Back to Analog

Enter Your Electronics & Design Project for a chance to win a $100 Shopping Cart!

Back to The Project14 homepage

Project14 Home
Monthly Themes
Monthly Theme Poll


As you all know I have been stripping a SONY Cassette Player/Recorder TC-188SD. After stripping I still have tons of room to stuff my 3 toroids,  power supply, 6 relays, Caps, 4x 12AX7, in nice colored shields, a TI MSP430 for control, and a partridge in the pear tree.  I had some four-inch wide plexiglass laying around from some drawer dividers and fits between the aluminum front panel and the tin chassis. The chassis is a real Rube Goldberg device (right & left sides, rear, bottom cover, front, and switch mounting plate and don't forget the nice front panel), all held together with lots of screws.

I originally going to use 4 Crydom SS Relays rated at 600vac @ 90amps for a small load and use zero crossing plus delay for startup. But I most likely will just use a TI MSP430 to control all my relays 3 of them are for the toroids with a resistor and a MOV in series with the transformer. This is the startup procedure:

  • The user pushes in the main power switch.
  • MSP430 Comes online
    • Opens Relay_1 (line power)
    • Closes Relay_2 (resistor short)
    • does the above for the three channels a light  a LED (ready)
  • MSP430 waits for the start switch to go low momentary.
  • Releases Relay_1 and turns on the power at the top or bottom of the curve.  So there is 0 current flow.
  • Releases Relay_2 after 5 seconds a LED will light
    • Does this for the three channels.

Why am I going through this for a power supply? Well, the problem with inrush current only applies to toroids. As you know Wattage between the Primary and Secondary should be the same (minus losses) but toroids have almost no loss and they saturate very fast where older iron transformers have a little loss and are not that quick to saturate. Given this problem, I don't want to be replacing the dang fuses all the time. So I will try three techniques for limiting inrush current.

  1. A relay and a current limiting resistor. (very old school)
  2. A MOV or a Metal Oxide Varistor, which is used with great success.
  3. An MSP430 for time delays for the resistor relay and a second relay which turns on a toroid at zero crossing.

Rember that W = I x E So:

Let us do the Secondary loads first.

On the heater side, I need 150ma per tube or 600ma at 12vac.  So, 0.6 x 12 or about 12 watts.

On the HV side, I need 6ma per tube or  0.06 x 2 @ 220vac. So .12 x 220 before the bridge. or about 26.4 watts

Now we can do the Primary loads. now we can use I = W/E.

For the heaters: 12 / 115 or about 0.10434 amps. So for the Heater fuse, I will need a 1/4AMP Slow Blow

For the Hv:  26.4 / 115 or about 0.2295 amps. So for the HV fuses, I will need a 1/3AMP Slow Blow

For the Mainline fuse well lets jus add up the loads, 1/4 + 1/3 + 1/3 + Wallwart.  So a nice 1.5AMP Slow Blow.


And yes there is a main power switch but I can't find the symbol in KiCAD



Back to the Monthly Index