Currents and Transformers

Salvaged Transformer candidate

So not feeling like assembling another annoying voltmeter tonight, I thought I’d do some testing to get a real idea of parts of the control voltage circuit. As I may of mentioned, I had a couple of extra transformers hanging about, so I thought I’d rectify their outputs and see if it could work for me. My requirements again are that I have at least one output capable of 26.5V rectified DC. Even better if it had multiple outputs that could be rectified and regulated to give 24Vdc, 12Vdc, and 5Vdc.

So I have one with three pairs of secondary wires that I had measured before, and they seemed close – but the only way to tell is rectify it and take readings. I used a simple integrated bridge rectifier and a single 1000µF cap for filtering. Probably excessive, but more fun anyway.

On the first pair, I measured 22Vac and got 27.4Vdc, perfect! I can get 24V from that. On the second I had 11Vac and got a disappointedly low 13.12Vdc. Linear voltage regulators like the LM7812 have a large dropout voltage of around 2.5V so it’s best to exceed that to get a stable regulation at your target voltage. I would need at least 14.5Vdc preferably higher by a volt or two. The last pair originally measured 8Vac and 9.22Vdc, perfectly fine for 5Vdc. Dang, if only the middle one was two volts higher I would be rejoicing.

Now I know what your saying, I could mess with combinations of secondaries in series to get a larger Voltage on the middle one. Fact is, I was hoping to keep them all separate, and if I am to combine them anyway, I might as well just buy a new transformer and save this (salvaged) one for another project. Gives me an excuse to go shopping, not like I need a reason.

Relays and current consumption

The largest draw on my control voltage board will definitely be my relays. There are five of them in the proposed circuit. Two for output of the power supply to whatever project I connect to it, and three to switch the current shunt between the three rails. I suppose I could have put the shunt on the positive rail and call it a day, but I really wanted the flexibility of seeing the current flow on any of the V+, V and V0 rails. The relays are used since I’ll be passing up to 3A of current through them, so the more traditional band switch would not work.

Anyway, I powered up my relays, both singly and together and got the readings I was expecting, more or less. The larger pair of relays for the output are real power hogs. They are 40A rated (total overkill) and suck 130mA each when actuated. The trio of smaller relays are 10A rated and consume a more modest 75mA each. They measured in at a total of 450mA which is close to the projected 485mA value from the simple arithmetic.

The conclusion of both of these tests are that I will need to go transformer shopping. I’m not about to waste a centre-tapped transformer, or a three-secondary one for this application. I’ll probably just grab a 24Vac one. 1A should be more than enough for my needs since it doesn’t look like I will top 600mA anyway. Always best to have breathing room.

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