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Lee Roberts
24-11-2008, 08:57 PM
Thanks for the info john, I’ve got a few TT's laying about that I had planned to use as a bench supply for testing bits and bob's.

What I would really like, is something that would allow me to turn a knob and change the voltage output to different amounts.

As and when I need to for supplying and testing different things, can you please recommended what it is I should use, what kind of "knob" I need to be looking at.

Panel mount voltage regulator?, I would more than likely use a little digital screen of sorts so I can fine tune the “knob” to just the right volt output each time.

Thanks guys !

Lee Roberts
24-11-2008, 09:10 PM
I'v just had a quick look on ebay and found this:

Link: http://cgi.ebay.co.uk/50Vdc-BLUE-LED-Dual-Battery-Panel-Meter-Regulator_W0QQitemZ380083553639QQcmdZViewItemQQptZ UK_BOI_Industrial_Tools_Generators_ET?hash=item380 083553639&_trksid=p3286.c0.m14&_trkparms=66%3A2%7C65%3A15%7C39%3A1%7C240%3A1318

This is the kind of thing i was thinking, i would prefer to change the output via a front panel using a knob, over poping inside the unit to change the dip switches everytime i need a new output amount.

irving2008
24-11-2008, 09:21 PM
I'v just had a quick look on ebay and found this:

Link: http://cgi.ebay.co.uk/50Vdc-BLUE-LED-Dual-Battery-Panel-Meter-Regulator_W0QQitemZ380083553639QQcmdZViewItemQQptZ UK_BOI_Industrial_Tools_Generators_ET?hash=item380 083553639&_trksid=p3286.c0.m14&_trkparms=66%3A2%7C65%3A15%7C39%3A1%7C240%3A1318

This is the kind of thing i was thinking, i would prefer to change the output via a front panel using a knob, over poping inside the unit to change the dip switches everytime i need a new output amount.
Well its a pretty voltmeter, but its not a voltage regulator...the way its described is misleading. It has a built-in regulator allowing it to be powered from a 5 - 15v supply - while reading that supply... so it can be used as a supply monitor.

Its not autoranging tho, so not much use for monitoring a truely variable power supply.

So lets start by listing what you want...

A power unit with an adjustable output voltage from 0 to 25v is a good range for hobby stuff, switched in 2.5 volt steps maybe ?

Output current at 5A maximum, current limited. Maybe a switched limiter, for .1,.5,1 and 5A?

Output voltage and current monitoring, current limit warning (LED, buzzer?)

What else?

What transformers etc have you got to start the ball rolling?

Alternatively...look here (http://cgi.ebay.co.uk/BENCH-POWER-SUPPLY-VARIABLE-0-20vdc-0-5A_W0QQitemZ120338909164QQ)

John S
25-11-2008, 01:54 AM
Try a Variac Lee.

Good idea but YOU MUST use an isolating transformer downstream of the Variac as these are at mains potentional.

Here's a Variac
http://cgi.ebay.co.uk/CLAUDE-LYONS-VARIAC-2-5A-0-to-270V-panel-mounting_W0QQitemZ110313909476

and here are some isolating transformers that can handle 110 volt.

http://cgi.ebay.co.uk/12-x-BBH-Isolating-Transformer-250VA-AC-Type-TU250-7YA_W0QQitemZ350125103969

John s.

irving2008
25-11-2008, 08:37 AM
A variac will give variable AC volts, but that's not what Lee was looking for if I understood his original post correctly - he wanted a bench supply for trying out bits and bobs.. I read that as a DC supply.. Lee?

Jeremy
25-11-2008, 09:08 AM
What sort of voltage and current range are you looking for?

What I think you need is something like an ordinary lab bench supply. These often come up on ebay for a reasonable price.

Alternatively you can build one easily enough. I have three or four variable voltage bench supplies that I've built, the most recent being one to power my home made capacitive discharge battery tab resistance welding machine (see pic below - it's the silver box on the left with the digital voltage readout).

http://farm4.static.flickr.com/3257/2617482299_8cf441ffcf.jpg

Jeremy

Robin Hewitt
25-11-2008, 12:40 PM
For my DC suds pump speed control I used an LM338T, just google for LM338 and get the data sheet with circuit diagrams.

Up to 5 Amps, needs a few extra components, but with a bit of jiggery pokery you can solder them on to the pins and enclose with shrink wrap.

Bit of an industry standard, costs a quid.

irving2008
25-11-2008, 01:06 PM
For my DC suds pump speed control I used an LM338T, just google for LM338 and get the data sheet with circuit diagrams.

Up to 5 Amps, needs a few extra components, but with a bit of jiggery pokery you can solder them on to the pins and enclose with shrink wrap.

Bit of an industry standard, costs a quid.LM338 is good device for a fixed supply, but not ideal for a variable unit unless you have a big heatsink. if you want 0 - 25v off a 30v supply at 5A then you are outside its thermal characteristics - it can dissipate 25W max so thats 5A at 25v output, dropping to 1A at 5v output. Has the advantage of simplicity tho with only 5 other parts needed (excluding bridge rec and smoothing cap)

Although slightly more complex (as in needing 6 more parts) a switch mode supply based on LT series devices needs no heatsink and gives variable current limit as well as voltage.

A similar spec bench supply on ebay is about 60 so by the time you bought the bits and made it up and boxed it for safety its marginal to make them really... except for the fun/satisfaction of knowing you did it yourself.

Lee Roberts
28-11-2008, 01:50 AM
A variac will give variable AC volts, but that's not what Lee was looking for if I understood his original post correctly - he wanted a bench supply for trying out bits and bobs.. I read that as a DC supply.. Lee?

Yes it DC i am after !


capacitive discharge battery tab resistance welding machine

Ermmm...lmao...O one of them :rolleyes:, that unit looks like a nice little thing to have, thats the kind of thing i was thinking about making.

The TT i have are:
Primary x2 120 Volt
Secondary 2 x 15 Volt

I think they maybe a bit big for what i want tho, given how small you can get them and given i only want to test things in the 0 - 30v range maybe.

i also just whanted to make a little gadget as well for somthing to do one night, i cant remember what the ampage is on the TT's but i think it was somthing like 160va, right now i'v got one hooked upto two (big) caps:

http://www.mycncuk.com/forums/showthread.php?t=261

as youll read that little box is nolonger going to be used and the contents going to waste, so i just wanted to make use if it all really, maybe you guys could point me the a direction to go with so as to make the most of what i'm hoping could be a very nice bence supply ? in terms of being able to go from a low out put, to a high out put both in DC and in amps should i need to.

I had a look on ebay and for 15 a ready made one dose look appealing, although i would rather design and make a home made system, i'v got leads BR and so on so all i would need really is an enclosure, display and maybe a few other bits.

I dont know, ill leave it with you guys for ideas and thoughts on the above please.

Cheers,

irving2008
28-11-2008, 08:57 AM
Yes it DC i am after !

The TT i have are:
Primary x2 120 Volt
Secondary 2 x 15 Volt

...
Cheers,
Well you have options... wire secondaries in parallel 15v AC 10A will give a 0 - 20v 10A supply, or in series will give 30v 5A will give a 0 - 40v 5A supply.

To make it variable you need to go down the switched mode supply route, else the heatsinking arrangements will be extensive (ask if you want explanation)... this will mean milling a PCB, you can't bird nest a switcher (any more than you can a high current stepper driver, indeed the two are very similar). A suitable fully-integrated chip for this will cost around 10 or a cheaper one with external MOSFET around 3 (again same issues as steppers) with about 10 worth external parts. You might be able to get the chip as a sample tho...

If you want we can do this as a design/learning exercise and I'll take you through it - then you'll have the satisfaction of both building it and designing it. And of course others can chip in with their views...

regards,
Irving...

Robin Hewitt
28-11-2008, 09:17 AM
I'd still go with the LM338T, it might run a tadge warm but it's so cheap and easy. Heck, just PM me a mailing address and I'll send you a freebie :beer:

irving2008
28-11-2008, 01:36 PM
I'd still go with the LM338T, it might run a tadge warm but it's so cheap and easy. Heck, just PM me a mailing address and I'll send you a freebie :beer:Robin, it'll run rather more than warm. On a decent 4degC/W heatsink the dissipation limit is 12.5W (at 25degC ambient) so at 20v input and 5V output the allowable output current before it goes into thermal shutdown is less than 800mA, rising to 2.5A at 15v output. Maximum output would be 18.8v at 5A.

Also there's no current limit, only short circuit protection, again not meeting expectations...

I'm not saying that you couldn't use this, and its cheap and simple, but Lee will have to downgrade his expectations and thats not making best use of his 160VA transformer when a 50VA would do.

regards,
Irving...

Jeremy
28-11-2008, 02:18 PM
re: using an LM338.

I guess it depends as to whether or not current limiting is really needed and whether or not there is a need to supply a constant high current.

The power supply in that pic I posted is a good example. It uses a 5A linear voltage regulator IC rather similar to the LM 338, but only needs to deliver full current for short periods when charging up the 1.445 F, 20V capacitor bank. The same might be the case when driving steppers with PWM chopper control, for example, where the average current might actually be pretty low much of the time.

Jeremy

irving2008
28-11-2008, 03:07 PM
I agree Jeremy. The point is that Lee wants a bench supply with a specific capability. If I was making a fixed supply then the LM338 is an ideal candidate. IF you want full output current to a given spec over a wide voltage range then linear regulators arent going to cut it. All depends on whats required. If Lee wants to reduce his expectations to 0 - 20v at .5A max guaranteed then the LM338 will do the job without overheating across the range. If he wants >1A output guaranteed then the LM338 isnt the way to go.... as the heatsink will be more expensive than the rest of it all together....

There is a secondary issue - is the goal just to produce something that'll 'just about do' or also to learn more about how to design a proper supply?

Irving...

Robin Hewitt
28-11-2008, 04:09 PM
Robin, it'll run rather more than warm. On a decent 4degC/W heatsink the dissipation limit is 12.5W

OTOH if he's bolting it to an aluminium enclosure he's going to get heaps better than 4degC/W.

I'm not saying there isn't anything "better", but does this particular pudding really require that many eggs? :rolleyes:

irving2008
28-11-2008, 04:36 PM
OTOH if he's bolting it to an aluminium enclosure he's going to get heaps better than 4degC/W.

I'm not saying there isn't anything "better", but does this particular pudding really require that many eggs? :rolleyes:
4degC/W is 300 sq cm of 1.5mm ali sheet held vertically and supported at corners only - I'd be surprised if the enclosure gave anything close to around 10degC/W - aluminium boxes are incredibly poor heatsinks (the type of alloy used makes a huge difference as well).

Eddy
28-11-2008, 06:13 PM
A small fan will do wonders for heat dissipation. You can pick up small fans that are 5v and 12v at several surplus shops.

I placed a 50mm fan over a dual 15A motor controller with no heatsink and it kept it sufficiently cool even at the full 15A pull. The most current I could pull with a heatsink and no fan was about 8-9A.

Eddy
www.wrighthobbies.net

irving2008
28-11-2008, 06:26 PM
A small fan will do wonders for heat dissipation. You can pick up small fans that are 5v and 12v at several surplus shops.

I placed a 50mm fan over a dual 15A motor controller with no heatsink and it kept it sufficiently cool even at the full 15A pull. The most current I could pull with a heatsink and no fan was about 8-9A.

Eddy
www.wrighthobbies.net (http://www.wrighthobbies.net)
You are indeed correct, however there is a world of difference between a pulse width modulated motor controller with a high peak current but low RMS value and a linear regulator with a constant high dissipation.

Eddy
28-11-2008, 09:38 PM
You are indeed correct, however there is a world of difference between a pulse width modulated motor controller with a high peak current but low RMS value and a linear regulator with a constant high dissipation.

You missed my point. The motor controller has to dissipate the heat caused by the voltage loss of the high and low side Vsat. A loss of 2V at 15A is 30 watts of heat to dissipate per IC (there's 2 on the board), a total of 60 watts with no heatsink. The movement of the air across the surface of the IC and the PCB is able to dissipate the 60 watts.

Eddy

irving2008
28-11-2008, 11:15 PM
You missed my point. The motor controller has to dissipate the heat caused by the voltage loss of the high and low side Vsat. A loss of 2V at 15A is 30 watts of heat to dissipate per IC (there's 2 on the board), a total of 60 watts with no heatsink. The movement of the air across the surface of the IC and the PCB is able to dissipate the 60 watts.

EddyEddy, I didnt miss the point - if its a motor controller is not dissipating 60w continuously, as its almost certainly a PWM arrangement. Its only dissipating that wattage during the 'on' cycle at a high frequency. take for example a common controller chip from ST. Its capable of 30A without a heatsink... but thats because the switching frequency is 10khz. for a single pulse at that frequency the thermal characteristic is <0.1degC/W so a heatsink of only 16sq cm of copper on the PCB will suffice to keep the mean junction temperature below 125degC. You could never run a linear circuit at that sort of load. The actual static chip thermal resistance is typical at 15decC/W... at 60W the jiunction temp would be 15*60deg over ambient = 900deg... it would have long since melted!

For the record I have designed systems for both miltary (-55 -> +125degC) and space applications (-100 -> +250degC)!

Eddy
29-11-2008, 12:20 AM
Eddy, I didnt miss the point - if its a motor controller is not dissipating 60w continuously, as its almost certainly a PWM arrangement. Its only dissipating that wattage during the 'on' cycle at a high frequency. take for example a common controller chip from ST. Its capable of 30A without a heatsink... but thats because the switching frequency is 10khz. for a single pulse at that frequency the thermal characteristic is <0.1degC/W so a heatsink of only 16sq cm of copper on the PCB will suffice to keep the mean junction temperature below 125degC. You could never run a linear circuit at that sort of load. The actual static chip thermal resistance is typical at 15decC/W... at 60W the jiunction temp would be 15*60deg over ambient = 900deg... it would have long since melted!

For the record I have designed systems for both miltary (-55 -> +125degC) and space applications (-100 -> +250degC)!

I assume you are referring to the VNH3SP30. This chip cannot handle 30A without a heatsink. It's thermal protection kicks in when more than 8A continuous is output using the small PCB footprint. It has a PWM input capable of 10K but it is supplied externally, fully under the control of the user. There is no internal PWM capability. This IC switches high and low side power MOSFETs as instructed by the inputs and PWM may be used if you chose to use it.

Again, my point is that airflow over the chip is much more efficient at dissipating heat than a hunk of metal.

Eddy

irving2008
29-11-2008, 01:23 AM
I assume you are referring to the VNH3SP30. This chip cannot handle 30A without a heatsink. It's thermal protection kicks in when more than 8A continuous is output using the small PCB footprint. It has a PWM input capable of 10K but it is supplied externally, fully under the control of the user. There is no internal PWM capability. This IC switches high and low side power MOSFETs as instructed by the inputs and PWM may be used if you chose to use it.

Again, my point is that airflow over the chip is much more efficient at dissipating heat than a hunk of metal.

EddyEddy, you are right in respect of the specific implementation as done by Pololu, but thats because they didn't provide the board area that ST recommend for full current operation - they say it needs a heatsink, ST's application note say otherwise if the PCB layout is right.

However, I am not disagreeing with you that airflow is a good dissipator of heat. The effect is still governed by surface area and all the airflow does is effectively increase that surface area. By way of example a typical 20mm x 20mm device fully exposed to a ducted 5m/s airflow (120cu ft/min 100mm dia fan) with no interruptions in the airflow is roughly equivalent to the device being fitted with a 32degC/W heatsink - i.e. better than no heatsink by a factor of 3 or 4, but airflow alone is not the answer to good cooling - good thermal management is the effective use of both properly designed heatsinks (not just hunks of metal) and properly directed airflow.

We are getting off topic here. The original issue was how to provide a variable voltage, high current, regulated supply. A solution based on linear devices could be made to work but the cooling becomes problematic leading to reduced expectations of what can realistically be achieved. I still contend that the right answer is not to generate the heat in the first place - a switch-mode low loss solution is the right approach to this requirement and need not be significantly more expensive or difficult to make.