MY Power Supply Has Me Confused - can't control Current

[smithb98]

First post here. Love the forum. Wish there was more traffic! First off, I set up my Copy Cad system last night. Plated a couple of nuts and bolts that came out surpirisingly nice considering I am having this issue. I understand that it is current that matters, not voltage. I have a 10Amp constant current/constant voltage power supply (Mastech - appears identical to the units Caswell sells) hooked up. Tried to plate a couple nuts and bolts last night that totaled around 22 square inches which at .14 amps/sq-in should have needed about 3 amps. Problem is, I could not get the power supply to work in that range. It will operate between .00-.04 amps or between 6 and 10.

If I turn it on with the knobs cranked up, the current will show up to 10amps. I then try to adjust the current knob down and it will let me get to about 6 amps. If I try to go any lower, it'll drop down to .4 amps and won't go back up. The only way to get it to go back up above .40 amps is to turn the unit off, adjust the current knob back up and turn the unit back on, it will show 6+ amps. Again, try to turn it down and at 6 amps, it'll drop down to .40. Make sense?

The power supply didn't come with instructions. Any ideas?

[overcashier]

See the response I got on my post "3 amp power power supply". My supply didn't come with a manual and I consider Sean's post my manual.
Good Luck
Greg

[seanc]

Your adjustment procedure should work. That it does not may due to either:

1. the unit is defective, or
2. (more likely) the load resistance is too low for the power supply

Many of these low-cost power supplies do not operate properly into very low load resistances. And our "electrolyte cell" plating tanks are very low, often less than .1 ohm.

What do you have connected for a load when you try this adjustment? When you have it adjusted to 6 amps, what is the voltage?

Voltage for CopyCad plating is usually very low. For your 22 sq-in, I'd guess you'll see .4-.5 volts, maybe less.

If you can get hold of a low-value resistor (1 ohm is ideal) rated 5 watts or more, connect it with both controls turned down. Gradually turn up each control in small increments, see if it will get up to 1 volt and 3 amps.

If so, then the power supply is operating correctly, but you'll need to keep the resistor in series while plating. If you plan to plate at the full 10 amps, you'll need a resistor rated 20 watts or more (bigger is even better).

Sean

[smithb98]

I just had the tank bar with about 22 sq in of nuts / bolts on it. Am I supposed to put a resistor in line with it? The power supply is a 10amp constant current type, identical to the ones Caswell sells. I'll check out what the voltage numbers are doing (I was only pay attention to amps). Is there a way to figure out what kind of resistor I need? A mathematical equation or any resource that has rules of thumb? Also, is it as easy as picking one up from Radio Shack or are they a special type? Thanks for the quick responses above. I had read the thread on the 3amp power supply.

[seanc]

Originally Posted by smithb98

Am I supposed to put a resistor in line with it?

No, but sometimes it may be necessary to compensate for limitations of the power supply.

Keep in mind, that "power supplies" are designed to operate electronics circuits on the test bench, and electronics circuits typically have much higher load resistance than an electrolyte cell.

One significant difference between expensive, high-quality power supplies, and the low cost supplies (such as Mastech) is their ability to handle low-resistance loads. Good power supplies will handle a dead-short circuit, others will not, but will have protection circuitry to kick in when it thinks it has a short circuit. I think this is what you're experiencing.

Using your adjustment procedure, as you turn the current down, the voltage will also drop (see Ohms laws below). When voltage reaches the design threshold at which the power supply thinks it is shorted out, the protection circuit kicks in, and all readings drop out.

Adding an in-line resistor keeps the load high enough that the power supply never thinks it's shorted out.

Is there a way to figure out what kind of resistor I need?

Any low value resistance w/adequate power rating will do. Even a small 6 or 12v auto bulb will work, but then you don't know what the resistance is, so you can't tell if your meters are reading correctly or not. That's where a resistor of known value is useful.

A mathematical equation or any resource that has rules of thumb?

Ohms laws are what you're dealing with, very simple math

• Ohms voltage law: V = I x R (volts = amps x resistance). Knowing any 2 of the three terms will let you calculate the third. This is why I recommend a 1-ohm resistor: you can eliminate it from the math, so V always = I.
• Ohms power law: P = V x I (power(watts) = volts x amps). eg. if you expect to run 5 amps at 5 volts, the resistor (or any other component) has to be able to handle 25 watts, otherwise it will burn out.

Also, is it as easy as picking one up from Radio Shack ... ?

It's that simple, if they have something suitable in-stock. Resistors of more than 2 watts are typically called "power" resistors. Just be sure to get one that will handle the maximum power you expect to use. I doubt you'll ever see more than 3 volts while plating, so a 30 watter would handle the full 10 amps. A bit bigger would be better though, as it won't get so hot (a 30 watt resistor dissipating a full 30 watts will get hot enough to give you an near instant 3rd degree burn!)

OTOH, you probably won't need one for anything above 6 amps, as you've already found out!

Make sense?

Sean

[smithb98]

sean, simply put, you rock! Ok. I got my resistors and ran my test all before I got your last post so you may have answered my question.

Stopped at radio shack on my way home and picked up two different resistors. One was a .47ohm, 5 watt resistor, the other was a 1ohm 10 watt resistor. First used the 1ohm/10 watt and just placed it in line. Hooked up the power supply and sure enough, I had control over the amps from .00 to 10.00. SWEET! So then I notice a tiny little wisp of smoke coming from the big resistor and that the volts are pretty high It started to turn dark grey and was incredibly hot so I figured I'd try the smaller resistor. Put in the .47ohm resistor and it worked great though it only went down to 3 or 4 amps and then defaulted to .40 (reading a short as you suggested below?). Volts stayed low (makes sense with your equation below of V=IxR) but I'm not sure why you said I shouldn't see more than 3 volts if I am plating at 6 amps with a 1ohm resistor. Wouldn't my volts be 6? Sorry if I am just missing the obvious.

So I ran a 1ohm at 6 amps and 6 volts so that is why it burned up, it was only rated for 10watts and needed to be rated for 36 watts right? Do they make a 1ohm resistor rated at 100watts? 100 watts=10amps x 10 volts?

Sorry for all the questions. Starting to make sense though. Again, THANK YOU!!!!

[smithb98]

though re-reading your post, you pointed out the obvious. If I'm not having problems, say, below 6 or 7 amps, I would only need a 1ohm resistor rated to handle 36 or 49 watts.

[seanc]

Originally Posted by smithb98

So then I notice a tiny little wisp of smoke coming from the big resistor .... It started to turn dark grey and was incredibly hot

Good heating elements!

Put in the .47ohm resistor and it worked great though it only went down to 3 or 4 amps and then defaulted to .40

So it looks like your power supply needs at least 1/2 ohm or more to operate.

but I'm not sure why you said I shouldn't see more than 3 volts if I am plating at 6 amps with a 1ohm resistor. Wouldn't my volts be 6?

You're right, I didn't explain that fully.

You're never likely to see more than 3 volts at the plating tank (from anode to cathode). Any inline resistor will have it's own voltage drop. The 2 voltages, added together, are what the power supply has to furnish.

eg, if you take 2 resistors of equal value in series (actual value doesn't matter, as long as they're equal), and connect them to a 12v battery, each resistor will drop 6 volts. It's this voltage that you use in calculating the wattage.

Unequal value resistors will each see a different voltage, depending on the actual resistances, which will sum up to the applied voltage.

Do they make a 1ohm resistor rated at 100watts? 100 watts=10amps x 10 volts?

You bet they do, but you won't find it at Radio Shack.

If you're in or near a big city, you might find an electronics supply house where you can get one quick.

Otherwise, shop online. They're not cheap, nothing that big ever is:

• Aluminum housed (best)
• wire-leads

You might find cheaper by searching surplus stores.

though re-reading your post, you pointed out the obvious. If I'm not having problems, say, below 6 or 7 amps, I would only need a 1ohm resistor rated to handle 36 or 49 watts.

You got it! Figuring this stuff is not cut-n-dried, may take a little experimentation. That's why higher wattage is better, it may be overkill, but you won't have to wonder if it's enough.

Sean

[smithb98]

if anyone else is following along, and needs to do this same experimentation, I did manage to find an aluminum housed/finned resistor here for $13 shipped (shipping was $7, more than the resistor itself). It is rated at 50 watts, 1ohm.

http://www.action-electronics.com/

I'll post up how it works.

Again, Sean, your help has been invaluable! Thank YOU!!!!!

[smithb98]

hey sean, another question for you. I'm starting to get my arms around this electrical stuff a little bit but wasn't able to find an answer to this question anywhere else.

I have two .5ohm, 50 watt resistors and two 1ohm, 50 watt resistors. My goal is to hook my wiring up up and leave it, not put it in and take it out each time so I use the same setup regardless of my amps. If I put the two 0.5ohm resistors in series, I will see 1ohm resistance from them both. Question is, will they share the power (watts) between them so I can handle 100 watts or is my handling still 50 watts? How about if I put the two 1ohm resistors in paralell. I should see .5 ohms so my 10amp max would be 5 volts (though I realize my part being plated will have some additional resistance and my volts will likely be even less). But based on the known values here, a max of 50 watts. The question again is, do they share the wattage between the two?

Not sure if that makes sense.