Hey guys
I have purchased a 220VAC->24VAC transformer. I have used a 800V 4A bridge rectifier to change it to 24VDC and one 3300uf 25V Cap to smooth the output. an image:

But I got a problem. when I measure the output, it is around 33v! why?

P.S. What's the matter with the CMS of the forum? I just get this error "500 - Internal Server Error"

 

24V ac is a RMS value so the peak value is 24V*1.41 = 33.84Vpeak. And the capacitor will be charge to this peak voltage.
But after you load your circuit the voltage will drop depending on the current load.

 

Hey guys
I have purchased a 220VAC->24VAC transformer. I have used a 800V 4A bridge rectifier to change it to 24VDC and one 3300uf 25V Cap to smooth the output. an image:

But I got a problem. when I measure the output, it is around 33v! why?

P.S. What's the matter with the CMS of the forum? I just get this error "500 - Internal Server Error"

The transformer rating is RMS voltage. The capacitor charges to peak voltage - 1.4 X RMS.

 

IOW for 24vdc you need a 17v/18v secondary.
Max.

 

Normally, you would put an IC regulator between the filter capacitor and the final 24V load...

You must estimate the power dissipation in the regulator, and heat-sink accordingly. For example, if your ultimate load was 1A, the regulator would be dissipating E*I = ~(32-24)V*1A = 8W, which is a pretty big heat-sink...

 

Thank you so much guys.
I want to use this Buck module to supply my MCU and I want to turn on my heater(it needs 24v 2A). Can I reduce its voltage?

 

Also the need for a regulator will depend on the type of anticipated load, for e.g. if this is for a DC motor PWM drive etc, regulation is not needed and at least 10% over plate voltage is not only acceptable but common practice.
Lamp loads are another.
I see you are using it for a heater, this does not require a smoothing cap and can be ran off directly after the bridge.
If you need a cap for the SMPS module you can place a single diode between the rectifier out and the cap.
Max.

 

...I see you are using it for a heater, this does not require a smoothing cap and can be ran off directly after the bridge.
If you need a cap for the SMPS module you can place a single diode between the rectifier out and the cap.

Thanks Max but I have to reduce the voltage to 24v because I have a thermocouple in series with heater. then I MUST reduce it to 24v.

 

And please keep in mind that that's not supposed to turn on the heater always. then sometimes it is turned off.

 

then I MUST reduce it to 24v.

You will if the output of the bridge is 24vdc without the Capacitor, or the series rectifier before the cap.
Max.

 

Ok, then if I remove the Cap from circuit, each peak would be 24v. am I right? if so, then alone problem would be the buck module. you said a diode would solve this problem? right? if so, a 1n4007 could be enough?

 

Ok, then if I remove the Cap from circuit, each peak would be 24v. am I right? if so, then alone problem would be the buck module. you said a diode would solve this problem? right? if so, a 1n4007 could be enough?

A 24v sec would give you around 24vdc with no cap, a in4007 is good for 1amp so as long as the buck module and its load is less than this OK, O.W. you will need one with a higher current capacity.
Max.

 

I think I need less than 0.5A for the output of my buck module. then it's ok but if one diode couldn't reduce the output, can I increase it to two or three?

 

The diode is not for reduction but to isolate the heater from the smoothed DC.
The module takes up to 40vdc input.
I assume you want output for your MCU.
Max.

 

Thanks Max but I have to reduce the voltage to 24v because I have a thermocouple in series with heater. then I MUST reduce it to 24v.

Thermocouple in series with heater? Can you provide a drawing of what exactly you are trying to do?

Ron

 

He means thermostat, not thermocouple.

The maximum allowed voltage input to the buck module is 40V. Here is what is proposed:



I would need to know the heater Wattage (or resistance), and the actual current into the buck module to do this more accurately.

Note that with my guess of a 24V, 24Ω, 24W, 1A heater, if you feed it full-wave rectified, unfiltered 24Vrms, the actual power is only 20.4W. This makes sense, because of the two diode drops in the fwb. See the light blue trace. I let LTSpice integrate that waveform to find the 20.4W.

For the buck regulator, I guessed the input current.

 

I have found that typically the output of a 24vac secondary is often something higher than this, depending somewhat on the supply voltage.
Max.

 

He means thermostat, not thermocouple.

Thank You Mike. I should have known.

Ron

 

The transformer rating is RMS voltage. The capacitor charges to peak voltage - 1.4 X RMS.

In addition to that; there's the question of the transformer's regulation factor - the figure mentioned by the TS isn't disasterously terrible.

Smaller transformers often give even higher off load voltage, and don't always hold up to the listed voltage when fully loaded.

AFAIK: any spikes that arrive with the mains can pump up the reservoir capacitor a little when there's nothing to dissipate it.

 

I have yet to see any mention of heater current and Transformer Va rating?
Max.