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| Current Limiting for a Cheap SMPS |
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| rampra:
Hello All, I am a Mechanical Engg. and I have very little idea about circuits. But I have a hired an electronics guy who can help me with the electronics portion of my project. He is not an Engineer so he cant really "think". He can do what is told and differ between safe and dangerous. Well I have a cheap Chinese SMPS (notebook style) which has 24V and 10A output. Everytime I use to for my application which is as good as short circuiting (brush electroplating), the SMPS fuses and then we have to switch OFF the SMPS and wait for capacitor to discharge and then switch it ON again (I guess this is a safety feature). I guessed, what i needed is a constant current device and SMPS is a constant voltage device maybe. I have 2 questions for all the knowledgeable members: 1. If i add a simple limiting resistor like 1K pot or resistor in series to the output of the SMPS will I be able to use it for my application? 2. Is is possible to add a current limiting circuit outside the SMPS to regulate the current? I used the "Search function" but found no queries on this topic. Related queries were either too complicated or not concerned to me. If there is any thread on this topic please feel free to guide me. Thanks! |
| Siwastaja:
SMPS can be either voltage (CV) or current (CC) source, or both (CC/CV). You need a current source. Most random cheap supplies are voltage sources, and even though they could implement the constant-current mode as well (being CC/CV), they often implement a latching or "hickup" protection mode instead, meaning that while they can and do limit the current just as you need them to, they only do it for very short bursts, and, depending on device, either perform these short bursts with low duty cycle, or turn off requiring a total restart. This is hard or almost impossible to "mod". "Lab supplies" are CC/CV, but they tend to be a bit expensive for such a simple job. LED power supplies are CC, and may work for your purpose - if you find one for a single low-voltage led. They may have protections for too low output voltage, however, so that they think the LEDs are shorted and turn off. Adding a resistor after the voltage supply kind of works, and regulates the better the more voltage difference over the resistor. On the other hand, the efficiency is poorer, the more there is difference. With 24V, 10A, and 0.5V electroplating output, you'll burn 23.5V @ 10A = 235W in the resistor, and 5W doing the actual electroplating! At 2% efficiency, this is a non-starter. If you can find a very low voltage supply, like a high current 3.3V supply, you'll get the efficiency up to 10-20% range. Still a lot of heat to dissipate in the resistor, but possibly workable. You need a huge, fairly expensive resistor. You can sometimes make substitutions, like using large steel sheets, possibly submerged in water... One practical solution to get you started, if you want to play with Chinese Ebay modules, is to try a CC/CV mode buck module from there. They do exist, pick one with an explicit current limit, meaning a separate potentiometer for current. You can use your 24V brick or similar (such as a 19V laptop power brick) as an input for such a module. Using a safe, existing mains supply makes life so much easier. I mean something like this: https://www.ebay.com/itm/DC-300W-20A-CC-CV-Constant-Current-Adjustable-Step-Down-Converter-Voltage-Buck/401342111666?hash=item5d71da9fb2:g:nscAAOSwSeVaL7gG , but do note that these tend to be of low quality, and with a minimum output voltage requirements around 0.8 or 1.2V, they may still need a bit of resistance in the output to work in electroplating. In any case, such step-down (buck) module multiplies the current manyfolds and easily offers >50% efficiency. |
| rampra:
Thanks for replying. I have this type of converter with CC/CV mode. Can you please explain how to provide this resistance? --- Quote ---they may still need a bit of resistance in the output to work --- End quote --- Please consider that my application is actually equal to short circuiting. The process gives results with welding machine but is definetely an overkill and doesnt provide great results. Regards! |
| David Hess:
A power resistor in series with the output is the easiest solution but there is something even better. Get some big 12 volt incandescent automotive lamps, use them in series pairs for 24 volts, and put as many pairs in parallel to get a high enough current rating without tripping the power supply. Use these instead of a power resistor to limit the current. The advantages are that they will have no problem with the power and the non-linear resistance of the tungsten filaments means that at lower output currents, the resistance is also lower and more current is available than if a fixed resistor was used. |
| Siwastaja:
By all means use incandescent lamps, but at least find some lower voltage supplies first, 12 or 5V supplies are even more ubiquitous than the 24V supplies, and 10% efficiency (with a 5V supply) makes a lot of sense over 2%... (Assuming 0.5V output voltage, which is approximately what I saw when doing electroplating (through hole copper plating).) Using resistors (or lamps - a big 12V bulb is fine for current-limiting 5V) in series to limit current, allows you to easily parallel multiple 5V supplies as well so you don't need to try to find a single massive supply. Use one lamp/resistor per supply, and connect them in parallel after that. If you do the paralleling right at the electroplating mount site, this allows you to use distributed cabling with smaller gauge cables, as well, making the cabling more flexible and, again, cheaper and easier to source. Note that when wasting power in the resistors, a 5V 10A supply is as good as the 24V 10A supply, but it's much smaller and a lot cheaper, and easier to find due to lower power. |
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