Hi,

I am trying to repair a relay driver board that has dual 78SR112HC regulators on it (which are obsolete) for power regulation I believe. The board mainly consists of ULN2803 drivers and relays.

1) is there some advantage over the 78SR112HC vs. say a "standard" 7812? I don't see anything in the data sheets (or I am overlooking it).

2) trying to understand the reason behind how they are hooked up together. Pin 1 is the input, 2 ground, and 3 the output of the 78SR112HC. See attached picture

- Now the manufacturer has connected the devices together on a daughter board with 3 connections: input, output and ground. Same configuration as the 78SR112HC (or 7812) pinout.

- What is the purpose of device #2? Is it there for some sort of feedback control?

3) can I simply install 2 7812's (or even just use 1 in place of this daughter card?

Thank you for any info/suggestions.

 

They big difference is the efficiency. They are switch mode regulators against analog reg.
Look at ....
http://au.element14.com/tracopower/...-5w/dp/2280247?MER=bn_search_2TP_LastViewed_1
There are now quite a lot. yeas ago I used the same as you have to replace and thought they were "magic".
Tracpower, Recom Power and others are similar.

Have a look at...
https://www.digikey.com.au/en/artic...ntages-and-disadvantages-of-linear-regulators

 

They look like they are wired to produce a +24 volt output.

 

Sorry, I didn't read it fully. We had a visitor arrive so I just quickly posted an example of a 5V one.
Still, if it looks to just be a 24V supply, some of the regs I posted earlier have a range of output voltages available so select a 24V out version.
What is the input voltage to the card? You will need to make sure the replacement reg can handle the supply.
These are good for 72V in so will be fine, but only 300mA out.
This is to use just a singe reg for 24V out.
http://au.element14.com/recom-power/r-78hb24-0-3/converter-dc-dc-0-3a-24v-sip3/dp/1793168?st=non isolated 24v DC/DC

But you may be able to get a couple of 12V regs and do the same a the original.
http://au.element14.com/tracopower/tsrn-1-24120/dc-dc-converter-smd-1a-12v-12w/dp/2280250?st=TSRN 1-24
I've not run them this way but going by the circuit you posted, it look like it will work.

 

Thank you for the replies.

1) I suspected that they may be wired to supply 24v output since all the relays are 24v, but was not sure. Makes sense since that type of regular was not available as a 24v device. Just never seen anyone do it this way before.

2) input voltage I believe should be within the range of 24v (looks like it is coming off a 24v winding transformer). The OEM device has a max of 30v listed. I don't have access to an actual schematic of the board - so it is guess work.

3) I do have a couple 7812 regulator handy, does anyone see a problem if I just install them in place of the 78SR112HC, even if temporary? Will they work hooked up that way and provide 24v output?

Cheers

 

3) I do have a couple 7812 regulator handy, does anyone see a problem if I just install them in place of the 78SR112HC, even if temporary? Will they work hooked up that way and provide 24v output?

A big problem may be heat. The switch mode regs don't waste anywhere as much power as the linear.
Have a look at these..
https://www.ebay.com.au/itm/LM2596-...686302?hash=item3614a6715e:g:j3EAAOSw4A5YzI3H

This type are very handy but the input volts limit will be to low for your application..
https://www.ebay.com.au/itm/10-PCS-...400436&hash=item3af2a5d444:g:essAAOSwIINZzHl-

 

Give it a try, I think it will work. You may have to add a resistive load cross the 47 uF cap as the first stage will need to be able to sink some current through pin 2. Try 1K.

 

You may need heatsinking. Just remember, the first regulator tab will be over 12V and not 0V so will need to be insulated if you use a common heatsink.

 

Ok thanks.

I will add a resistor and I was already planning on adding separate heat sinks.

Cheers,