Why are any caps necessary? We're driving a relay coil which doesn't require filtered power.

 

Why are any caps necessary? We're driving a relay coil which doesn't require filtered power.

au contraire mon frere, DC relays do not have the copper shorted turn like AC relays do. They buzz loudly if not fed filtered DC

 

Not in my experience, the back emf, especially if a diode is fitted retains them when the ripple transitions to zero. 160Hz is too fast for the armature to respond.
With AC, a copper shading ring is introduced to provide a phase shift so there is current flow through the zero point reversal.
Heavy current DC brakes and clutches have been used for years in industry without the use of capacitors.
Max.

 

...
Heavy current DC brakes and clutches have been used for years in industry without the use of capacitors.
Max.

Their inductance is much higher than a small relay...

 

One experiment I did once was to time the delay of a small control relay fed from a unfiltered supply at switch off, but had a BEMF diode across the coil, I don't recall the actual time, but it greatly exceeded 8msecs.
Also no audible or visual chatter was seen.
Max.

 

au contraire mon frere, DC relays do not have the copper shorted turn like AC relays do. They buzz loudly if not fed filtered DC

Je l'ai! Learn something new every day. Relays buzz when you don't expect them to.

 

One experiment I did once was to time the delay of a small control relay fed from a unfiltered supply at switch off, but had a BEMF diode across the coil, I don't recall the actual time, but it greatly exceeded 8msecs.
Also no audible or visual chatter was seen.
Max.

Since the TS is building a tester for (among others) DC relays, wouldn't it be better to drive them with DC?

 

It is DC, just pulsating DC.
If a tester is being built for both 12v & 24v AC/DC, then the same transformer will work for both, otherwise you would need a 8.5vac and a 16.5vac for the DC relays, as well as 12vac and 24vac for the AC relays if DC filtered supplies are used.
Incidentally I have 6 different DC relays on hand from large to almost contactor size down to ice-cube and automotive type, both 12vdc & 24vdc and tested them on non-filtered supplies, non of them exhibited any buzzing even without diode, and also taking them down to the hysteresis point, where I would have expected it, if it was going to happen.
Max.

 

I agree on skipping DC filtration, un-necessary...

To create universal power source for your box, you need something to produce various voltages (transformer). Since some of them are DC, you need rectifier. And of course there need to be way to select voltage (either selector switch or use separate relay sockets).

I would not want to carry heavy box so one transformer would be ideal (as shown) but winding one would be painful (small transformers use a lot of turns of thin wire). There are companies that would make custom transformer but this would not be cost efficient. Practical workaround would be to use two or more small transformers with multiple taps.

 

I've been flip-flopping ala Kerry, on the topic of using capacitors. I have landed on the side of them not being necessary. As MaxHeadRoom noted, the transformers would have to be 8.5vac and 17vac to get 12 and 24 vdc filtered. There is another advantage with capacitors, as they can provide an extra surge of power when first activated. But as this is a tester for one relay at a time, I don't think this is a major requirement.

Nice neat schematic, vu2nan. It is similar to what I had in mind and corrected many of my mistakes. Great job.