Linear Battery Eliminator for VTVM's (REV 1.0)

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In this video we take a look at a design that I am working on I would like to build a battery eliminator that works on a linier supply instead of a switching supply. Tolerances are tight engineering is bad and an adventure issues. As always more is on the way, and I will see everyone in the comments.

** Repair exposes the Tech to mains electric and should only be done by trained personnel. Errors can be fatal. **

[td]

I have also designed this same circuit to (universally) tame wall warts, in a user friendly format. Meaning, it is small, has sockets to change component values, a barrel connector input and pads for the output. Super handy.

[johnbonham]

Hi, very interesting little project, just as an exercise of constructive criticism, I think you are having a little mathematical issue here. If the nominal output of the transformer is 2.5 VAC, the rectified voltage would be 2.5 x sqrt(2) - 1.4 (the dropout voltage of 2 diodes of the full bridge rectifier), so it would be quite close to 2.13 VDC (loaded). The regulator has a dropout voltage of 0.066 volts, so in order to calculate the value of the capacitor, the equation is: i=C*dV/dt so at 0.3A load, C=0.3A*0.0083s/(2.13-1.566)v, so C=4389uF. (dt=1/freq, actually the period of the rectified wave). If you want to make it less sensitive to low mains voltage, you could try a bridge rectifier made of Schottky diodes, and the capacitance may be reduced to around 1800uF. Good luck.

[davorst]

When i saw you put 220u cap and at that low mA current draw regulator stopped to regulate i was really wondering when you gonna bump up that bulk cap. But i'm really surprised that at 50mA with 220u you got that big ripple. I also checked regulator datasheet and it's low dropout so i knew problem wasn't in transformer cause you should get enough voltage after rectifier if bulk cap was big enough. This is a great video for beginners to show them how to properly design a regulator, how to troubleshoot it if something is wrong and that stuff. Big thumbs up!

Btw speaking of VTVM's i bought recently some german VTVM that doesn't have battery inside. It's probably getting voltage from mains transformer for resistance measurement, but i didn't checked schematic to see how that voltage is generated. I checked what i think it's some reference voltage for calibrating and after all this years it is still spot on, -10V reading on my DMM.

[Edisson.]

Hi, I kind of didn't understand why you didn't try two-way rectification when you have a transformer with a center tap, the capacitor could be smaller and the ripple would be reduced.
Your transformer seemed to me too big to fit in the space for the D battery, so I looked at the availability and found HAHN 21.7x22.6x15 mm 1x4V 0.250VA, so if it was enough to have approx. 60mA at the output, the whole system would had it built directly into the battery pack D 34.2 x 61.5 mm.
Or like I did when I was making a new power supply for Tektronix 212, I didn't want to make an original modification from Tektronix, I didn't want batteries working with the original power supply as a filter in the oscilloscope. I made a linear power source, I placed it in one of the original holders instead of batteries, and I built the transformer into a small adapter that plugs directly into the mains socket. The adapter also serves as a 230V supply for my small curve tracker for which the TEK 212 is a monitor.
I made several attempts to replace the battery with a source, but in some cases I had a problem with the capacitive coupling to the network thanks to the transformer. When I made a quadruple digital independent voltmeter 5digit +/- 200V, I used a B1212S DC/DC module to separate them from each other and it works perfectly (if interested, I can send some photos to my e-mail)

Nice day 🙂 Tom

[td]

Please know that lin v regs attain their best ripple rejection at a v headroom that is significantly greater than their dropout voltage. Say 3v verses 1.5V. Using (only) the v dropout value as headroom to supply the v reg is a common error. This is usually stated more clearly in the app notes. The DS usually only shows the higher value as the headroom for ripple testing. See the graph. Why? Because the marketing department can claim the higher ripple rejection rate, while not pointing out that additional headroom is needed to attain it. So, depending on the your particular v reg's specs, you might be starving it for ripple headroom/voltage. Chasing the extra voltage with a cap may not work. Diminishing returns? Or, it may. What do I know; I'm just a hobbyist... HTH.