1
\$\begingroup\$

I'm just beginning in hobby electronics and have made a power supply with a bridge rectifier and a smoothing capacitor (4700uF). I want to put a toggle switch in to turn the supply off and on. Should i put it before the capacitor so the supply stays on for a few (15) seconds after I turn it off? Or should i put it after the capacitor so it turns off straight away but the capacitor stays charged? Is it dangerous if the cap stays charged?

Cheers, Michael

\$\endgroup\$
  • \$\begingroup\$ Neither. You should put it on the primary side of the power transformer. \$\endgroup\$ – user207421 May 3 '15 at 1:33
  • \$\begingroup\$ What is powering your power supply? Since you have a recifier, I assume it is AC. And since it is AC, I assume it is mains. As a starting hobb-est. Why are you working with mains power, at all? \$\endgroup\$ – Lyndon White May 3 '15 at 2:44
  • \$\begingroup\$ Hi, the power I'm working with is from a 24VAC plug pack. I used a plug pack instead of a transformer due to my inexperience and safety issues. \$\endgroup\$ – Michael Malek May 3 '15 at 3:12
5
\$\begingroup\$

It depends on the voltage of your circuit. If you're using a transformer and you're producing under, say, 20VDC, there's no problem.

But if you're actually using the line (120VAC in America, 220VAC elsewhere) directly, you'll need a 'bleeder resistor' in parallel with your smoothing capacitor. Otherwise, the cap will remain charged and will discharge into the first resistor bridging its output, which can be YOU.

THe resistor will discharge the cap when the rest of your circuit is off. You choose the value low enough so that while ON, you aren't drawing an 'unacceptable' amount of current (this is up to you), but you also want the RC constant on the order of a few seconds or so (so that a few seconds after switch-off the cap is discharged). For your cap, 4700uF and say a 2 second rc constant, that's about 425 Ohms.

Just keep in mind the name: it's a 'bleeder resistor' because while your circuit is on, it's always uselessly drawing some current. So, account for this when you are budgeting your maximum current draws.

What you could do if this is a problem is to use a multipole switch; on switch off, send the cap through the resistor. I'll leave it as an exercise what kind of switch (DPDT, SPDT, etc.) ;)

\$\endgroup\$
  • 3
    \$\begingroup\$ Note that "dangerous" needs to be defined. If you've got 20 volts on the cap it won't shock you (much), but if you bridge the voltage with a screwdriver or test probe the resulting spark may cause problems. \$\endgroup\$ – WhatRoughBeast May 2 '15 at 23:57

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.