Voltage drop on SSR01 under load

[zidor28]

Per Anders,

I have installed a +12V SSR01 regulator to power up a DDDAC board. At no load, the regulator provides 12.3V but when the DAC board is connected, the output voltage from the regulator drops to 2.5V.

I was wondering if other users have already faced this issue, and how to solve it. Thanks for any input you could provide!

Operation conditions are as follows:
On the SSR01 board, both J1 and J2 are bridged.
The DAC board should be drawing around 100mA.
Input voltage is 30VDC
There is a large heatsink over the LM317
Upfront DC regulation is done with a LC circuit (diode bridge, 17H in series, 100uF polypro in parallel)

[peranders]

How much current are you able to take out before you'll get a voltage drop?

[zidor28]

Well, I don't know how much current can I draw before voltage drops. What would be the best way to actually measure this (variable CCS, resistor, ... )?. Thanks.

[peranders]

I'll suggest that you try with 120 ohm or something. This would create 100 mA.

If the regulator works you should have a voltage drop of a few millivolts only and the voltage at pin 2 and 3 on the opamp should be within 1-2 mV, and that is 6.25V if the output voltage is 12.5 V.

[zidor28]

I've taken some measurements and with a 100 Ohm resistor, the voltage drops from 12.3V to 5.2. With a 1k resistor, then there is no drop.

[peranders]

When you load with 100 ohm, what is the voltage between emitter and collector of Q2? Voltage between in and out of IC1?

[zidor28]

With a 100 ohm resistor, I get a 0.9V difference on Q2 (pins 2 and 3), and a 1.67V difference on IC1 (LM317, pins 2 and 3). Thanks again.

[peranders]

The input voltage is? Something isn't right here.

Output = 5.2
Q2 = 0.9
IC = 1.67

= 7.77 V is that what you have as input voltage? In order to work properly you must have 18V in min.

[zidor28]

That's strange indeed. Here are the DC voltage measurements.

Rectified B+ (before even connecting the SSR01): 30VDC
Input voltage on the SSR01 when connected to B+ but with no load on the output: 18.3V
Input voltage on the SSR01 when connected to B+ with a 100R load: 7.3V

[peranders]

Why do you have a huge inductor and a very minimal capacitance after the rectifier bridge?

[zidor28]

In my (limited, certainly) experience, choke rectification works well with a good amount of inductance (within current limits of the choke of course) and can handle lower values of capacitance. I've used this topology in a good number of projects (although never before in a 12V power supply). Let me try to increase capacitance and see how it goes.

[peranders]

If you use a super regulator with 120 dB in PSSR you don't need any inductor.

[zidor28]

This is a good point. I will try and let you know. In the meantime, thanks for your support.

[zidor28]

Per Anders, here is what was at play. The DAC board was actually drawing more than 100mA. The choke was designed for this rating, which wasn't enough. Hence the choke saturated and behaved as a resistance. Installing a choke with higher current capacity has solved the issue. Thanks again for your support and for the regulator.