I accidentally purchase a larger 24vac transformer

Primary 120v/208v/240v Secondary 24 Output 40VA 50/60 HZ

This transformer is not center tapped. Rather has only 1 green and 1 red wire on the secondary. Can this be used as a linear power supply with bridge rectifier?

  • \$\begingroup\$ Yes, if you only want a single output voltage. \$\endgroup\$ – Finbarr Apr 25 '19 at 21:10
  • \$\begingroup\$ So you cannot use somthing like the LM317 variable with it. Is that what you mean? \$\endgroup\$ – queennikki1972 Apr 25 '19 at 21:11
  • \$\begingroup\$ You can use a LM317 with it too. What are you trying to do? \$\endgroup\$ – Finbarr Apr 25 '19 at 21:15
  • \$\begingroup\$ Center taps give you a positive and a negative rail, or you may use only two diodes instead of a bridge rectifier. \$\endgroup\$ – Janka Apr 25 '19 at 21:16
  • 2
    \$\begingroup\$ You already have a question about the transformer selection for what you yourself explained is the very same project at electronics.stackexchange.com/questions/435460/… DO NOT REPOST. Instead, edit the actual requirements into your original question. \$\endgroup\$ – Chris Stratton Apr 26 '19 at 0:29

My transformer is not center tapped. Can this be used to build a linear power supply with bridge rectifier?

Yes you can build a linear power supply with your transformer.
Note: You should always add extra information into the question rather than add as a comment. This makes it easier for people to help you.

There is nothing wrong with the general schematic you posted in a comment, but it's worth pointing out some restriction you will have in building a linear power supply.

enter image description here

Your specifications (5A 0-30VDC) CANNOT be met however. There are various problems preventing you achieving the results.

  1. The transformer is able to provide only 40VA. So for a 24VAC transformer this would equate to an RMS current of about 1.6A.
  2. Because the conduction angle of the bridge rectifier is low, the voltage drop at peak current will impact the unregulated voltage you can get. This depends on the resistance of the transformer windings, but with the schematic shown you are likely to see no more than 32VDC lightly loaded, and probably in the 27-28VDC range when fully loaded (and that will not be 5A).
  3. In a linear regulator where the unregulated voltage is (relatively constant) and the output voltage is much lower there will be considerable heat dissipation in the LM317.
    For example if you draw just 500mA at 5V output voltage the LM317 would dissipate about:
    (32 - 5) * 0.5 = 13.5W.
    IF you could (and you cannot) draw 5A @5V the power dissipated would be approximately 135W !!!!
  4. There are very few linear regulators that can be adjusted to zero volts output. In the case of the LM317 the lower limit is about 1.2 - 1.3 volts (it depends on the internal reference voltage).
  5. There are very few linear regulators that can regulate all the way to the unregulated voltage. They have some minimum Vunreg - Vreg limit. In the case of the LM317 its about 1.5V.

So in summary the schematic you show is ok (I'd add a fuse in the mains side to protect the supply), but won't meet the specs you want (5A @0-30VDC).
To get even a 1A output supply over the full adjustable voltage range would require a large heatsink, and the output short circuit limits won't prevent you overloading the transformer which is only rated to about 40W.

If you want to build a flexible small bench power supply you may be better advised to use a pre built switching regulator instead of the LM317. The advantage is that the power dissipation in the regulator is much less. A switching regulator also transforms the input into the output side and provides a much higher current output when used at lower voltages.

For example, I use small units based on the LM2596 switching regulator for variable supply use. I also tend to use computer laptop 19VDC (readily available in 45 -240W range) supplies since they are so much cheaper than transformers (and regulated to boot).

enter image description here

These are extremely low cost and would work over the same voltage range.

You might (depends entirely on your transformer) be able to achieve close to 1A at 25-27V and close to 3A at 5VDC.

  • 1
    \$\begingroup\$ Very helpful answer. Thank you \$\endgroup\$ – queennikki1972 Apr 25 '19 at 22:35

Not the answer you're looking for? Browse other questions tagged or ask your own question.