How do I calculate the voltage needed to plate with zinc?

[adamthomas]

Major noob here. I searched for an answer but didn't find anything that appeared to be relevent to my situation. I know this has probably been addressed before so feel free to point me to any relevent threads.

Here's the stiuation. I just purchased a 25amp CV Arbe Pro-Plater to do some zinc plating. I now realize I should have purchased a CC/CV unit, but I'm trying to make due with what I have. My question is this.

What calculation can I use to determine the correct voltage setting for a particular size part? The rectifier allows for the adjustment of voltage, but not current, although it has both a voltage and current meter. The voltage meter registers fine, allowing me to dial in a specific voltage, but the current meter always reads <1amp, regardless of the voltage setting or the size of the part being plated. Should I be seeing a reading on the current meter? Am I missing something here. The current meter seems totally useless to me.

[seanc]

Adam:

What calculation can I use to determine the correct voltage setting for a particular size part?

You can't really. For any desired current, the voltage will vary, depending on the bath temperature, spacing between part(s) and anode, and size of part(s). Unless you carefully control these parameters every time you plate, the voltage will be different. That's why constant current is the way to go.

Having said that, you should still be able to get the current you desire:

The rectifier allows for the adjustment of voltage, but not current, although it has both a voltage and current meter. The voltage meter registers fine, allowing me to dial in a specific voltage, but the current meter always reads <1amp, regardless of the voltage setting or the size of the part being plated. Should I be seeing a reading on the current meter? Am I missing something here. The current meter seems totally useless to me.

I'm not familiar with this type of rectifier, but would assume that both the voltmeter & amp meter should indicate the actual operational values.

If you calculate the current you want, you should be able to adjust the dial until the amp meter reads your desired current. The volt meter will simply tell you what the final voltage is, but it's value is irrelevnat.

How large are the parts you're plating, and what current density are you shooting for? It's possible that your surface area is too small to get an accurate reading on the amp meter.

Is this amp meter single-scale, 0-25 amp? Or can it be switched to read a smaller amperage full-scale?

It is possible that the amp meter is bad. You can test that theory by hooking up a low-value resistor, and dialing the voltage up. As the voltage increases, so will the amperage. If the amp meter doesn't properly read, then it's bad.

eg. take a 1-ohm resistor: at 1 volt it should draw 1 amp, at 5 volts it should draw 5 amps, etc.

Be sure to use a resistor capable of the power you put into it.

eg. at 5 volts & 5 amps it needs to be rated 25 watts or more.

Sean

[adamthomas]

Thanks for the reply Sean. I'm just attempting to plate nuts and bolts and some small bracketry at this time, nothing a 25 amp plater shouldn't be able to handle.

I'm using the new Caswell Copy Cad/Zinc formula so I"m shooting for 140ma/sq in., and running the bath temp at ~110F. I purchased the 1.5gal kit so I have 2-4x8 zinc anodes that are ~9" apart when in place, which puts the parts ~4.5" from each one when centered in the bath.

The largest thing I have tried to plate so far was no more than 4 sq in(big M20 x 30mm bolt). My results obviously vary depending on the voltage I set initially. Fun thing is that I have to set the voltage using the dial and voltmeter before completing the circuit, or I get nothing. Once the circuit is completed, the voltmeter shows the voltage drop across the circuit, and the ammeter moves slightly above 0. Not sure what this is supposed to indicate. Attempting to adjust the voltage dial once the circuit has been completed does nothing. The primary issue is that I have no idea what's actually going on since both gauges are reading < 1 amp or volt, and no way of determining what voltage should be applied to being with. Not having the ability to vary the voltage once plating starts is also a pain.

I can't be the only one who has come up against this.

[seanc]

Adam:

The largest thing I have tried to plate so far was no more than 4 sq in(big M20 x 30mm bolt).

This may be the problem for a couple of reasons:

1. at 140mA/sq-in, you're only after 560mA total (.56 amp). From the photo of the rectifier, it looks like the amp meter resolution is @ 1 amp. The pictures not real clear, so it's hard to determine the graduations, but you get the idea. The amp meter will barely be deflecting.
2. at very low resistance values, some power supplies won't even activate. What does the documentation say about minimum load? Minimum voltage? Minimum current?

Fun thing is that I have to set the voltage using the dial and voltmeter before completing the circuit, or I get nothing. Once the circuit is completed, the voltmeter shows the voltage drop across the circuit, and the ammeter moves slightly above 0. Not sure what this is supposed to indicate. Attempting to adjust the voltage dial once the circuit has been completed does nothing.

That's how power supplies that don't like low resistance loads behave. You may have one of them.

The effective resistance of the electrolytic cell (anode/electrolyte/cathode) is quite low, sometimes less than .5 ohm (depending on all the variable factors). Many low-cost power supplies just will not drive that kind of load, the protection circuits cut in.

The workaround is to wire a low value resistor in series with the positive lead. That gives the PS enough load to operate. It will also let you measure current more accurately. Measure the voltage across the resistor, and do the math to calculate current.

That's why 1 ohm is a great value: voltage across 1 ohm = current. You'll be able to set low currents more accurately than your meter permits.

And having that resistor will allow you to confirm proper operation in the first place.

The primary issue is that I have no idea what's actually going on since both gauges are reading < 1 amp or volt

for parts that small, both voltage and current will be less than 1. even much larger parts will rarely get above 2 volts.

and no way of determining what voltage should be applied to being with.

Again, you don't care what the voltage is, only the current. I don't even have a working volt meter on my PS, it's broken. Set the current, and the voltage will follow, settling in at whatever it needs to be to provide the current.

Not having the ability to vary the voltage once plating starts is also a pain.

Since that's your only adjustment, it is a problem.

You really need to figure out whether the PS is functioning or not. Get that resistor & test it. If it works as expected w/the resistor, leave it in permanently.

I can't be the only one who has come up against this.

You're not. Most of the low-cost offshore CC/CV power supplies have the same problem. The resistor is the workaround.

Sean

[adamthomas]

Would it make sense to just put more parts in the bath to increase the load and get a reading I can actu
\ally work with?

I'll try adding the resistor as well. I assume I'll need a 1ohm resistor that can support the maximum amperage the unit supports? In this case, we're talking about a 25amp continuous unit with a peak output of 30amps.

[seanc]

Adam:

Would it make sense to just put more parts in the bath to increase the load and get a reading I can actually work with?

Simple answer: No.

Contrary to what common sense would seem to tell you, adding more parts (increasing the total surface area) actually DECREASES the effective resistance of the electrolyte cell.

The electrolytic cell is a strange beast, it does not behave like a either a fixed or variable resistor.

However, larger parts will need higher voltages, so it may work anyway, particularly if your PS is designed a voltage source, and has a lower limit. You'd have to try it and see.

I assume I'll need a 1ohm resistor that can support the maximum amperage the unit supports?

It doesn't HAVE to be 1 ohm, but 1 ohm is a very handy value, since voltage across 1 ohm is exactly the same as current through it. Using any other value resistor, you'd have to do a bit of math to calculate the current.

eg, with a 2 ohm resistor, if you're measuring 5 volts across it, then current would be 5÷2, or 2.5 amps. 1 ohm eliminates the need to do math.

In this case, we're talking about a 25 amp continuous unit with a peak output of 30amps.

You'll only need a resistor big enough to handle the highest voltage & current that you expect to ever need.

You haven't said how big your tank is, or how large your anodes are, and you will be limited by those.

25 amps is enough for 180 sq-in. at 140 ma/sq-in. Are your anodes that big? Do you forsee doing parts that big? Even with parts that large, your voltage will likely not exceed 6 volts.

The largest piece I've done was @120-125", at 80 ma/sq-in., and it came in @ 2.5 v. Using 140 ma/sq-in would still have been less than 5 volts.

So if you want to assume 6v, 25 A as most you'll use, you'd need a resistor rated at 150 watts (6x25). That's a fairly good size resistor. If you want "worst case" for anything you might throw at the PS, then 15v x 30A = 450 watts! That's a HUMONGOUS resistor!

You can get a 10 watt, 1 ohm resistor at radio shack ($1.80). That would be enough to plate at 3 amps for now, and to test your PS meters functioning.

Anything bigger than that you'd need to go to an electronics catalog house.

Sean

[adamthomas]

Sean, thanks for your excellent responses and your patience.

Originally Posted by seanc

You haven't said how big your tank is, or how large your anodes are, and you will be limited by those.

Actually I did in an earlier post but for the sake of clarity, I'm using a 1.5 gal tank with two 4x8 zinc anodes and running the bath at 110F. This is the basic 1.5 gal Copy Cad/Zinc Kit from Caswell with the new electrolyte formula.

For now the largest object I expect to plate is probably in the neighborhood of 7" sq-in. Doing the math gives me a current requirement of 980ma or (7" sq x 140 ma/sq-in), far below the lowest range on my ammeter.

To answer your earlier question, the ammeter is a single scale meter with 5 amp gradations up to 30 amps, and the voltmeter has 1 volt gradations up to 20 volts. Not very useful for plating small items, or plating in general it seems.

This rectifier really seems geared towards automotive uses since the voltage dial and voltmeter both have markings for 13.8 volts. It did not come with a manual, and looking at the manufacturer's website indicates that plating machines is not one of their primary areas of expertise. In fact, I can find no reference to plating machines at all, save a single picture of the unit in the site's banner image.

I will try the 1 ohm resistor test as you suggested, but based on what I have learned so far from you and in reading this forum, this unit is not very well suited to my needs so I will probably end up returning it and looking for a better unit iwth more granular control and metering.

Maybe something like this: http://www.contenti.com/products/plating/350-604.html

or this: http://cgi.ebay.com/COPPER-NICKEL-TI...em170051568385

[adamthomas]

OK, I decided to return the crappy Arbe Pro-Plater CV rectifier and buy a laboratory grade Lambda unit. See attached image. It's supports both CV & CC and has some decent meters.

"THIS POWER SUPPLY PROVIDES A SINGLE, 0 - 10 VOLTS @ 0 - 10 AMPS OUTPUT WITH VARIABLE COURSE AND FINE CONTROL KNOBS FOR VOLTAGE ADJUSTMENT, AND A CONTROL KNOB FOR CURRENT ADJUSTMENTS. THE DC SUPPLY PROVIDES, 0 - 10 VOLTS @ 0 - 10 AMPS, CONSTANT VOLTAGE AND / OR CONSTANT CURRENT (C/V, C/C). THIS IS TRULY A LABORATORY GRADE, DC PLATING POWER SUPPLY WITH HUM AND NOISE SPECIFICATIONS MEASURED IN MILLIVOLTS AND SUPPLY REGULATION ACCURATE TO +/- .01%.

This should make things much easier and provide more consistent results. Thanks for all the help Sean.

[seanc]

Adam:

You can't go wrong with a quality unit like the Lambda (or HP, or many of the other brands that seller had). That was going to be my recommendation, not the other "PEPE" unit.

The only other thing I would have looked for is a switchable meter, so you could read low current more accurately. But as long as you're using 1 amp or more, the Lambda meter will be fine.

You're on the right track now!

Sean

[superglide56]

Adam, I have the identical Lambda model. No complaints from me.