I need someone to ID a doohickey.

[websterz]

I recently replaced the DC motor control on my home-brew mini mill power-feed with the 30 amp module pictured below. The controller runs a 14.4 volt cordless drill motor utilizing a 12 volt 6 amp battery charger for the power supply. I reverse polarity with a heavy duty DPDT toggle switch. The old controller was only rated for 5 amps and eventually the traces on the board gave out. It was just a proof of concept fix, and since it worked reasonably well I changed out for the heavier controller which I felt would have more than enough cojones for the intended purpose. I installed the board, wired everything up and did a few dry runs with my multi-meter in place of the motor. Everything looked good, and ran quite well when I finished assembling it. I was running the table to the left under power to line up for a cut when the doohickey circled in pink popped like a firecracker. Scared the crap out of me! I cut the power and pulled the cover off and had doohickey guts all over the controller...looked like firecracker paper. I am no electronics whiz but I guess the doohickey overloaded? At any rate the board was in a fully sealed aluminum enclosure, mounted on brass PCB stand-offs with fiber insulating washers. I blew out the enclosure before I installed the board to make sure there was no swarf in it, and covered the ventilation holes with fiberglass window screen to keep the chips out. Cooling is supplied via a 12 volt muffin fan drawing air though the enclosure and venting through the side of the motor housing, again covered with screen. Here's where I get confused. Even with the dookickey guts blown out, and a couple of tabs sticking up where the doohickey USED to be, the controller still runs just fine, both directions, nothing seemingly affected by the pop. Any ideas why? Do I need to replace the doohickey? Do I even NEED it?

[John Hill]

Yes, you should replace the doohicky, the designers were careful not to put anything surplus in there.

The doohicky is a capacitor and it will have it value clearly written on the side.  It will be easy to find a replacement at any electrical/electronics parts store or repair shop and it will not cost more than a cup of coffee.  Just take note of which way it goes on the board, that black strip is the key.

John

[websterz]

Yes, you should replace the doohicky, the designers were careful not to put anything surplus in there.

The doohicky is a capacitor and it will have it value clearly written on the side.  It will be easy to find a replacement at any electrical/electronics parts store or repair shop and it will not cost more than a cup of coffee.  Just take note of which way it goes on the board, that black strip is the key.

John

Well, the side of the capacitor is now shrapnel. Guess I need to locate a schematic for the board, huh? Soooo...why does the controller still work? And any idea why the cap popped? The board is rated for 12-30 VDC and my power supply doesn't exceed 12. In fact it may be a tad lower than 12. Surely that couldn't cause it? The ammeter on the charger showed about a 2 amp draw so I know it wasn't an issue with the current.

[websterz]

Okay, found a schematic and it shows a 35 volt 47 microfarad capacitor. Guess I'll hit radio Shack tomorrow and grab a replacement. Actually I think I'll look through my vast PCB collection first and see if I can't get lucky and find one I can recycle. Damn I'm cheap! 

[websterz]

This is the one I need, right?

http://www.radioshack.com/product/index.jsp?productId=2102512

[dsquire]

John

That looks like it should do the trick for you.

Cheers 

Don

[Bluechip]

Websterz


What is the output from your Batt. Charger ?

IF that cap. is across the input AND you charger is not a smoothed DC, it will be having a desparately hard time, and will fail catastrophically.

You need a Low ESR cap across the input, if this is the case. One about 1000uF per Amp of output is about right. Maybe a bit more.

A lot of older types of Batt. Charger were 'Rectified AC' output. Not a smoothed DC.

Ther's some stuff on Wiki  .. Look for low esr, dissipation factor, tan d.

Did it on an electronics forum months ago, either enquirer sorted his problem or he gave up. Heard no more.   

Just a thought ..

Dave BC

[andyf]

The reversing switch is downstream of the board, rather than between the power supply and the board? These electrolytic capacitors are polarised, and don't like being subjected to reverse voltage. A salvaged one should be perfectly OK, if it's 47uF, with a voltage rating of at least 35V. One with a bigger voltage rating would be fine, if it will physically fit. Make sure there are no bits of foil etc left from the old one, ready to cause a short-circuit.

I speak as one who once accidentally reversed the polarity on a 4" diameter, 470 MILLIfarad one in the kitchen    . It took hours to clean up the mess, and there were consequential losses - dinner for two, to stop the  over what she had carefully prepared and left simmering on the stove.

Andy 

[NickG]

That's what I was just going to say. I've bought what looks a very similar PWM board from ebay to control an electric locomotive. I noticed that they do ones with reverse built it, but I thougt, why do I need that, I will just revers polarity after the output of the circuit board. As Andy said, electrolytic capacitors are polarised so you have to keep your input as intended and when you solder it back on, make sure it's the right way around!

They sound pretty dangerous in that size Andy! I remember when we were at school purposely connecting little ones the wrong way around to get them to and get the puff of white powder ... no idea what it was!

Nick

[Bluechip]

I think the DP switch must be after the PSU.

I doubt if the chip would work at all on reverse polarity, he does say it worked for a bit.

Dave BC

[No1_sonuk]

The pic was off the bottom of the window while I was reading, but from the description I could tell it was an aluminium electrolytic that popped.

Why no photo of the mess? 
 

I remember when we were at school purposely connecting little ones the wrong way around to get them to and get the puff of white powder ... no idea what it was!

That sounds like a tantalum type.  They're polarised, but some look like dipped ceramic caps.  They usually smoke rather than pop.

[andyf]

I'm sure Dave BC is right on both counts - it wouldn't work at all if polarity was reversed, and I don't think the output of car battery chargers is smoothed at all, so they produce a dc voltage with a lot of ripple. The "average" voltage must be greater than 13.8V to charge a battery (a "12v" car battery is actually 13.8v fully charged) and the peaks of the ripple might exceed the rating of the deceased capacitor. If you can find a CB radio power supply, they have basic smoothing. The PSUs in scrapped computers are rated for lots of amps, have 12V outputs and are easy to convert as shown here:
 http://www.instructables.com/id/Convert-an-ATX-Power-Supply-Into-a-Regular-DC-Powe/

Andy.

[Bluechip]

I've been finkin'    

If you just want to drive a low voltage DC motor, they don't require a smoothed DC. In my projects, they get what I give 'em.

You can knock out a circuit similar to  below, but ..

This will not work with steppers !

Thick lines are high current for motor, the rest only carry 12mA or so. SE is Switching Element:  FET or whatnot.

You might have to ensure the input to 78xx does not exceed limit.

As shown max. AC input is about 18V r.m.s. otherwise 100uF 35V is pushing it's luck.





Andy is spot on with PC PSU's, to get more wallop it is often possible to parallel 12V outputs, although it may be you have to ballast them to get the PSU to fire up.

Have fun, check your fire insurance.    

Dave BC

[websterz]

Okay, I have pulled my back-up ATX power supply out of storage and will use it to run my power feed. It is a 450W rated for 17 amps @ 12 VDC. From what I read on Instructables this should be sufficient for my needs and provide more stable output as was suggested above. I will salvage, or buy new, a replacement capacitor for the controller and see what shakes loose. Thanks guys! 

[websterz]

I think the DP switch must be after the PSU.

I doubt if the chip would work at all on reverse polarity, he does say it worked for a bit.

Dave BC

I have :

PS --> CONTROLLER --> DPDT --> MOTOR

Is this not right? Like I say even with the blown cap the controller still runs in the above configuration.

[andyf]

Yes, PS --> CONTROLLER --> DPDT --> MOTOR is right 

Like Dave says, some sort of load is often needed to wake an ATX up. A high wattage resistor from +12v to 0v would do it, but you seem to have amps to spare with your ATX, so how about 12v halogen illumination permanently on while the PSU is plugged into the mains? A 12v, 50 watt lamp pulls about 4A, giving you somewhere to warm your hands in winter. And burn your forehead  if you put the lampholder in the wrong place.

BTW, you may find your PCB marked "+" to show where the +ve terminal of the capacitor should go. But the caps themselves usually indicate which is the -ve leg, often by an arrow-shape down the side, with a thick minus sign inside it.

Andy 

[Bluechip]

W

That's OK ..

The point that was made is that the controller must be correctly polarised.
If the Switch was before the controller, it would almost certainly go Pop.

If you want to use a PC PSU, I wouldn't bother replacing the blown cap. Plenty of smoothing in the PSU.

Edit, Sorry Andy posted this when your post was on Page2  

PC PSU's have quirks, don't I B****y know. Some like the 5V loaded min. about 200mA, otherwise no go.
Some I have met have more than 1 12V o/p. If you just parallel the things, the PSU won't come up.
But, if you put a 0R1 10W resistor on each 12V o/p, then parallel them after the resistors, they will.
10W is overkill, but I've got a lot of the screw down sort, easy to fit.
Just the same as putting LM317T's etc. in parallel, 'cos the price of LM338K? ( the TO3 5A jobbies ) is beyond belief  



Dave BC

[websterz]

Thanks for all your help guys! I salvaged a 50v 47uF cap off of an old blown power supply board I had out in the scrap box  I transplanted it and wired in the modded PC power supply. Smoooooooth! I have much more low end control now. I'm going to learn this electronics stuff whether I want to or not I guess.  I'll try and get a video of the power feed in operation later today...right now it's time for breakfast. 

[jatt]

Bluechip --- hopefully I am reading this right.  Yeah a LM338K is exxy.  What I have done in the past is put a TO3 PNP pass transistor in when I need more current than a LM7812 can handle.

I only needed 1 PNP.  Here is a more extreme case with multiples used, but u get the general idea.

http://www.zen22142.zen.co.uk/Circuits/Power/1230psu.htm

Hopefully this helps those in need of more O/P grunt.

[Bluechip]

Jatt

Thanks for posting that cct. I was aware of the general idea, used it myself. Boost circuits are often on the data sheets, there is also, usually, a cct. for npn bypass transistors.

As you say, quite right to post the info. it may well help others who are into this stuff.

As an aside, apart from data sheets, quite a few manufacturers of devices also do 'Application Notes'.
Worth while doing a search on these, far more eg. type circuits that the (few) on the data sheets.
Not always successful, but sometimes a lot of good stuff.

Try this for my favourite reg. chip ..

http://www.st.com/stonline/books/pdf/docs/1678.pdf



Dave BC