SUPPLYING 24VDC TO A 24VAC INPUT THAT GETS RECTIFIED. AN ISSUE? - Page 1
Forum section: electronics / projects

author: accesscontrolforum
author_rank: Contributor
timestamp: September 16, 2014, 10:31:27 pm
content: We have a device that is powered by a 120VAC stepped down to 24VAC transformer. The AC appears to be rectified to 24VDC immediately then DC powers all the components. I can only power this device via solar panels, 2 12VDC in series with batteries.1. Are there any fundamental issues supplying DC to a rectifier?2. In the circuit board shown do you see any long term issues supplying this board with DC?I have labeled the circuit board front and back as best I could pertaining to this matter.I know I am putting to much thought into this, my guess is the diode doesn't have to do any work it's already done so there's no issue. But I need to set it and forget it in this installation so I am covering my butt.ThanksACF

author: Phaedrus
author_rank: Frequent Contributor
timestamp: September 16, 2014, 10:56:01 pm
content: I haven't looked at your circuit, but your 24VAC line is actually +/-34V. 24VAC is the RMS (root mean square) value. Your device could be taking as high as 30VDC depending on the filter capacitor. Of course, it could still work on 24VDC.

author: David Hess
author_rank: Super Contributor
timestamp: September 16, 2014, 11:17:59 pm
content: That is usually the case but 24 volts AC will produce 1.414 * 24 - Vdiode or about 33 volts DC peak when rectified so I do not see any way that the 24 volt DC out can actually be 24 volts DC but I doubt the absolute value is critical. From the looks of it, any DC voltage from 12 to 33 volts will work.

author: accesscontrolforum
author_rank: Contributor
timestamp: September 16, 2014, 11:21:15 pm
content: Phaedrus, Not sure if this relates to sending DC to a rectifier but it seems like the RMS value is all that I would care about. Isn't that the effective DC equivalent supplied to a load?Thanks

author: accesscontrolforum
author_rank: Contributor
timestamp: September 16, 2014, 11:30:03 pm
content: I think I see what you are saying. My transformer could be (is) outputting more than 24VAC. In turn my device may need more than 24VDC. I just supplied it with 120VAC as a test and the transformer is putting out 28.14VAC and the DC is 35VDC.Also that 35VDC drops to 28VDC under a 100ma +/-10% load

author: onlooker
author_rank: Frequent Contributor
timestamp: September 17, 2014, 12:00:30 am
content: QuoteDo you really mean to "SUPPLYING 24VDC TO A 24VACINPUT.."? Then, you can't use the transformer any longer.A better way is to parallel your new DC supply to the DC side of the original power supply, but, also serial a diode in you new DC supply to prevent back charging the new supply. In this way, you can have both supplies on at the same time or individually. Even if you are getting rid of the transformer permanently, this is still better, though you do not need to serial the diode now.

author: accesscontrolforum
author_rank: Contributor
timestamp: September 17, 2014, 12:10:51 am
content: The 24vac transformer will no longer be used. It will be bypassed and the only power source I will have available is 24VDC via 2 12V solar panels in series.

author: SeanB
author_rank: Super Contributor
timestamp: September 17, 2014, 04:38:11 am
content: Your 12V solar panels typically deliver 16-19V depending on load, so they might actually deliver close to 40V for the board. This will work most likely.

author: accesscontrolforum
author_rank: Contributor
timestamp: September 17, 2014, 02:51:05 pm
content: Supplying enough DC won't be an issue. I am still trying to figure out:"Are there any fundamental issues supplying DC to a rectifier?"

author: 0xfede
author_rank: Regular Contributor
timestamp: September 17, 2014, 03:47:26 pm
content: Hi accesscontrolforum.For what I can see this module seems an HF superetherodyne receiver for automatic door opener or something like that.The only issue that I see is if the circuit uses the mains frequency for doing stuff (but is unlikely IMHO).That diode will not be extra loaded by DC.Regards,0xfede

author: Whales
author_rank: Super Contributor
timestamp: September 17, 2014, 04:20:39 pm
content: Potentially this could overload the bridge rectifier, depending on how much headroom was designed in.AC waveform -> heat loss is shared over 4 diodesDC waveform -> heat loss is shared over only 2 diodesConsidering how little power this circuit probably takes (motors are not driven off it directly, are they?) it should not cause you issues.

author: accesscontrolforum
author_rank: Contributor
timestamp: September 17, 2014, 09:47:31 pm
content: 0xfede, It is a 412MHZ receiver for a "emergency" ingress device. It is in part a designed for a garage door opener. It takes the incoming Rf and send it to different receivers. If they match access is granted. Thanks!

author: accesscontrolforum
author_rank: Contributor
timestamp: September 17, 2014, 09:53:02 pm
content: Whales I am going to assume this is a half wave rectifier. I only see one diode for the power side. However I never thought of heat dissipation. Could be wrong though? No motors or "hi"current devices from my test the highest load will be 130ma +/-5%.Also I temporarily powered it via 26VDC from a regulated power supply and it worked. I am going to take some temperature measurements with an AC source and compare with a DC source and see if the are any issues.

author: Whales
author_rank: Super Contributor
timestamp: September 17, 2014, 10:10:32 pm
content: Quote from: accesscontrolforum on September 17, 2014, 09:53:02 pmAh woops -- I made an assumption

author: accesscontrolforum
author_rank: Contributor
timestamp: September 18, 2014, 11:56:55 pm
content: I will get some pictures in a week or so. But the system is installed and working. I will check my voltage readings to see if I got my panel size correct. I went with 5w panels thats about 300-350ma by my guess just barely enough to recharge the batteries.Thanks for all the help!

author: G7PSK
author_rank: Super Contributor
timestamp: September 19, 2014, 02:43:24 pm
content: Probably dose not apply to your board here but I have done the same sort of thing in the past and supplied DC into a bridge rectifier only to have half the bridge blow, I think due to the rectifier being run close to its limits on AC and the DC loads one side of the rectifier and causes it to overheat the problem has always been overcome by fitting a bigger bridge or supplying the DC to the other side of the bridge.

author: David Hess
author_rank: Super Contributor
timestamp: September 19, 2014, 07:35:30 pm
content: Quote from: G7PSK on September 19, 2014, 02:43:24 pmI have always wondered if this could happen but have never run across a practical implementation where it did.On one hand, the power dissipation with DC occurs over half as many diodes. On the other hand, total power dissipation is greater with AC because the loss occurs at higher RMS current with conduction only at the peaks. On the gripping hand, the original design was probably not derated sufficiently.