seware said:

I am wanting to build a power supply and need a bit o' help. I have an

"Electronics for the hobbiest" type book that goes into fair detail on the

subject, but where it talks of filtering and bleeder resistors it speaks of

their use and suggests values for an example power supply but not how to

calculate the values for your own. Their example uses a transformer with a

12.6V 3A secondary wth a full-bridge rectifier. The bridge outputs have 3

1000uF 35V caps in parallel and then a 1K 1/2W resistor in parallel with the

caps for the bleeder. How do I calculate my needs for a transformer with a

15V 5A secondary? A worked example would be nice but I can plug in numbers

if someone will explain the reasoning. I understand that the size of the

bleeder depends on how fast I want to discharge the caps, but I don't know

what kind of time is reasonable. Should I reverse engineer the RC constant

in the example and work forward from that or are their some better rules to

follow? Thanks to all you professionals who sustain the questions of all of

us wannabees.

Steve

There are no hard and fast rules for these things. More capacitance

produces less ripple voltage at full load current but a bigger start

up surge and a longer bleed down at turn off. Personally, I hate to

wait for a lab supply to fade out after I turn it off, because I may

do that dozens of times a day when working on circuit variations. So

I like serious bleeders on a lab supply. The supply for an audio

amplifier, I am not in such a hurry about, and there is some minimum

load there that pulls the voltage down, anyway.

But here are some formulas starting with the transformer.

A capacitor input filter (cap directly to the bridge rectifier) and

load heats the transformer more than a resistor that draws the same

average current, connected directly to the transformer.

The bigger the cap, the worse this gets, because the cap voltage tends

to stay near the peak of the rectifier output wave, so all the current

has to occur in a small blast right at the top of the wave, when the

rectifiers forward bias. So don't expect more than about 75% of the

transformer's current rating coming out as DC. Your transformer will

supply about 4 amps continuously from a capacitor input filter.

That peak voltage is almost 1.414 times the RMS AC voltage of the

transformer, so your 15 volt transformer will pump an unloaded cap up

to about 15*1.414=21.2 volts. How low the cap voltage sags to before

another peak comes along depends on the ratio of capacitance to load

current. If you want the output to be regulated to no more than about

12 or 15 volts, you can afford a lot of ripple on the capacitor, but

if you want to regulate something like 18 volts or use the supply

unregulated, you will probably want to keep the full load ripple less

than a volt or so.

To simplify the math a bit, lets assume that the 60 hertz line

frequency (if that is your line frequency) charges the cap

instantaneously every half cycle or every 8.3 milliseconds. That

means that the 4 amp output current is running entirely from the cap

all that time. The formula that relates current to rate of change of

voltage is I=C*(dv/dt) or current (in amperes) equals capacitance (in

farads) times the rate of change of voltage (in volts per second). So

to have a 1 volt sag before the next recharge, the 4 amps has to cause

a rate of change of voltage of 1 volt per .0083 seconds or 120 volts

per second. So the required C is 4/120 farads = 0.033 farads = 33,000

microfarads. If you can tolerate 2 volts of ripple, half that

capacitance will do. Or you can substitute your available capacitance

and calculate the ripple voltage.

Bleeders are really only essential for safety reasons (to dump the

voltage to a safe level before you can get the case open). But you

can decide what is a reasonable wait if you decide you want the supply

to fade out on its own, even if it isn't a shock hazard. The time it

takes to fade it to 37% of full voltage is R*C seconds where R is in

ohms and C in in farads (or R in meg ohms and C in microfarads, etc.)

After you get that all working to your satisfaction and you have load

tested it with some pairs of 12 volt bulbs in series (or other 24 volt

capable loads) to see if the ripple voltage is as expected and the

transformer doesn't overheat, etc. you can start thinking about

regulation.