DIY Power supply kit suggestion (Fan & pull-up resistor, cap)

Question

I bought this 0-30v power supply DIY kit... There is three suggestion I need...

1. I want to use a 12V fan, so I'm changing the 7824 to 7812. So, I'm thinking to change the pull-up resistor (2.2K) to 1K/1W. Is it right? (as \$V =I * R\$)

2. I don't know about this heat-sink plate... Should I use heat-sink paste also?

3. For C1, can I use 2200uF instead of 3300uF? The provided capacitor is okay but came dented, and I have some 2200uF capacitors handy. (Is there any equation to get capacitor value?)

Answer 1

R1

You name this a pull-up, but if it's really between a 7824's Pin 1 (input) and Pin 2 (ground), it's an input-loading pull-down resistor. Since you don't change the input voltage, don't change the resistor.

Replacing 7824 with 7812

That means you'll have 12 more volts that the resistor has to drop. Also, a 12 V fan of same power as the original 24V fan needs to draw about twice the current – in sum, you will have more than twice the heat produced in U4.

It's unlikely that the heatsink was sufficiently oversized (these things are relatively expensive) to support that.

So, don't do that, unless you want to toast your 7812. Instead, you'd connect a switch-mode voltage regulator specified for the input voltage, 12 V output and the current you need. It's easy when you have such through-hole components – you can simply solder in wires to a sufficiently beefy buck converter module (don't go ebay/aliexpress on that – only buy things with proper datasheets).

heat sink "plate"

That's an isolating barrier that still lets thermal energy pass, typically made from mica. Yes, you'll need that if you want to attach your heatsink to a 78xx-style regulator.

2200 µF instead of 3300 µF

Obviously, someone designed in a 3300 µF capacitor because they decided that using a smaller value wasn't appropriate. That alone should tell you that, no, just reducing capacitance by 50% isn't a good idea.

Then again, this is just a buffering capacitor to smooth us the irregular and potentially very noise current draw of the fan. It is kind of usual to slightly overdimension that to allow for degradation and variability and noise-sensitive environments, so you might be fine. However, again, when going from a 24 V fan to a 12 V fan with the same power, you're doubling currents and hence probably also significantly increase current ripple – and that might be so much that it makes the life of your regulator hard.

So, I'll go with a "no, unless you know very well through measurements that the ripple you see is negligible and you don't run into EMI problems, you can't just replace a component with one 2/3 of the values".

Answer 2

Assuming it's a linear regulator...

the effect of a 2200uF cap will be to increase ripple on the UNregulated supply, especially at full load current. The regulated output will probably be fine up to about 27V but may drop out (start showing ripple) somewhere before 30V. (Assuming all else is OK ... correct transformer, nominal mains voltage, etc.).

This would be a serious defect in a piece of lab equipment for general use (which is supposed to be trustworthy anywhere within its ratings).

But if this is for your own use, you may decide you're OK with that.

Just check the supply voltage looks clean on a scope whenever you're using it above 25V. (Actually, do that anyway, whatever the voltage setting, as a confidence check).