has anyone got a DC load tester?

hi, i purchased a 78H05 off ebay and its not regulating its load in a 5A circuit. i want to know at what load does it fail.

If murata/traco did a mini package 5v 5A i’d look to that.


Have you got a heatsink on the regulator? A 5Vdc 5A linear regulator is going to get hot! Most devices have an automatic overtemperature shutdown circuit to prevent damage to the device. In terms of testing it a simple high power low value resistor will work just as well as an electronic load.


V / R = I

5 Volts / 1 Ohm = 5 Amps.


5 Vdc * 5 Amps = 25 Watts

According to the datasheet the case temperature of the device needs to be below 85 °C in order to provide a 5 Amp output without the over temperature shutdown occurring.

So you need a method of keeping the case of the regulator cool in order to keep the case temperature at 70 °C in order to sustain the 5 Amp current.

So you need a very large heatsink or a fan assisted heatsink in order to achieve that. I would get a heatsink similar to the one below and test it again.

Assuming your input voltage is 8 Vdc then the calculation is:

Power disspation (Pd) = (8 Vdc - 5 Vdc) * 5 Amps

Pd = 15 Watts


Ambient Junction Temperature = (150 °C - 27 °C) / 15 Watts

Ambient Junction Temperature = 8.2 °C / Watt

So you need a heatsink capable of dissipating 9 °C / Watt to attach to the regulator to prevent it overheating at 5 Amps.

You haven’t mentioned the regulator case style so I cannot suggest a specific heatsink however one of these will do:


Get a suitable heatsink and attach it to the regulator with heatpaste and then retest.

Strap a high power 1 Ohm resistor across the outputs to test with an ammeter in series to measure the current. Be careful though as the resistor and regulator will get hot.

That should probably do it. If you have any more questions please get back to me.



hi Alex,

so its bolted to a large metal case and my original 78H05 has no issue with this, but i have added a heatsink as well for good measure. I had this heatsink before i obtained this new 78H05

On the metal case, with my Raytek IR gun, i can see it doesn’t really get that hot for a good 20 minutes. I can touch the heatsink, anodized, for about 10 seconds - this is around 65C ambient 19C

This is the tin can TO-3

My input voltage is around 12v

The new 78H05 i purchased, it doesn’t deliver the 5v from the get go…

I believe both are Fairchild but it’s hard to discern the branding on TO-3s as there is no logo.

I wanted to try with a load meter so i can gradually increase the current load and monitor when it goes into shutdown.

Hopefully the seller will accept what i say and give me a refund.


It sounds like your new 78H05 is a fake. A T03 package bolted to a large ducted heatsink should not be shutting down when used in the normal fashion.

I don’t think we have an electronic load in the hackspace. It is on my list of things to make / buy but it’s quite a niche piece of test equipment. Most people don’t need one very often. It’s more a development / production device for people testing battery capacity or power supply output.

If you bought it from an online auction vendor I would ask for your money back. I can help you design a switch-mode version or try and help you source a genuine part.

Hope that helps.



i have raised with the italian supplier… you know i’m not sure if its a fake or its a part that should never have got out of the factory, a raggy doll if you are old enough to know the reference.

Re. SMPS… i want to start using SMPS for my TTL but keep my audio analog side linear because i find it to be lower noise.

I still have that Kurzweil K250 which i asked you about, back in May '15, and its now in a rack chassis with the power TDK Lambda linear 15/-15 and i’m running the 5v off an old ATX PC supply.

Nice Kit Kurzweil, I’d forgotten about our converation. Apologies a lot happens when time passes.

I agree with keeping audio kit linear as much as possible.

I got the Raggy dolls reference - ‘made imperfectly’