Power supply woes

[kenrinc]

Doing zinc plating. I've had a 3amp digital jobber for awhile, similar to the one Cas sells. I moved up to a similar unit only it puts out up to 10a. Problem is even though the manual says it does CC or PC mode, if I hook it up and try and plate, it will not let me adjust current flow. It just stays at .04 a. If I unhook the load, put the negative lead in the plating solution and then reconnect the load I can plate the lead just fine; lead bubles like crazy and the amps move up on the meter. Connect the lead back to the part and then reconnect, nothing.

If I hook it back up to my old 3a supply, it works fine and I can adjust output current as usual. What gives? The supply is one of those Mastech 10a units like this one: http://www.multimeterwarehouse.com/hy3010Df.htm

The old unit I had, that worked fine, was this:

http://www.multimeterwarehouse.com/hy3003dSingle.htm

Ken-

[engineerscott]

Re: Power supply woes

That unit will function in both constant current and constant voltage mode. You must have the output voltage set high enough to supply the current you set for a given load resistance. If it is not set high enough it will operate in constant voltage mode rather than constant current mode. Try turning your output voltage up. In fact, when I am running a power supply in CC mode I normally start out with the voltage turned all the way up and the current turned all the way down. I then connect my circuit and dial up the current to the desired target. As an experiment you can then start dialing the voltage down. What you'll see is that the current will stay constant until you dial just below the voltage required to supply the set current for you load resistance. At this point you are operating in CV mode.

[engineerscott]

Re: Power supply woes

Just to clarify a little further I thought it might be useful to give an example with some real numbers.

Say that you want to plate an item at 3 amps constant current. Lets assume that the resistance presented by your plating system (anode to cathode) is equal to 6 ohms. Using ohms law (V=IR) we know that the voltage across your plating system must be equal to 18 volts to achieve a 3 amp current. This means that if your power supply output voltage is set for anything less than 18 volts you can not achieve your target plating current of 3 amps. If the current limit is set for 3 amps and the output voltage is less that 18 volts your power supply will operate in Constant Voltage (CV) mode. If we now turn up the voltage (while keeping the current limit set at 3 amps) the output voltage will rise as will the current. When the output voltage hits 18 volts the power supply will transistion to Constant Current (CC) mode with the output current equal to 3 amps. Notice that we could continue to turn up the output voltage dial, but nothing would happen. The output voltage would stay constant at 3 amps and the output voltage would remain steady at 18 volts. In reality, the voltage and current dials on this type of power supply set the MAXIMUM voltage or current. Depending on these settings and the load resistance, your power supply will either operate in CV or CC mode. If you hit the max current setting before you hit the max voltage your supply will be in CC mode. Likewise, if you hit the max voltage before max current you will be in CV mode.

Now in the real world you don't typically measure the resistance of your plating circuit. The easy way to set up your supply is to turn the voltage up to its maximum setting (the only exception would be if you have a DC supply capable of producing hazardous voltages, say greater than 50V. In this case you would set your max voltage to something reasonably safe, perhaps 40V). Turn the current to its minimum setting and connect the power supply to your plating setup with the power turned off. Turn the supply on and then dial up your current until you reach your target plating current.

An interesting side note is that in a real plating environment, the resistance of the plating circuit will not be constant throughout the plating cycle. As the plating solution is depleted the resistance will increase. This is the nice thing about a constant current supply. It will adjust it's output voltage to achieve the current you have set. As the resistance increases the output voltage will also increase and your current will remain constant. The only exception being that if the resistance increases enough you may hit the maximun output voltage that your supply can produce. In this case you would be back to CV mode. A simpler supply, for instance a battery and a current setting resistor will not behave like a CC supply. When you initially set up your plating system with a simple supply like a battery you would normally use an ampmeter and a variable resistor to set the current to your target. As the plating solution is depleted the current will drop and the rate of plating will slow. In many cases this is not an issue however if you are trying to achieve a certain plating thickness a CC supply is a big plus.

[kenrinc]

Re: Power supply woes

Thank you for the info it was very useful !! I need to go try this. Although when I was trying to set the output current, the unit never switched back to CV mode. But there are other variables that I will look at tonight and let you know. I know when the unit switches modes because there is "click" sound and the CV LED will light. I need to get home and try this out ....

Ken-