.) will work for a few reasons.

.

Les,

I hope my last post to Mick has clarified this !!

… I was NOT suggesting a Mains Dimmer.

MichaelG.

.

Edit: for reference … here is the schematic.

Edited By Michael Gilligan on 15/08/2014 19:24:03

[Ian PParticipant @ianp]

A variable voltage DC supply is not the best way of driving a brushed motor, far better for your purpose in this case is a variable pulse width controller. eBay has lots of them (371047950882 is the first one I saw)

Search for 'DC motor speed controller'

Ian P

[michael howarth 1Participant @michaelhowarth1]

 

 

 

Edited By mick H on 15/08/2014 19:39:21

[Neil WyattModerator @neilwyatt]

> None of this makes sense, he has been a Ebay member since 2005, and good feedback does not pay the bills.

Ebay ratings depend on feedback in the last six months (IIRC) and good feedback gives buyers reassurance, hence greater sales of higher value items. It also encourages people to click on 'see sellers other items' so if you want to buy some hair…

Neil

[michael howarth 1Participant @michaelhowarth1]

I had to edit out my last post because posts were coming in so fast that it made mine redundant!

Thanks for all the information…. as is usual when I ask for assistance on this site, I have got loads to get stuck into and hope to report back soon.

Mick

[Ian PParticipant @ianp]

Posted by mick H on 15/08/2014 19:35:55:

 

Thanks Les, I will measure the current that it uses, over the weekend. I have been powering the file temporarily with a plug in type 6V 1amp charger, but it runs the motor a bit too fast and the charger gets a bit warm. I stumbled across a brand new Bermec 230V – 12V 5amp regulated power supply last weekend for which I paid the princely sum of £4 and thought that this would be more suitable for this and other applications if I could vary the voltage. Perhaps the dimmer switch idea in the mains feed to the Bermec might work?

 

Mick

Posted By mick H on 15/08/2014 19:35:55.

Edited By mick H on 15/08/2014 19:39:21

I have no idea what a 'Birmec' is but presuming its a linear DC PSU, putting a 'dimmer' in front of it is not going to work very well, if at all.

Using low voltage DC motors to drive workshop machines is at best a compromise. The lower the voltage the higher the current required for a give amount of power. The only thing I would consider if I wanted to drive something like a small drilling machine or grinder (say about 1/4HP) would be to use a 12V car battery and charger. Making a PSU to drive a 3 or 6 Volt motor that is going to produce any useful power is not a trivial task and speed control is also more complicated at very low voltage.

Even if your battery screwdriver originally had a 3V battery it would operate perfectly correctly off a 12V supply using one of the PWM speed controllers off eBay.

Ian P

[Ian PParticipant @ianp]

This is getting confusing!

I've just replied to a post that's disappeared.

Ian P

[Les Jones 1Participant @lesjones1]

Hi Mick,
Now I understand correctly the type of dimmer Michael was referring to I think it should work. The dimmer is a PWM controller but for use with a motor I would suggest adding a diode across the motor terminals (In the correct polarity.) to reduce the risk of damage from back emf spikes when the controller switches off. (Which it does many time a second.) The "5 amp rated regulator" does not say which switching regulator IC it uses so I would not risk buying it. One other concern with using one of the 3,6 volt screwdriver motors paticualarlly if you plan to use it with 12 volts is that it will not last very long. These screwdrivers would normally only be used for a few seconds at a time.

Les.

[BazyleParticipant @bazyle]

These items are probably made for something else and this is just the remnant dumped at cost by the manufacturer say 25p. Production will be fully automatic on a line usually used for computers etc with most labour being to put the final item in a plastic bag. Production rate about 60 thousand per shift and they aren't going to set up the machine if you only want one shift's worth.

BTW the regular bench filing machines like the Excel, Bustterfly etc had about a 1/3hp motor, but not saying it needs the power.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Les Jones 1 on 15/08/2014 20:27:30:

… The dimmer is a PWM controller but for use with a motor I would suggest adding a diode across the motor terminals (In the correct polarity.) to reduce the risk of damage from back emf spikes when the controller switches off. (Which it does many time a second.)

.

Thanks Les … we're on the same page.

I have not yet investigated the need for back emf protection, but one brief test with a small motor was O.K.

… these controllers do look very useful.

MichaelG.

.

Edit: For info … I have just found a note from my testing with a small LED panel:

I have just checked one of my 12-24Volt 8A units, on the Oscilloscope.

Operating Frequency is 1.237 kHz and [with a load] it produces nice clean pulses which vary in width but not in level … i.e. it performs exactly as it should.

With my little 12V battery reading 13.2V, the pulse train is only at 9.28V … so it appears to drop nearly four Volts.

Hope that's useful; although, of course, I can't guarantee that all similar-looking units actually have the same component values.

Edited By Michael Gilligan on 15/08/2014 21:04:26

Edited By Michael Gilligan on 15/08/2014 21:20:49

[MuzzerParticipant @muzzer]

A variable voltage DC supply is not the best way of driving a brushed motor, far better for your purpose in this case is a variable pulse width controller. eBay has lots of them

The LM2596 actually is a PWM buck converter, so it wouldn't actually be a bad choice for controlling a small motor. The switching frequency is 150kHz (hence the tiny inductor which is the small grey cube). The main issue here is that the max output of 3A sounds rather puny.

On the other hand, the 8A units are rather crude in comparison – they don't even have any form of current limiting or voltage regulation. The lack of current limiting may not matter much if you can avoid wiring shorts and the speed regulation will be pretty poor when driving a brushed motor anyway, even if you had a regulated voltage. As suggested, you'd want to fit a flywheel diode for driving a motor with one of these. Even better get a proper motor speed controller….

Murray

[Author]

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