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Step down adjustable power supply

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mick H15/08/2014 17:26:53
761 forum posts
28 photos

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

Bazyle15/08/2014 17:32:04
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6042 forum posts
220 photos

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.

mick H15/08/2014 17:41:49
761 forum posts
28 photos

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 115/08/2014 17:45:37
1699 forum posts
8 photos

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

Tony

mick H15/08/2014 17:54:07
761 forum posts
28 photos

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 115/08/2014 18:09:14
2234 forum posts
153 photos

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 Gilligan15/08/2014 18:26:17
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18972 forum posts
944 photos

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.

mick H15/08/2014 18:30:03
761 forum posts
28 photos

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

Mick

Neil Wyatt15/08/2014 18:41:19
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Moderator
18777 forum posts
733 photos
80 articles

> 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 115/08/2014 19:06:48
2234 forum posts
153 photos

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 P15/08/2014 19:10:38
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2526 forum posts
102 photos
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 115/08/2014 19:14:16
1699 forum posts
8 photos
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 Gilligan15/08/2014 19:14:52
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18972 forum posts
944 photos
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 Gilligan15/08/2014 19:21:12
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18972 forum posts
944 photos
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 P15/08/2014 19:23:32
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2526 forum posts
102 photos

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

mick H15/08/2014 19:35:55
761 forum posts
28 photos
 
 

 

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

Neil Wyatt15/08/2014 19:41:31
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Moderator
18777 forum posts
733 photos
80 articles

> 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

mick H15/08/2014 20:15:20
761 forum posts
28 photos

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 P15/08/2014 20:16:38
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2526 forum posts
102 photos
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 P15/08/2014 20:18:41
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2526 forum posts
102 photos

This is getting confusing!

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

Ian P

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