Transformer Mixing

Alright, So I have a 19.5VAC transformer and a 5.2VDC transformer. Is there anyway I can link these two together so that they mix at a total of 24.7V? Any resistors I can use? Or can I break something by combining AC and DC power together? Anyone please tell me how to do this, thanks!

if you attempt any of this, I’ve got some words for you:


You are trying to mix an AC and a DC transformer. This will not work at all, especially when you later try to use it on a fire alarm system.

My best recommendation for you is to go buy a 24 VDC power supply.

Thanks! You saved me, I do have an old panel power supply, (Notifier SPF-400) I just don’t know how to wire it up. Can you help? Or do I need to post a new topic, I really don’t want to just to save space.

holy hell. what exactly are you trying to do?

Blow something up haha.


He’s trying to wire an AC and a DC transformer in series and then to a fire alarm.

Okay Captain Obvious… :mrgreen:

Thank you for bringing my self-esteem to an all time high, checking again it is 5.1VAC, not DC.

Well in that case you should be able to wire them in series.

I’d highly recommend just finding a 24 VAC transformer though.

Wait, do you want 24VDC or VAC?

VDC would be nice. But if it’s AC, I can just convert it with a rectifier. If anyone can help me find one.

Stop, just please, stop.

What you are attempting to do is build your own power supply but it appears to me you have very little knowledge of electronics. Building a power supply is not as simple as “one wire here, one wire there, rectifier in, rectifier out” – you need a lot more components.

First, you need a transformer that outputs at least 30 VAC. Why? The rectifier removes some of the voltage that passes through. If you use a 24 VAC transformer you’re only going to get about 18-20 FWR.

Second, straight out of the rectifier you’re going to get a messy output called Full Wave Rectified power – FWR. It’ll destroy any Simplex device, as well as ALL smoke detectors. To fix this you need a very large capacitor called a filter capacitor.

Third, the input voltage controls the output voltage. If you have a 120 VAC || 30 VAC transformer, and you connect 240 VAC, you’re going to get 40 VAC out. If the power increases or decreases at all, it will affect the output voltage. To fix this you need a linear regulator circuit. For 5V applications they are a piece of cake, but for something like 24V it is a lot harder to do. The schematic for a decent 24 VDC power supply is quite complicated.

120 VAC → Transformer → 30 VAC → Rectifier → 24-26 FWR → Filter Cap → 24-26 VDC Filtered → Linear Regulator → 24 VDC regulated

This is why I highly recommend you find a pre-made 24 VDC power supply at your current level of knowledge before you accidentally fry one of your alarms, or use the wrong capacitor and get sprayed by capacitor oil – pretty nasty stuff, or get shocked by high voltage.

Here’s one:
I need to warn you though that this is a switching power supply as opposed to linear. The difference is this one provides a more accurate voltage output by rapidly switching between regulator circuits. It’s not the best for this application but it will work until you can find a good linear power supply.

Hope this explanation helps.

Like I said before, I already have a 24VAC power supply. It’s from an old panel I had, If anyone can help me wire it then that would be great!


Just get an Altronix power supply and be done with it. no messing around or connecting things to each other. Altronix is the industry standard and is readily available.

Accidentaly posted this as a new topic.

Prog; VAC

Magnum Alert; I don’t have the money at the time.

Well, why don’t you wait until you HAVE money to do it the RIGHT way, without creating danger to yourself, others, and your house.

Because I need to make a new power supply cabinet. Like I said, I already have a 24V Transformer. I just need help wiring it up.

If you only want to use this for 24 VAC powered alarms then that’s all you can do with that transformer.

Read what I said above and you’ll see it won’t work.

I have an AC alarm, and yes, thanks for the tip on how to make it DC. I’ll hopefully do that someday, when I get the money.