Schematics and component values

[jeroddumas]

I have a question involving some of the component values shown on thee site. Would yF be the same as uF for capacitor values? Also for resistor values I am guessing that 6k8 is the same as 6.8k. Thanks for any help.

Jerod

[Mark Burnley]

Hi Jerod,

Yes, "yF" is sometimes seen in older schematics and means "uF".

For resistors, the suffix is often used in place of the decimal point to save confusion- a decimal point can easily be misinterpreted. So you get:

6k8 = 6.8k
2M2 = 2.2M
4R7 = 4.7R

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Mark

[mcs]

If you are talking about µF, yes that's the same thing as uF. If it is actually yF, I guess they are trying to write µF...

6k8 is the same as 6.8k. R, k and M are often used as decimal points to make schematics easier to read.

Best regards,

Mikkel C. Simonsen

[jeroddumas]

thanks to all. Does anyone know where i can get some 1000uF 450v electrolytic caps?

[Mark Burnley]

Well,

That's quite a high capacitance at such a high voltage- I don't think I've seen any of that value at 450V in the usual catalogues. If you really need that large a value cap, it may be best to parallel four 220uF caps (which I know you can get at 450V!) What's the application- tube circuits don't tend to need capacitors as large as that.

Mark

[mcs]

You can get caps as large as 1000µF 450V easily - Rifa makes 6000µF 450V caps for instance. But just like Mark I doubt you will need caps that large...

Best regards,

Mikkel C. Simonsen

[jeroddumas]

I want to build the slowblow mic pre mention on the tech talk pages. The power supply calls for 1000uf at 400v. Or is that a mistake? I haven't been able to find that value.

[Mark Burnley]

Jerod,

I've had a look at the schem- it needs 3 1000uF 450V caps- you're right. IIRC, the designer has been on this forum before, and he was always concerned with building very high quality low noise power supplies. This PSU has an RC filter followed by an LC filter with a 10H choke- pretty impressive stuff!

I think for now you could get the circuit up and running by replacing the 1000uF caps with 220uF or 470uF caps (rated at 450V of course). Another option is to use two 2200uF 220V capacitors in series for each 1000uF 450V. Now, there is a very important addition needed for connecting capacitors in series in an HT power supply. You must add a potential divider network to help equalise the voltage across the capacitors.

Due to manufacturing processes, the tolerance of electolytics varies widely. When connected in series, the voltage across the capacitors is related to the charge on them, which in turn is related to their value. So if you just connected two capacitors in series across the supply rails, it's highly likely that the voltage will not split across them exactly in half. If this happens, then the max voltage rating of one of the caps will be exceeded with explosive (and messy!) results :(

So to equalise the voltage, you wire two equal-value resistors in series alongside the series connected capacitors, and wire the mid-points together. So the centre connection of the capacitors and resistors is commoned. This way the resistor potential divider "forces" exactly half the PSU voltage across each capacitor. You also get the added bonus of "bleed" resistors- so when switched off, the filter capacitors are allowed to discharge through their respective capacitors, which can be an added safety bonus.

I usually use resistors of value 100k-330k, and rate them at about 1W (you can calculate exactly the power dissipation by doing P=V^2/R)

Hope this helps!

Mark

[mcs]

I think the 1000µF caps are overkill. Try the circuit using 100µF caps, and I think it should work fine. If not you can always add more caps later on.

In the powersupply for my tape head preamp (weaker signal than a mic) I use a 33µF cap followed by a 100µF cap and no chokes. And it is very quiet anyway...

Best regards,

Mikkel C. Simonsen

[Mark Burnley]

Yea,

I agree. I've been using two 220uF series-connected 220V capacitors in a "pi" network- two sets of caps with dropper resistor in between- for mic pre experiments, and ripple wasn't an issue. So that's a total of ~200uF!

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Mark